CompletPresentationA.an

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CEPATEC ABKnut-Erik Persson

PAPER MAKING, GENERALLY

1. Paper machine, different types2. Paper making, an introduction3. Fibre selection for different paper grades4. Interfibre bonding5. Chemical and mechanical pulps6. Stock composition, the furnish

Page1581012151. Paper machine, different types

The design of a paper machine can vary, but there are always a forming section, a press section and a drying section.

Fig. 1. Forming section, press section, drying section. (2-001.tif)

The sheet is formed in the forming section. The fibres in the stock are directed and spread, and during the dewatering they are fixed and formed in a connected web.

Fig. 2. Fourdrinier machine. (2-002.tif)

A formning section can have the original form

an ordinary headbox with perforated rolls

Fig. 3. Headbox with perforated rolls (KMV). (2-003.tif)

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a flat wire where the paper is formed.D-

Fig. 4. Illustration. Fourdrinier machine. (2-004.tif)

Genuine Fourdrinier machines are found mostly at old fine paper, kraft and at sack paper mills.

To increase the dewatering there is often a top wire on the Fourdrinier wire. This is particularly common on the machines producing printing and writing papers. Such machines are called hybrid (paper) machines.

Fig. 5. Illustration. Hybrid machine. (2-005.tif)

This headbox, called hydraulic headbox, is of a different type.

Fig. 6. Hydraulic headbox.(2-006.bmp)

Paper can be produced also on twin wire machines with hydraulic headboxes.

This one is for newsprint and a simplified sketch looks like this.

Fig. 7. Twin wire machine.(2-007.tif)

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Tissue is another grade, which in most cases is produced on double wire machines. The machines are smaller than those producing newsprint, but they run much faster.

Fig. 8. Illustration. Double wire machine producing tissue.(2-008.tif)

Some paper grades are produced both on double wire machines and on Fourdrinier machines. The genuine Fourdrinier machines are today rather unusual.

Fig. 9. Illustration. Fourdrinier wires on a board machine. (2-009.tif)

Paper board consists of several layers, so a board machine can have several Fourdrinier wires with headboxes of their own above a long wire. Each forming unit brings a layer. The layers are ”couched” together to a web with a multilayered structure.

In the press section the wet web is dewatered. The presses can be arranged in many ways. What determines the design is which sort of paper that is to be produced.

Fig. 10. Press section in a sack paper machine. (2-010.tif)

There may be two or more press nips, with one or two felts to each nip.

Fig. 11. Press section in a board machine. (2-011.tif)

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A press roll can be engaged in more than one press nip.

Fig. 12. Press section in a fine paper machine. (2-012.tif)

The press rolls are sometimes very big. Bigger rolls mean a longer press nip and a higher dry solids content after the nip.

Fig. 13. Press section with press rolls in the first nip and a shoe press in the second nip.(2-013.tif)

There may also be special cylinders which have very long nips, known as Shoe Presses.

tfi VALMET Paper Machinery

Fig. 14. Shoe press, Valmet.(2-014.tif)

One of the opposing rolls in a nip may be a hot drying cylinder, as on a tissue machine. A warmer nip means a drier web.

Fig. 15. Tissue machine. (2-015.tif)

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An ordinary drying section for most types of paper and board is a multicylinder section. Such a section can have more than 50 drying cylinders.

Fig. 16. Multicylinder section.(2-016.tif)

Sometimes the drying is a combination of a big drying cylinder and of blowing hot air from a hood covering most of the drying cylinder.

This method of drying paper is used preferably for soft tissue but also for drying some kraft paper and board. The cylinder is called yankee cylinder.

Fig. 17. Yankee cylinder, with a gas heated hood, in a tissue machine. (2-017.tif)

Conclusion: Paper machines can have different designs. Each paper grade requires its special concept.

2. Paper making, an introduction

Paper is a multi-layer network of paper fibres bound to each other. The fibres are very small. A sheet weighing one gram can contain several million fibres. Raw material for producing paper is mainly fibres from wood, but even various grass types can be used.

Fig. 18. Microscope photo. Newsprint. (STFI) (2-018.tif)

Actually, paper is produced in the same way in a paper machine as has been done manually during two thousand years. The process steps are similar.

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Before the dewatering, the fibres must be treated in a way making it possible to create strong bonds in the finished paper sheet.

During the dewatering the fibres form an even network. Manually this is done with a mould; a thin cloth of metal wire. The cloth catches the fibres and dewaters them to make them form a sheet.

Fig. 19. Manual paper production at Grycksbo Paper Mill. (2-019.tif)

The wet sheet is laid down, is couched, on a felt. The sheets and the felts are piled and the water is squeezed out of the sheets. The felts absorb the water. It is not possible to squeeze out all the water. To get the sheets dry they are hanged out.

The same steps still remain in the industrial process. In most cases different kinds of stock are used.

In a fine paper mill for example a blend of chemical soft- and hardwood fibres.

Fig. 20. A simplified process line, fine paper mill. (2-020.tif)

After a mechanical treatment, refining, the stocks are mixed in the blend or mixing chest. Normally different chemicals are added to the stock.

Fig. 21. Simplified process line, fine paper mill. The refining is marked.(2-021.tif)

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After cleaning, screening and perhaps also deaeration of the stock, the sheet is formed.

Fig. 22. Process line. Cleaning, deaeration, screening. (2-022.tif)

During the forming process the stock is first uniformly spread on a wire and dewatered to make a connected web.

Fig. 23. Headbox, wire section.(2-023.tif)

From the wire the wet paper web is brought to the presses, often with the help of a felt. The dry substance of the web is then about 20%.

Fig. 24. Paper web follows the top felt into the press section. (2-024.tif)

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In the press section the water is squeezed out of the web in a number of nips. Press felts support the web and maximise the water removal. When the web leaves the last press nip the dry solids content is usually between 40 and 50%.

Fig. 25. Illustration. Single felted press roll nip. (2-025.tif)

After the press section the web enters the drying section. The web is led above drying cylinders which on the inside are heated with steam. The water between and inside the fibres evaporates and the dry, solids content which is usually between 90 and 95%, is reached.

Fig. 26. Multicylinder dryer.(2-026.tif)

3. Fibre selection for different paper grades

At the production of paper it is important to utilise the specific properties of the fibres.

Long fibres from chemical softwood pulp are used for production of strong papers, e.g. sack paper, liner and wrapping paper.

Fig. 27. Illustration. Long softwood fibres. (2-027.tif)

Short fibres from chemical hardwood pulpgive the sheet a more even surface. Thus, they are suitable for finer printing paper or for the printing surface of the board.

Fig. 28. Illustration. Short hardwood fibres. (2-028.tif)

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4. Interfibre bonding

How is paper made? What makes the fibres in the wet web to keep together and what makes them bind so strong in the paper sheet? The fibres are hydrophilic. The water makes the fibres in the web to come close to each other. When the water evaporates hydrogen bonds are created between the fibres.

On the fibre surface and inside the fibre wall there are chemical groups called hydroxyl or OH groups.

The OH groups are bound to the carbon in the cellulose and the hemicellulose of the fibre wall.

Fig. 32. Illustration. Enlarged fibre surface with many OH groups.(2-032.tif)

The OH groups are attracted to each other. However the groups are not only attracted to each other but also to material with similar groups. Water is such a material.

Water is written with the chemical formula H2O. /0

\H

Fig. 33. Structural formula, water. (2-033.tif)

Water can help the OH groups to bind by forming bridges between them.

When the water evaporates the fibres draw closer. When they have come close enough bridges will be formed between the fibre surfaces with the help of water.

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Fig. 34. Illustration. Water molecules forming bridges between the OH groups on the fibre surfaces. (2-034.tif)

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When the water has evaporated, the water bridges disappear as well. But now the fibres are so close that the OH groups on the two fibre surfaces bind directly.

The OH groups bind to each other by hydrogen bonds.

Fig. 35. OH groups bind directly to each other with hydrogen bonds.(2-035.tif)

To develop the binding between the fibres as strong as possible, the fibres shall have a contact surface as large as possible. When the web is dewatered, pressed and dried in the paper machine the fibres come closer to each other. The area of direct contact between the surfaces becomes larger and more hydrogen bonds can be formed.

Fig. 36. Illustration. Soft, formable fibres. (2-036.tif)

The softer the fibres become and the easier they are to bend, the tighter they can fit to other fibres in a sheet and the larger becomes the area of direct contact between the adjacent fibre surfaces.

There must also be a great number of OH groups on the fibre surfaces which can bind to each other.

Fig. 37. Illustration. Hydrogen bonding between two fibres.(2-037.tif)

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The springwood fibre has a thinner fibre wall than the summerwood fibre and it collapses easier. Collapsed fibres bind more easily to other fibres.

Fig. 17. Illustration. Fibre collapse, springwood and summerwood fibres.(5-017.tif)

4. Refiner tackle, segments and fillings

The refiner tackle has bars and grooves. There are two parts. In most cases one of them is fixed and is called stator. The other one rotates and is called rotor.

Fig. 18. Illustration. Refiner filling: rotor and stator.(5-018.tif)

The refiner tackles may have different patterns. The design depends on the pulp grades and on the desired refining results.

The refining bars may have different length and width. The grooves between the bars may have different width, too.

STATOR

ROTORFig.19. Illustration. Refiner fillings. (5-019.tif)

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Wide bars are used for refining of long fibre pulp.

Fig. 20.1. Refiner fillings with wide bars. (5-020.1.tif)

Thin bars are used for refining of short fibre pulp.

Fig. 20.2 Refiner fillings with thin bars. (5-020.2.tif)

5. Gap clearance

The distance between the rotor and the stator bars is called gap clearance.

The gap clearance is changed according to how hard the fibres should be treated.

Fig. 21. Illustration. Gap clearance.(5-021.tif)

The gap clearance should be a bit larger than the fibre thickness. During the refining the fibres will pass through the gap.

If the gap is too small the fibres may be cut off. Normally, this should be avoided. Fig. 22. Illustration. Fibres

in a refiner gap. (5-022.tif)

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6. Refining machines

The refining, or beating, machines may have different designs but there are two main types; conical and disc refiners.

• The conical refiner is the oldest type.

Fig. 23. Classical conical refiner.(5-023.tif)

• The more modern conical refiners have a larger cone angle.

Fig. 24. Modern conical refiner: ”Conflo”. (5-024.tif)

• The disc refiner can have a single or a double disc gap. This one has double disc gaps and is called double disc refiner; two fixed discs and a rotor with bars on both sides.

Fig. 25. Double disc refiner.(5-025.tif)

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Main rt-7. Refining energy

The refining result depends on the applied energy as well as on how this is used within the process. The applied energy may be called the refining quantity and how it is used the refining quality.

The refining quantity is shown in kWh by multiplying the motor power by time. The power is read as kW and the time is one hour.

7.1 Idling power (No load power)

Not all applied power can be transferred into effective refining.

In all refining a certain amount of the applied power is consumed only to operate the rotor and to pump the pulp through the refiner. This is called the idling power.

Idling power, kW

The idling power is measured with water flowing trough the refiner and the gap clearance as narrow as possible without the bars touching each other.

The effective refining power, the net refining power, is estimated, if the applied power of the motor, the total power, is deducted by the idling power.

Total power, kW - Idling power, kW

= Net refining power, kW

7.2 Refining quantity

By refining quantity, kWh/ton, is understood the consumed net refining energy during one hour divided by the number of tons of fibres passing through the refiner during the same period.

Refining quantity:

Net energy, kWh/ton

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7.3 Refining quality

The added net refining energy can be used in different ways.

The energy can be divided either into

- a great number of small energy inputs

or into

Refining quality:

Net energy divided into many small or few large energy inputs

a few large energy inputs.

The different methods to use the net refining energy will influence the beating result and is consequently called the refining quality.

• Distance between refining bars

Tightly placed refining bars means that each fibre is treated many times while it passes through the refiner. The applied net refining energy is divided into many but small energy inputs.

- High number of low energy impacts gives a gentle refining.

Fig. 26. Illustration. Tightly placed refining bars.(5-026.tif)

If the bars are thinly placed the effect will of course be the opposite one. The applied net refining energy will be divided into a few but large energy inputs. There will be a greater stress on the fibres and the risk for damaging them increases.

- Low number of high energy impacts gives a more severe refining.

Fig. 27. Illustration. Thinly placed refining bars.(5-027.tif)

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The rotor speed is a factor influencing how many times a fibre will pass a refiner gap in the refining zone. A high speed means that the fibre passes through many gaps. The energy at every single treatment of the fibres will be low.

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- High number of low energy impacts gives a gentle refining.

Fig. 28. Illustration. Refining bars. Marked rotor speed.(5-028.tif)

• Bar width

Also the bar width influences the refining result. If the bar width increases it will take a longer time for a fibre to pass through the refiner gap. The net refining energy will be distributed during a longer time. The momentary energy input becomes lower; the stress on the fibres becomes less.

Fig. 29. Illustration. Wide refining bars. (5-029.tif)

- If the energy of impacts is constant, the severity of impacts decreases as the length of the impacts increases. The refining becomes more gentle.

Weak fibres, like e.g. hardwood pulp, require a gentle refining. By the same reason long fibre sulphite pulps are refined more gently than long fibre sulphate pulps.

The individual fibres are not treated but groups or networks of fibres are. The groups are broken and the fibres are redistributed to let other parts be treated.

Fig. 30. Illustration. Fibre in a refiner gap. (5-030.tif)

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Main rt-7.4 Fibre consistency

Until now we have talked about how the fibres, passing a refiner gap, are influenced by different quantities and qualities of energy. Ho w-ever, there is another factor influencing the refining result; the fibre consistency. The fibre consistency determines how many fibres will absorb the, in each impact, applied energy. Normally the fibres are refined at a consistency of 3.5 to 4.5% fibres in water.

Refining at a high fibre consistency means that there are many fibres in the refiner gap to absorb the applied energy.

Fig. 31. Illustration. Many fibres in a refiner gap. (5-031.tif)

If the consistency is decreased, the number of fibres being able to absorb the applied energy decreases. The stress on the individual fibres becomes greater. The risk for damaging the fibres increases.

Fig. 32. Illustration. Few fibres in a refiner gap. (5-032.tif)

8. What happens to the fibre during the refining?

When refining the fibre walls are treated. The primary wall, P, is more or less peeled off. The same happens to the outer secondary wall, S1. The secondary wall, S2, with its large

number of OH-groups, is released. The outer part of the S2 layer starts to

split up and the fibrils partially come loose. This is called outer fibrillation.

Fig. 33. Illustration. Refined chemical fibre.(5-033.tif)

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Even the inner part of the fibre wall, S2, is influenced by the refining. The

fibre wall starts to delaminate; called inner fibrillation.

When the outer fibre walls P and S1

are removed and the fibre wall S2 is delaminated, water can penetrate. The fibre wall begins to swell.

Fig. 34. Illustration. A split up fibre wall. (5-034.tif)

The water absorption increases the fibre’s softness and pliancy.

Fig. 35. Illustration. Soft, pliable fibres. (5-035.tif)

Later, when the water between and inside the soft fibres evaporates in the drying section fibres will easily collapse. When the web is finally dried the fibre surfaces come in close contacts and bind strongly to each other.

Fig. 36. Microscope picture. Liner. (STFI) (5-036.tif)

Thus, the main reason to refine is to attain an outer and an inner fibrillation but some undesired things happen.

Fig. 37. Microscope picture. Inner and outer fibrillation. (STFI) (5-037.tif)

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When the fibre wall is split up, parts of the wall are peeled off and a lot of fine material is created.

Fig. 38. Microscope picture. Fibre, surrounded by fibril-threads. (STFI) (5-038.tif)

The fibre is bent and wrinkled.

Fig. 39. Microscope picture. Fibre folds. (STFI) (5-039.tif)

The fibre may even be pressed together so hard that it will be cut off.

Fig. 40. Microscope picture. Fibres cut off during the refining. (5-040.tif)

A too intensive refining may be detrimental:

- The average fibre length decreases.- The fibre walls are weakened.- The fine material content becomes too high.

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J ■=;•The fibre changes may lead to:

A stock difficult to dewater. A lower paper strength.

During the refining some substances are released from the fibres. Small amounts of lignin, hemicellulose and wood resin will dissolve or disperse in the water.

9. The influence of the refining on the paper strength

The classical ”valley beater” is sometimes called a hollander. It existed before the conical refiner.

In the hollander the pulp circulates in a trough and at each turn it is treated by rotating knives working against fixed knives. The time is the measure of the energy used.

Fig. 41. Principle picture. Hollander. (5-041.tif)

When the properties of a paper produced from the stock is measured and related to the refining time, a diagram can illustrate how the properties are influenced by refining.

In the beginning the tensile strength increases sharply, but the more the fibre is refined the slower the increase.

Fig. 42. Diagram. Tensile strength after different refining times.(5-042.tif)

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At first, the paper’s tearing resistance increases, but later it decreases.

Fig. 43. Diagram. Tearing resistance after different refining times.(5-043.tif)

The more the pulp is refined the more the opacity decreases.

Fig. 44. Diagram. The opacity after different refining times. (5-044.tif)

The density slowly increases to an even level.

Fig. 45. Diagram. The density after different refining times. (5-045.tif)

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10. Measuring of the refining degree

It is desirable to be able to quickly check how much a pulp is refined. One well-tried method is to measure the stock’s drainage resistance. For a chemical pulp this is done in a standardised way with a Schopper-Riegler apparatus. The result is indicated as the SR number. Well-refined pulp has a great drainage resistance and gets a high SR number. Fig. 46. Principle sketch. Refining

grade gauge. (5-046.tif)

For a mechanical pulp another measuring method called Canadian Standard Freeness, CSF, or only Freeness, is used. On the whole the apparatus is similar and the procedure is about the same.

The Freeness method measures the drainabil-ity. Thus, a high Freeness number means a stock is easy to de-water.

The stock’s dewatering capacity can continuously be measured with a gauge installed in the process. The values are often reported as trend graphs.

Fig. 47. Connection between SR and CSF. (5-047.tif)

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11. Examples of products with different refining

A greaseproof paper is an extremely tight paper.

Fig. 48. Printed paper partly covered with greaseproof papers. (5-048.tif)

The paper is made from high-refined fibres with aim on inner fibrillation. The paper’s opacity is low. The more it is refined, the lower the opacity will be.

Fig. 49. Microscope picture. Greaseproof paper.(5-049.bmp)

The other extreme is a sack paper, which has to be very strong and resilient.

Fig. 50. Different types of paper sacks.(5-050.tif)

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Blend chest

-The sack paper stock is refined in two steps. In the first step the pulp is refined at a high consistency; HC refining. In most cases the refining consistency is about 35%. In the second step, the pulp is refined at a normal consistency of 3.5 to 5%; LC refining.

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Soft wood fibres

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Fig. 51. Flow diagram showing the refining of a sack paper. (5-051.tif)

At the high consistency the fibres are bent and lengthways compressed. At the same time the fine material formation is kept low.

The outer and inner fibrillation of the fibres, necessary to make them bind strongly to each other, is then achieved at the low consistency.

Fig. 52. Microscope picture. Fibres, refined at a high consistency. (5-052.tif)

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The freeness method measures the drain-ability. Thus, a high freeness number means an easily de-watered stock.

Fig. 57. Connection between SR and CSF. (6-057.tif)

6.2 Different freeness, influence on the paper properties

The freeness is measured to get an approximate apprehension of which properties a paper from the mechanical pulp will have.

Mechanical pulps always have a lower freeness than a normal, unrefined chemical pulp.

Fig. 58. Tambour with finished paper. (6-058.tif)

Because the chemical pulp is unrefined the amount of fine fraction is very low.

The mechanical pulps contain much more fine fraction.

Fig. 59. Unrefined chemical fibres. (6-059.tif)

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Of course, the freeness values can vary within the different types of mechanical pulps.

However, SGW always has the lowest freeness and CTMP the highest. The TMP freeness is somewhere between.

The difference depends on the amount of fine fraction, The greater it is, the lower the freeness value is.

6.2.1 Tensile strength and tearing resistance

A low freeness value gives a paper high tensile strength, whereas the tearing resistance is hardly influenced by a decreasing freeness.

Tensile index Nm/g

600 700ml CSF

Fig. 62. Tensile index at different CSF. (6-062.tif)

6.2.2 Opacity

The opacity of a sheet depends on how the fibres are separated and on the applied energy. Low freeness means that there is more fine fraction.

Fig. 63. Edge cross section, newsprint. (6-063.tif)


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Main reg Glossary iallery JThe fine fraction has few hydroxyl groups and forms few hydrogen bonds.

Thus, the more fine fraction, the larger the area of unbound fibre surfaces being able to spread the light will be. The share of transmitted light is low and the opacity is high. Fig. 64. Illustration. The

reflection and the refraction of a ray of light passing a fibre wall. (6-064.tif)

It is the other way round with chemical pulp. If the refining is increased, the opacity decreases.

Fig. 65. Picture covered with a low-refined and a high-refined paper. (6-065.tif)

The more pliable the fibres becomes and the more OH groups that can be exposed, the more hydrogen bonds will be formed. The free, not bound, surfaces in a sheet becomes less. The decreasing amount of free surfaces makes the opacity lower.

Fig. 66. Extremely high-refined paper, grease-proof paper.(6-066.tif)


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lain reg ^j6.2.3 Brightness

The mechanical pulp contains almost all the wood lignin. When the lignin is exposed to ultraviolet light, as from the sun, it becomes increasingly dark. Because of this, mechanical pulp can not be used for printing papers intended to be durable.

Fig. 67. Part of a newspaper page, which has become yellow in the sunshine.(6-067.tif)

6.2.4 The strength of the wet paper web

The wet paper web must have such a strength that it resists breaking.

Fig. 68. Side view: modern newsprint machine. (6-068.tif)

If the strength is too low, it must be improved by addition of a chemical, long fibre, reinforcement pulp. As the chemical pulp is more expensive than the mechanical one, the addition should be kept as low as possible.

Fig. 69. Chemical fibres (reinforcement fibres) in a newsprint pulp. (6-069.tif)


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6.2.5 The paper density and stiffness

The density is a measure of the fibre tightness in the sheet.

Fig. 70. Microscope picture. Newsprint. (6-070.tif)

The density is measured by weighing a sample sheet with a determined area.

Thereafter, the thickness is measured and the volume is calculated. The density is calculated by dividing the sheet weight by its volume.

Fig. 71.(6-071.tif)

Fig. 72.(6-072.tif)

Fig. 71 and 72. Determination of the sheet density.

Chemical fibres are soft. The fibres collapse and come close to each other. The paper density becomes high.

The high stiffness of mechanical fibres prevent the fibres to collapse. The paper thickness will be high and the density low.

Fig. 73 and 74. Illustration. Comparison between sheet made of chemical fibres and sheet made of mechanical fibres.


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JIt is just because of the lower density that mechanical pulp is used, for example, in the middle of a board.

The board becomes stiff and the price per volume becomes lower.

Fig. 75. Example of products with mechanical fibres in the middle layer. (6-075.tif)

The lower the energy input during the grinding or refining, the lower the density will be. Thus, pulps, which are to be used in board, must have a high freeness.

Fig. 76. Edge cross-section. Board. (6-076.tif)

7. Examples of products containing mechanical pulp

Formerly SGW was often used, for example in newsprint. Nowadays TMP is the main raw material for newsprint and periodicals.

Fig. 77 and 78. Examples of products containing TMP.


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SGW was often used for the middle layer in board. Today CTMP is primarily used.

CTMP is also mixed into simple printing and writing paper. The main reason for this is that it is less expensive than chemical pulp.

CTMP is an excellent fluff pulp for hygienic products and different tissue papers. Fig. 79. Example of products produced from

CTMP pulp. (6-079.tif)

8. Pulp for sale

SGW, TMP and CTMP are sold as market pulp in Sweden. To increase the possibilities to store and transport the pulp, it is dried. Normally it is dried by a flash dryer.

Recirculated air

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Fig. 80. Flash dryer. (6-080MJ)


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9. Grade properties, summary

In mechanical pulp the majority of the original wood material remains. The wood yield is high 93-98%.

In a chemical pulp most of the lignin and extractives are removed. A large part of the hemicellulose and some of the cellulose is removed as well. The wood yield has decreased to 45-60%.

Mechanical pulp fibre is shorter as well as thicker than the chemical. It has thicker fibre walls and these make it heavier. Because of this, there are fewer fibres in a sheet made from mechanical pulp than from chemical pulp.

Fig. 81. Illustration. Weight difference between mechanical and chemical fibre. (6-081.tif)

The mechanical fibres do not collapse as easily as the chemical. Few hydrogen bonds are formed in a sheet. The density will be lower and the paper weaker.

The original brightness of mechanical pulp is higher than that of the chemical one, but it can not be bleached to the same high brightness. Mechanical pulp yellows with time.

Mechanical pulp is less expensive to produce than chemical pulp.


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Glossary Gallery

CEPATEC AB Knut-Erik Persson

FORMING 1, GENERALLY1. Forming process..........................................................................................................2

l.lDewatering .............................................................................................................21.2 Wet web strength.....................................................................................................4

2. Forming and paper properties...................................................................................62.1 Formation................................................................................................................6

2.2 Fibre orientation....................................................................................................102.3 Distribution of the fine material in the Z direction...............................................15 |j

3. Factors influencing the forming process.................................................................16 [3.1 Forming of fibre floes...........................................................................................16 '3.2 Fibre orientation in the sheet.................................................................................273.3 Distribution of the fine material in the Z direction...............................................28 II

o

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1. Forming process 1.1

Dewatering

During the forming process, the stock has to be dewatered in a way which an even fibre network is be created. A web is formed.

Fig. 1. Forming section in a liner machine. (11-001.tif)

What influences the dewatering and what makes the fibres in the network keep together?

If there is a large amount of fine material in the stock or if the fibres are swollen and soft, the stock drains less and it takes a longer time to dewater it.

Fig. 2. Microscope photo. TMP pulp. (STFI) (11-002.tif)

Fig. 3. Microscope photo. Beaten chemical fibres. (Sunds Def.)(11-003.tif)

If a higher concentration in the head box is chosen, the amount of water leaving the stock decreases and the forming takes place more quickly. However, the increased concentration makes it more difficult to form the sheet. Therefore, this is normally not an acceptable way to speed up the forming process.

Fig. 4 and 5. Pictures illustrating how the stock volume decreases when the fibre concentration increases.

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Main re Glossary Gallery

The stock temperature is another factor, influencing the drainage.

Fig. 6. Dewatering on a Four-drinier machine. (ll-006.tif)

The higher the temperature is, the lower the water viscosity will be and the faster the stock will drain.

Fig. 7. Diagram showing the connection between temperature and viscosity of the stock.(11-007.tif)

Adding a retention chemical is another way to increase the drainage rate.

Fig. 8 and 9. Equipment for preparation and dosage of a retention chemical.

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The dewatering does not depend on the stock conditions, only. The design of the wire section is important as well. Dewatering in two directions is always faster than in one. Consequently, a two-sided dewatering is often used in new machines.

Fig. 10. Two-sided dewatering on a hybrid machine. (ll-010.tif)

1.2 Wet web strength

In the finished paper the fibres bind to each other with hydrogen bonds. To make these forces work, the fibre surfaces must be in direct contact with each other.

Fig. 11. Illustration. Enlarged surface section showing two fibre surfaces binding to each other with hydrogen bonds. (11-011.tif)

However, the wet web has a certain strength, too. The reason is the so called surface tension.

Fig. 12. Open transfer of the web from the wire to the press section in an old paper machine.(11-012.tif)


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Wet fibres are surrounded by a thin water layer.

Fig. 13. Illustration. Two fibres surrounded by a water layer.(11-013.tif)

When two such fibres get into contact with each other, the water layers will overlap in the contact point.

Fig. 14. Illustration. Two fibres in close contact. The water layers are overlapping in the contact point. (11-014.tif)

Forces, trying to keep the water layers together, arise and the fibres will keep together, too.

Fig. 15. Illustration. Forces arising between two wet fibres.(11-015.tif)


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2. Forming and paper properties

2.1 Formation

The forming process means a lot for all paper properties. In this part of the presentation, some paper properties strongly related to the forming are presented.

The local distribution of the fibres in a paper is called paper formation.

Fig. 16. Microscope photo. Fine paper. (STFI) (11-016.tif)

A simple, but not always correct, way to judge the formation is to view the paper in transmitted light. If the formation is bad, the paper seems to be patchy and is said to have a wild look-through.

Fig. 17. Visual judgement of the paper formation. (ll-017.tif)


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Of course, instruments in the mills can measure the formation more exactly.

Fig. 18. Sensor measuring the paper formation. (11-018.tif)

A better formation makes the paper more even and improves its printability.

Fig. 19. Picture from a printing office. (Norra Skåne) (11-019.tif)

The formation influences the paper strength, but how much depends on how a certain degree of formation is achieved.

Fig. 20. Reeling up of a kraft liner. (ll-020.tif)

A good formation will not only be of importance for the properties of the finished paper. It will also enhance the production of the paper.

Fig. 21. Paper machine for liner production. (11-021.tif)


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With good formation, it will become easier to dewater and press the web.

Fig. 22.Fourdriniersection.(11-022.tif)

However, above all, the formation means the most during the drying process.

Fig. 24. Drying cylinders in a multi cylinder dryer. (11-024.tif)

When the fibres dry, they shrink crosswise and become thinner. In the cross point the fibres are fixed together. A fibre shrinking crosswise compresses another fibre lengthwise. As a consequence, the whole sheet shrinks.

Fig. 25. Microscope picture showing how the overlying fibre is compressed lengthwise when the underlying fibre shrinks crosswise. (STFI) (11-025.tif)

If the formation is bad, the paper dries and shrinks unevenly. There will be tensions in the paper and it may get a cockled finish.

Fig. 26. An example of a paper with a cockled finish. (11-026.tif)


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A paper that should have a high gloss has to be calendered. At this operation the formation is most important.

If the paper sheet has a bad formation, the thicker parts of the sheet will be harder pressed than the thinner ones. As a result, these points will get a higher gloss. The paper will become top calendered.

Fig. 27. Calender. (Twin roll with a soft nip; soft calender.) (11-027.tif)

If the paper has a very uneven formation the thick parts may still be moist when the paper leaves the drying section.

Fig. 28. Drying section in a fine paper machine. (11-028.tif)

Moist parts are easier compressed in the calender. The free surfaces in the paper sheet, which can reflect the light are reduced on those points. The spots become more transparent.

Fig. 29. Paper that has been calendered and made transparent at certain points. (11-029.tif)


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In another lighting the transparent parts appear as dark spots. This is called blackening.

Fig. 30. Transparent parts appearing as dark spots. (11-030.tif)

2.2 Fibre orientation

During the forming, the fibres are not only to be distributed, but also directed or ”orientated”. This is another factor influencing the paper properties.

Fig. 31. Forming of sack paper. (ll-031.tif)

In many paper grades it is desirable to have the properties the same as possible in all directions. Examples of grades with such properties are fine paper and sack paper.

Fig. 32. Different types of writing paper. (11-032.tif)

Fig. 32 and 33. Examples of grades where the properties have to be as like as possible in all directions.


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In fine and sack papers, the fibres are to be equally orientated in all directions.

Fig. 34. Illustration. Sheet with the fibres equally orientated in all directions. (11-034.tif)

However, sometimes it is desirable to have the fibres directed, as much as possible, in the machine direction.

Fig. 35. Illustration. Sheet where the fibres are more orientated in the machine direction than in the cross direction. (11-035.tif)

Newsprint and tissue are examples of such grades. Fig. 36.

Newspapers.(11-036.tif)

Fig. 37. Tissue paper.(11-037.tif)

Fig. 36 and 37. Examples of grades where the strength has to be higher in the machine direction than in the cross direction.


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The more the fibres are orientated in the machine direction, the higher the strength in that direction will be. A high strength in the machine direction makes the paper more resistant to the tensile stress in a printing press.

Fig. 38. Paper web in a printing press. (11-038.tif)

With machine direction orientation the web becomes stronger and it will be easier to produce the paper.

M»Fig. 39. (ll-039.tif)

Fig. 41. (11-041.tif)

Fig. 39 - 41. Magazine paper machine: forming, pressing, drying.


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How the fibres are orientated in the sheet does not only influence the strength properties of the web and the finished paper. During the drying process, the fibre orientation effects the web also in another way.

Fig. 42. Paper web in the drying section. (11-042.tif)

The fibres always shrink more crosswise than lengthwise during the drying. If most fibres are orientated in the machine direction, the paper web will shrink mostly in the cross direction.

Fig. 43 and 44. Paper web during the drying. The main shrinking direction is marked.

Thus, if the fibre orientation is different in machine direction, a different shrinking is achieved in that part.

Fig. 45 and 46. Paper web during drying. The main shrinking direction is marked.


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The fibre orientation is often different in the edges. This is one reason, why the edge rolls may sometimes create problems in the paper use.

Fig. 47. The edges of a paper web often have a different fibre orientation compared to the other part of the web. (11-047.tif)

How the fibres are orientated in the paper can be estimated by measuring the tensile strength in various directions. However, this is a detailed and most time consuming procedure.

Fig. 48. Measure of the tensile strength. (11-048.tif)

With the help of new, modern instruments the fibre orientation can be defined safely and quickly.

Fig. 49. Apparatus for determination of the main fibre direction. (11-049.tif)

Sometimes, the fibre orientation is different on the two sides of the paper, that is one side is more lengthwise orientated than the other.

Fig. 50. Illustration. Paper with different fibre orientation on the two sides. (11-050.tif)


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If the humidity in the atmosphere surrounding the paper is changed, the paper will shrink or widen differently on its two sides.

Fig. 51. Sheet pile in a printing office. (11-051.tif)

The result will be a curly paper and the phenomenon is called curl.

Fig. 52. Illustration. Curl.(11-052.tif)

2.3 Distribution of the fine material in the Z direction

It is not only the paper formation and the fibre direction in the sheet that have a direct influence on the paper properties. The stock contains filler and fine material, too. How that material is distributed in the thickness direction of the paper, the Z direction, is of great importance.

Fig. 53. Microscope picture. Cross-section of a paper sheet. Note the even filler distribution. (STFI)(11-053.tif)


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A usual problem, in one-sided dewatering, is that most of the fine material will be localised closest to the top side of the paper. The paper is unequal-sided or two-sided.

Such a paper may give curl.

Fig. 54. Microscope picture. Cross-section of a paper sheet. The filler share is here higher closest to the paper’s top side. (STFI) (11-054.tif)

The surface strength and the printing properties are also strongly influenced by an unequal-sided paper.

Fig. 55. Offset printing. (Norra Skåne) (11-055.tif)

3. Factors influencing the forming process

3.1 Forming of fibre flocs

We have seen how some important paper properties are influenced by the forming process. Now, we have to take a step backwards and study what influences the process as such.

The quality of the supplied stock and the conditions during the forming are both of great importance in the forming process.


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3.1.1 Stock fibre properties

The fibres in a stock easily tangle and bind mechanically to each other, they create flocs.

Such fibre flocs can be quite stable, so rather great forces are needed to split them up.

Fig. 58. Photo. Fibre flocs in a stock. (11-058.tif)

During the forming, fibre flocs are always created.

Fig. 60. Forming section in a board machine. (11-060.tif)

If these flocs are not broken down, they will remain in the finished paper. The paper will get a bad formation.

Fig. 61. Example of papers with better and worse formation. (11-061.tif)


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Thus, fibre flocs cause bad formation.

What determines how much fibre flocs there will be in the stock?

Fig. 62. Forming in a Fourdrinier machine. (ll-062.tif)

A stationary fibre will occupy a space equal to its own volume.

Fig. 63. Illustration. Fibre.(11-063.tif)

The fibres in water follow the water movements. If there are whirls, or turbulence, in the water the fibres will rotate.

The largest possible volume the fibre can sweep over, corresponds to a sphere with a diameter as large as the fibre length.

Fig. 64. Illustration. Fibre rotating in water. The sweep volume is marked. (11-064.tif)


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The fibre in a softwood pulp is about three times as long as the fibre in a hardwood pulp.

Fig. 65. Microscope picture. Long softwood, chemical fibres. (STFI) (11-065.tif)

Fig. 66. Microscope picture. Short, hardwood chemical fibres. (STFI)(11-066.tif)

The sweep volume increases with the cube of the fibre length. So if the softwood fibre is three times as long as the hardwood fibre, it will sweep over a volume of 3 3 3, thus 27 times ∗ ∗larger than that for the hardwood fibre.

Fig. 67. Illustration. Comparison between the sweep volume of a short hardwood fibre and a long softwood fibre rotating in water. (11-067.tif)

However, the hardwood fibre is lighter than the softwood fibre. In order to get the same weight there must be about four short hardwood fibres to each long softwood fibre.

In spite of the number of hardwood fibres being four times higher, the short fibres sweep over a smaller volume than the long fibres do.

Fig. 68. Illustration. Four short hardwood fibres have the same weight as one single softwood fibre. (11-068.tif)


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The ideal way to form a sheet would be to have enough space to let the fibre move freely in the stock until they are deposited in the created fibre net work.

Fig. 69. Illustration. Few fibres in water. Here, the fibres can move freely. (11-069.tif)

However, forming a paper under such conditions is not realistic from practical or economical reasons.

Consequently, the fibres, not being able to move freely, will tangle and form flocs.

Fig. 70. Illustration. Many fibres in water. Here, the fibres can not move freely. Fibre flocs are formed. (11-070.tif)

The risk for such floc formation increases with the fibre length. Therefore, forming a sheet from long fibres requires a lower stock concentration than forming it from short fibres.

Fig. 71 and 72. Illustrations. Many short and few long fibres in water.


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Fibres easily form flocs. However, what really happens when a floc is formed?

If there is not enough space for the fibres to move freely, the fibres penetrate into each other’s rotation zones. Then, the risk that the fibres tangle increases.

Fig. 73 and 74. Illustrations showing how the movement space decreases when the fibre concentration increases.

The turbulence force in the stock make the fibres move. As long as the moving force is greater than the force hooking the fibres, no flocs will be created.

Fig. 75. Illustration. Turbulence whirls in the stock. (11-075.tif)

The fibres are elastic and therefore, they bend when they move.

However, if the turbulence decays, the fibres stop moving. An immobile fibre will take back its natural form.

Fig. 76. Illustration showing fibres straightening out when the turbulence whirls have disappeared. (11-076.tif)


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The fibres, however, prevent each other from straightening out completely. As a result, they bind mechanically to each other, a floc is then created.

Fig. 77. Illustration showing how the fibres lock each other when they straighten out. (11-077.tif)

Refined chemical fibres are softer and more elastic, than unrefined ones. These properties make them more disposed to entangle and lock each other. The longer the fibres are, the greater the risk for floc formation.

Fig. 78. Chemical fibres. (STFI)(11-078.tif)

Fig. 79. Conical refiner.(11-079.tif)

The fibres in mechanical pulps are short and stiff. Such fibres are not so easily entangled and and interlocked.. Consequently, mechanical, fibres have less tendency to form flocs.

Fig. 80. Mechanical fibres. (TMP). (STFI) (11-080.tif)


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The choice of fibres depends on which paper that is to be produced. Thus, the furnish for each paper grade is fixed.

Fig. 82. Magazine paper (periodicals).(11-082.tif)

Fig. 84. Kraft paper (sacks). (11-084.tif)

It is during the forming process in which the floc formation could be prevented.

Fig. 85. Flow diagram. The short circulation. (11-085.tif)

Before the web is formed, the stock is always highly diluted. This dilution is done in the short circulation. The longer the fibres are, the more the stock has to be diluted.

Fig. 86. Dilution of the stock in the short circulation.(11-086.tif)


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Main re Glossary Jallery

Diluting the stock is the ideal way to prevent fibres from forming flocs.

Fig. 87 and 88. Photos. Stock before and after diluting.

A paper produced of a highly diluted stock gets a good formation.

The better the formation is, the stronger the paper will be.

Fig. 89. Liner is one example of a strong paper formed from a highly diluted stock. (11-089.tif)

However, the stock can not be diluted too much. The lower the fiber concentration is, the larger the stock flow becomes. Soon, an upper limit will be reached.

Fig. 90. Diluting before the fan pump. (11-090.tif)

Thus, there is maximal limit beyond which the stock can not be diluted. The next step is to limit the size of those flocs which in spite of all are formed.


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3.1.2 Stock turbulence

The way to prevent the fibres from forming large flocs is to have enough stock turbulence.

If the turbulence is strong, the shearing forces tearing up the flocs becomes greater than the forces keeping them together.

Whether the fibre floc is decreased only, or dispersed totally, depends on the character of the turbulence.

Fig. 92.(11-092.tif)

Fig 91 and 92. Photos showing stocks before and after generating turbulence.When the turbulence is estimated, the intensity and the size of the

whirls must be taken into consideration. How to define the intensity and the size of the whirls?

The intensity is the velocity difference between two adjoining whirls.

Fig. 93. Illustration. The intensity of whirls. (11-093.tif)

The size is the area influenced by every single whirl.

Fig. 94. Illustration. The size of the wirls, the scale.(11-094.tif)


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When the diameter of a simple whirl is approaching the fibre length, micro turbulence is created.

Fig. 95. Illustration of micro turbulence. (ll-095.tif)

If the turbulence is more coarse, the flocs are only partly broken down.

Fig. 96.(11-096.tif)

Fig. 97.(11-097.tif)

Fig. 96 and 97. Illustrations showing a fibre floc being broken down by a coarse turbulence.

The smaller the turbulence whirls are, the greater the probability to release single fibres will be.

Fig. 98 and 99. Illustrations showing a fibre floc broken down by micro turbulence.


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However, what will happen to the fibre flocs does not depend on the turbulence, only. The fibre properties are also of importance.

If the fibres are long and elastic the number of points locking each fibre increases and the forces keeping the fibres together become greater. Then, the force needed to separate the fibres is increased.

Fig. 100. Illustration. Long fibres lock each other in many points. The floc strength becomes great.(11-100.tif)

Thus, long fibres do not only form flocs easily. The fibre length, too, makes the flocs difficult to break down again.

The stock turbulence can never totally prevent the fibres from forming flocs, but it limits the size of the flocs. The smaller the floc size, the better the paper formation will be.

However, the strength of the paper never becomes as high when the flocs are broken down again, as when flocs have never been created.

3.2 Fibre orientation in the sheet

During the forming process, the fibres tend to orientate in the flow direction of the stock.

The longer and stiffer the fibres are, the more they tend to orientate.

Fig. 101. Illustration. Alignment of the fibres in a flowing stock.(11-101.tif)


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If a local cross flow is generated during the forming, the fibres will orientate in the same direction. Thus, the fibre orientation will become different compared to the rest of the paper web.

Fig. 102. Illustration. The orientation of the fibre in a flowing stock. Note the cross flow. (11-102.tif)

The fact that the fibres tend to orientate in the flow direction depends on laws of physics. However, how much the fibres orientate and which dominating direction they will get in the finished paper may be influenced during the forming process.

3.3 Distribution of the fine material in the Z direction

Stock properties

The more fine material there is in a head box stock, the greater the risk becomes to get an unequal-sided sheet.

Single-sided dewatering

When the stock is dewatered, the fibres form a connected network. The fibre network is finer than the wire cloth, and the thicker it is, the better it will catch the fine material of the stock. Conseqently, the content of fine material will be higher on the top side of the paper than on the wire side.

Fig. 103 and 104. Illustration. Single-sided dewatering.


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The large quantities ofwater, flowing throughthe initially formedfibre layers, wash offsome of the finematerial, too. This isanother reason for thedifferent amount offine material in the Z direction.

Fig. 105. Dewatering over foils. (11-105.tif)

A single dewatering causes the content of fine material in paper to be lower closest to the wire side.

Two-sided dewatering

When dewatering between two wires, the same thing will happen.

Fig. 107. Twin wire former. (11-107.tif)

However, in this case the fine material becomes more symmetrically distributed. It will be lowest closest to the surfaces and at the highest in the middle of the paper. How much depends on how the dewatering is done. There are different ways to counteract the effect and on modern formers the fine material is quite evenly distributed.

Fig. 108. Microscope photo. (STFI) (11-108.tif)


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When dewatering in two directions, the two fibre networks are only half as thick as when the dewatering takes place in one direction. This, together with a higher pressure in the dewatering zone, makes the stock drain very quickly. However, the pressure pulses needed to get this quick dewatering at a good formation, increases the risk for breaking up the already formed fibre nets. If this happens the retention will become lower.

Fig. 110.(11-110.tif)

Fig. 109 and 110. Illustrations. One- and two-sided dewatering.

Retention chemicals (Retention aids)

To help bind the fine material to the fibres, retention chemicals are often used.

The wire retention becomes higher. Besides, when the fine material forms flocs which bind to the coarser fibres, the fine material is more evenly distributed in the thickness direction of the sheet. The paper becomes less tight. The porosity is higher.

Fig. 111. Illustration showing how the fibre surfaces catch up the fine material at the use of retention chemicals.(11-111.tif)

During the forming process, the more even distribution of the fine material is enhancing the stock drainage.

Fig. 112. Dewatering over foils. (11-112.tif)


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Thus, the retention agent improves the retention and makes the stock to drain more quickly. However, it is necessary to be very careful when selecting the retention agent.

What could happen is that the fine material binds together and creates flocs, which can destroy the sheet formation .

Fig. 113 and 114 illustrate how the fine material flocs together and fills the vacant space between the fibres.

Forming a paper means that the stock is to be dewatered and that the fibres are to be directed and distributed in the formed network.

However, the quality demands on each paper grade is specific and the supplied furnish has its special character.

Of course, the demands on the section forming the net work becomes specific as well. The design of the forming section is to be treated in the following chapters.


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Main reg C^ Glossary ^ Gallery


FORMING 2, SHORT AND LONG CIRCULATION

1. White water system ........................................................................................... 21.1 Short circulation .............................................................................................. 21.2 Long circulation .............................................................................................. 51.3 White water cleaning ....................................................................................... 6

2. Wet end system ................................................................................................. 82.1 Broke system ................................................................................................... 8

■=>

3. Approach flow system ..................................................................................... 123.1 Grammage regulating valve .......................................................................... 123.2 Fan pump ....................................................................................................... 14

4. Cleaning and screening ................................................................................... 164.1 Hydrocyclone ................................................................................................ 164.2 Stock deaerator .............................................................................................. 22

5.1 Feeder pump .................................................................................................. 245.2 Screens .......................................................................................................... 255.3 Feeder pipe .................................................................................................... 28

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Main reg C^ Glossary Gallery

This chapter is about the stock and process water flows around the paper machine. The systems may be designed in different ways depending on the type of paper produced, but the main principle is still the same.

1. White water system 1.1 Short circulation

During the forming, the water in the stock suspension drains through the wire and the the fibres form a connected network; a web.

Fig. 1. Dewatering on a Fourdrinier machine.(12-001.tif)

The water passing through the wire is called white water. Some parts of the stock solids are not caught in the fibre network and therefore the white water contains a great deal of fine material. How much depends on the wire retention.

r^<]<- *-

< ;• u ,i >Fig. 2. Illustration. One-sided dewatering. (12-002.tif)

By wire retention it is meant how much of the solids in the stock from the headbox that is to be found in the web leaving the wire section.

Fig. 3. Fourdrinier section in a liner machine. (12-003.tif)

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The wire retention depends on how much fine material the stock contains from the beginning, and if retention chemicals are used or not.

Fig. 4. Microscope photo. Headbox stock in a fine paper mill.(12-004.tif)

An additional important factor is of course the mechanical conditions on the wire section.

Fig. 5. Fourdrinier section.(12-005.tif)

The short circulation is a term used describing the process flows around a paper machine.

By the short circulation is meant the flow loop from the wire tank, through pumps and cleaning systems, to the headbox and back to the wire tank.

TMachine W chest <j

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^ ^^^v^^viMaskirlHIKSZ "

Ikar IBEffl grc:1

-; i

ii i

Fig. 6. Process schedule. The short circulation. (12-006.tif)

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In the short circulation the stock from the machine chest is diluted.

The dilution is done before the fan pump with water from the wire tank.

Fig. 7. Dilution of the machine chest stock, before the fan pump.(12-007.tif)

The wire tank is the chest where the white water is collected when the stock is dewatered on the wire. The level in the wire tank must be constant.

The constant level is obtained as some of the white water always spills over. Fig. 8. Wire tank with flooding

white water. (12-008.tif)

The reason why there is always a water surplus in the short circulation is that the stock in the machine chest has a consistency of 3 to 4%.

When the paper web then leaves the wire it has a consistency of about 20%.

4 1 18-20%

IIIII kar 1

1 Vlra-E

II9rop E

chB9t 3^%

Fig. 9. The short circulation. Marking of typical stock consistencies. (12-009.tif)

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1.2 Long circulation

The excess water from the short circulation goes to the long circulation.

Fig. 10. Process schedule. The short and the long circulation. (12-010.tif)

In the long circulation the water is first collected in a white water tower. The white water tower is a big tower used to level out the differences between the supply and the consumption.

Fig. 11. Process schedule. Water flood to the white water tower. (12-011.tif)

The water in the white water tower is later used for regulation of the stock consistency.

Fig. 12. Process schedule. The long circulation with marked dilution points.(12-012.tif)

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The water in the white water tower is also used to slush broke or dry pulp bales.

Fig. 13. Process schedule. A part of the long circulation with marked dilution points for wet and dry broke or pulp bales. (12-013.tif)

1.3 White water cleaning

The excess water from the white water tower is cleaned from fibres and fibre fragments. The cleaning is often done on a disc filter.

-i fk-^vUJ___

Blond Machln* J chfffil chfffil 1

H Bak- ■Xi'li v i

Fig. 14. Process schedule. The white water flood from the long circulation. Marking of disc filter. (12-014.tif)

To catch the fine material better, pulp from some of the stock chests is usually mixed into the white water. The fibres and the fine material separated on the filter are led back to the machine chest. Fig. 15. Disc filter for cleaning of the

white water. (12-015.tif)

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The cleaned white water is used for various shower water on the paper machine.

Fig. 16. Wire showers in a Fourdrinier section. (12-016.tif)

The remaining water leaves the process and goes to the exterior cleaning.

Fig. 17. Process schedule. The white water surplus from the long circulation. Marking of the exterior cleaning. (12-017.tif)

The separated fibres and the fine material are normally not led back to the paper mill. In most cases the sediment is dewatered in a centrifuge or on a band press.

Fig. 18. The exterior cleaning of the white water. (12-018.tif)

The energy content in the dewatered sludge is made use of when it is later mixed with other material and burned.

The cleaned water leaves the process.

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You always try to minimise the amount of fresh water to a paper machine system. A small amount of fresh water means a small amount of contaminated water to lead out. The system is said to be more closed.

Normally, the system must not be too closed. Dissolved organic substances and dissolved salts (called disturbing substances and disturbing ions) can be so accumulated in the white water that it will be difficult to keep the system clean. The impurities could even interfere, making the additives less effective.

2. Wet end system

2.1 Broke systemThe broke taken back may be ”wet” and come from, for example the couch pit.

Or the broke may be ”dry” coming from broke pulpers.

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The conditions in the pulpers vary and it is difficult to keep an even consistency in the broke leaving the pulpers.

Consequently, the consistency in the broke tower where the different pulped broke is stored will vary.

Fig. 21. Process schedule. Recovery of wet and dry broke. (12-021.tif)

To get an even consistency the pulp is pumped further and dewatered on a thickener.

The thickened pulp goes further to a second broke chest.

The water leaving the pulp at the dewatering on the thickener goes back to the white water tower.

Fig. 24. Process schedule. The broke chest after the thickener is marked. (12-024.tif)

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The pulp from the broke chest is diluted with white water to the correct consistency. It is then mixed with other stocks in the blend chest to the desired proportions.

Fig. 25. Process schedule. The blend chest after the thickener is marked. (12-025.tif)

If the broke is difficult to slush, it may first pass through a deflaker before it lands in the blend chest.

Fig. 26. Deflaker. (12-026.tif)

2.2. Stock preparation

The stock in the blend chest contains the correct balance of fibre chosen to give the paper the desired properties.

Fig. 27. Process schedule. The refiners are marked. (12-027.tif)

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Chemical fibres have always passed a number of refiners before they land up in the blend chest. Sometimes mechanical fibres, too, are treated in a refiner: they are post refined.

Fig. 28. Conical refiner. (12-028.tif)

After the blend chest the stock is forwarded to the last stock chest: the machine chest.

On the way to the machine chest there is a final regulation of the stock consistency.

Fig. 29. Process schedule. The addition point for the white water is marked. (12-029.tif)

Fig. 30. Measuring of the consistency after the dilution with white water. (12-030.tif)

If dry strength agent is added it is often brought to the stock in the machine chest. If rosin size is used, alum, too, may be found in the stock. The other chemicals are often added a bit later. Fig. 31. Process schedule. Ordinary addition points

for alum and dry strength agent (starch) are marked.(12-031.tif)

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3. Approach flow system 3.1

Grammage regulating valveWhen the stock leaves the machine chest, the consistency is constant. The flow from the machine chest is measured by a flow meter.

Fig. 32. Process schedule. The machine chest pump is marked.(12-032.tif)

Fig. 33. Machine chest pump.(12-033.tif)

By regulating the flow from the machine chest the disired amount of fibres is supplied to the headbox.

Fig. 34. The grammage regulating valve controls the stock flow mixed in before the fan pump.(12-034.tif)

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A computer informs about the correct fibre amount.

The computer receives information about the machine speed, the web width and the grammage. Then, it calculates the amount of fibre needed to achieve the right grammage. Based on the fibre flow the amount of chemicals and fillers is estimated.

Fig. 35. Measuring device, grammage. (12-035.tif)

The stock size is often added just when the stock leaves the machine chest.

Fig. 36. Machine chest pump. The addition point for size is marked. (12-036.tif)

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If filler is added, it is in most cases done just before the fan pump.

Fig. 37. Fan pump. The addition point for filler is marked. (12-037.tif)

The stock is later cleaned in hydrocyclones and in screens. It is not possible to avoid that some part of the filler disappears out of the system. However, the loss is not very extensive and it is preferred rather than getting over-sized filler particles in the stock entering the headbox.

3.2 Fan pump

The consistency in the machine chest is too high to make it possible to clean the stock and to form a sheet. Therefore, the stock must first be diluted with white water from the wire tank. This dilution is done immediately before the fan pump. The larger the flow after the pump, the higher amount of white water is added and the lower the stock consistency becomes.

To get a good admixture it is important how the dilution of the stock with the white water is done.

Fig. 38. Process schedule. The short circulation. The fan pump is marked. (12-038.tif)

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In most cases the stock pipe from the machine chest is led into the wire tank. Then, the stock is injected with a velocity that is higher than the velocity of the white water.

Fig. 39. Illustration. Arrangement for stock dilution. (12-039.tif)

Sometimes the same thing is done with the pipe collecting and bringing back the flow from for example a hydrocyclone and a deaerator. The flow velocity in the outer tube is always a bit lower than the velocity of the inner one.

Fig. 40. Illustration. Arrangement for dilution of the returned flow from for example a hydrocyclone. (12-040.tif)

Previously the flow was in most cases controlled by throttling it after the pump.

Fig. 41. Controlling the flow by a throttle valve. (12-041.tif)

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Nowadays, the flow is controlled by regulating the rotation speed of the pump motor

Fig. 42. Controlling the flow by a pump motor with variable rotary speed. (12-042.tif)

4. Cleaning and screening

4.1 Hydrocyclones

After the fan pump the stock is cleaned in small cyclones: hydrocyclones.

Fig. 43. Battery. (12-043.tif)

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The reason why the hydrocyclones are so small, is that the smaller the diameter, the better it separates very small impurities.

Fig. 44. Cyclones in a cleaner battery.(12-044.tif)

A hydrocyclone is cone-shaped. The incoming stock, the inject, is fed into the thick end.

Fig. 45. Exploded view of a hydro-cyclone. The inject flow is marked.(12-045.tif)

The stock, when it comes into the cyclone, starts rotating.

Fig. 46. Exploded view of a hydrocyclone. The rotation direction of the stock is marked.(12-046.tif)

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The quicker the stock rotates, the more the centrifugal force will influence the stock.

The centrifugal force influences particles with a high density more, than particles with a low density. The higher the density, the higher the force driving the particles towards the cyclone wall will be.

Fig. 47 and 48. Exploded view of a hydrocyclone.

The fact that the centrifugal force influences heavy particles more than light ones and that they end up closest to the cyclone wall is rather obvious. However, what then happens to the particles is not so easy to predict.

Closest to the wall the stock streams in a downwards spiral. The heavy particles come along in the stream and leave the cyclone as reject.

dr

1r > Reject

Fig. 49. Exploded view of a hydro-cyclone. (12-049.tif)

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However, in the centre of the cyclone the flow spiral is directed upwards and its content, the accept, leaves the cyclone.

Fig. 50. Exploded view of a hydrocyclone. (12-050.tif)

Thus, heavy stock particles, for example grains of sand and very big filler particles, can easily be separated in the hydrocyclone.

Fig. 51. Microscope picture. Grains of sand. (12-051.tif)

The stock might contain other contamination, even lighter than the fibres. In spite of that, they can be separated as well. Bark is such a contamination.

The reason why just this contamination is separated is that it is larger than the fibres. A large contamination has a smaller surface compared to its volume than the fibres have. The smaller the surface is compared to the particle volume, the easier the particle moves towards the cyclone wall.

Fig. 52. Microscope picture.Bark particle in a stock reject. Note the size difference between the bark particle and the fibres.(12-052.tif)

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An important thing to remember is that the fibres easily form flocs, particularly when the fibre concentration is increased.

The flocs may surround the contamination making it more difficult for the contamination to move towards the cyclone wall. The cleaning becomes poorer.

Fig. 53. Illustration. Fibre floc. (12-053.tif)

Because of this, the fibre concentration in the inject is seldom above 0.8% and often as low as 0.5%.

The accept from the first cleaning step is ready to go further into the process.

In most cases the reject volume is about 10% of the inject volume. The reject leaving the cyclone normally has a consistency almost twice as high as the inject consistency.

Thus, the weight of reject leaving the cyclone may come up to 20% of the weight of the inject.

Fig. 55. Exploded view of a hydrocyclone. The numbers denote parts by weight shares.(12-055.tif)

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First the reject is diluted with white water and then pumped to a recycling step, step 2, where it is reseparated.

Fig. 56. Step 2 is the first recycling step in a cleaning system. (12-056.tif)

The amount of reject from step 2 is still too high to be allowed to leave the process. It is therefore diluted a second time and separated in a third step: step 3.

Fig. 57. Step 3 is the second recycling step in a cleaning system, etc.(12-057.tif)

The procedure is repeated and in most cases the reject passes from step 3 through another two or three recycling steps. First after that the amount is so low that the reject is allowed to leave the process.

Fig. 58. Step 4 and 5 in a five step cleaning system. (12-058.tif)

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The reject from a recycling step is always led back to the inject of the previous step.

When the hydrocyclones are connected in this way they are said to be cascade connected.

Fig. 59. Five cascade connected steps.(12-059.tif)

4.2 Stock deaerator

The stock leaving the hydro-cyclones contains air, which may disturb the forming process. The air bubbles adhere to the fibre surfaces and obstruct the breaking up of the fibre flocs.

Fig. 60. Microscope picture. Air bubbles in a stock. (Cellco AB) (12-060.tif)

The air bubbles may expand and disrupt the stock jet from the headbox.

The higher pressure in the box, the more the bubbles are pressed together and the more they expand in the jet.

Fig. 61. Stock jet in a twin wire former.(12-061.tif)

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A way to get rid of the air is to pump the stock to a deaerator. A deaerator is a closed vessel, placed vertically or horizontally.

Fig. 62. Exploded view. Horizontal deaerator tank. (12-062.tif)

The air bubbles in the stock are fixed to the fibres. To remove them, the stock is ejected to the upper tank wall.

There is an underpressure in the deaerator making the stock boil. When it boils the air bubbles disappear.

Fig. 63. Exploded view. Enlarged part of a deaerator. (12-063.tif)

The deaerator must be placed on a high level. The outgoing stock pipe should not only overcome the underpressure in the deaerator. The stock must also have a certain pressure before the feeder pump.

Fig. 64. View showing the placing of the deaerator on an upper floor.(12-064.tif)

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5.1 Feeder pump

The fibre concentration in the stock is now low enough to form a sheet. However, if there are mainly long chemical softwood fibres, the stock must often be diluted with more white water. Sometimes the stock is diluted to a fibre concentration as low as 0.15%.

The water from the wire tank used for dilution of the stock also contains air. If there is a deaerator in the system, the dilution water passes through it before it is mixed into the stock before the feeder pump.

Fig. 65. Flowsheet showing a system where there is a second dilution with white water before the feeder pump. Note that the white water used for dilution passes through the deaerator. (12-065.tif)

Normally the feeder pump has a variable rotary speed and the flow determines the concentration in the headbox.

Fig. 66. Feeder pump. (12-066.tif)

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Main reg C^ Glossary

The feeder pump must give a flow free from pulsations. The pump is therefore designed in a way that makes it particularly suitable to meet that demand.

Fig. 67. Exploded view. Feeder pump. Note the double pump wheel and the mutually displaced shovels. (Ahlstrom) (12-067.tif)

5.2 Screens

The stock is not ready for the paper machine. First it has to be cleaned a last time in a screen.

The task of the screen is to remove remaining particles that may contaminate or damage the machine.

Fig. 68. Fourdrinier machine. The screened stock is ejected on the wire.(12-068.tif)

The screen drum in a machine screen can be single or double. Figure 69 shows an exploded view of a screen with a double screen basket.

Fig. 69. Exploded view. Machine screen with a screen basket. (Ahlstrom) (12-069.tif)

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2001-09-11

25


The drum plates can have round holes or slots. The slot width is always much smaller than the hole diameter. The slots are said to be more effective in eliminating shives.

Fig. 70. Sketch. Screen plates with slots and holes.(12-070.tif)

The clean stock passes through the drum’s perforated plates. Coarse elements, such as shives, fibre knots or foreign objects stay on the pressure side of the drum and can be separated.

Fig. 71. Exploded view. Screen. The inject, accept and reject flows are marked. (12-071.tif)

The screen is equipped with rotating wings.

Fig. 72. Exploded view. Screen, rotating wings. (12-072.tif)

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When a wing rotates it first creates a pressure pulse and later a suction pulse.

The pulses break up the fibre layer formed on the pressure side and prevent plugging of the screen plates.

Fig. 73 and 74. Illustrations showing the effect of the pressure and suction pulses arising when the wing runs over the screen plate.

Thus, the rotating wings simplify the screening. However, the pressure pulses generated in the screen must not be transmitted to the headbox.

Fig. 75. Outgoing pipe after the screen; the feeder pipe. (12-075.tif)

One way to decrease the risk for harmfull pressure pulses is to twist the rotating wings.

Fig. 76. Screen rotor with twisted wings. (Valmet) (12-076.tif)

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To make the screen work in a right way the reject must not be too small. A reject up to 5% is rather common.

Fig. 77. Exploded view. Screen. Normal amount of reject is marked.(12-077.tif)

The reject is rescreened in a secondary screen. The accept of the screen is brought back to the fan pump and the reject leaves the process.

5.3 Feeder pipe

The inner surfaces of the screen are thoroughly polished. The pipe after the screen must be polished, too.

Fig. 78. Exploded view. Screen. The inner surfaces are thoroughly polished. (12-078.tif)

The polished surface is there to prevent deposits. If the surface is too coarse, the friction between the fibres and the metal surfaces may be too high. This would make the fibres closest to the surfaces to rotate and to hook up to each other and form long threads, known as spinnings.

L:12-3.doc KEP 2001-09-13 28


Normally it is desirable to add the retention agent as close as possible to the headbox. Often it is added in the feeder pipe after the screen.

One problem with the selected addition point is that it is difficult to distribute such a small flow evenly in the big stock stream. In most cases the strongly diluted solution is injected through a number of radially directed pipes. To get a good admixture, it is important that the velocity of the added flow is sufficient high.

Fig. 79. A common arrangement for dosage of a retention agent. (12-079.tif)

Before the headbox the flow should be as uniform or laminar as possible.

Pipe bends easily create whirls, which may follow the flow into the headbox. Actually, the feeder pipe always has a certain straight section before the headbox.

Fig. 80. Feeder pipe to the headbox.(12-080.tif)

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______^_

Main reg Glossary Gallery c^>


FORMING 3, HEADBOX

1. Headbox, generally...................................................................................................2

2. Flow spreader (cross direction distributor)...............................................................4

3. Middle chamber......................................................................................................10

4. Headbox nozzle......................................................................................................14^>

5. Hydraulic headboxes..............................................................................................24

6. Hydraulic headboxes with air cushion....................................................................29

7. Hydraulic boxes with local stock dilution..............................................................31 |C

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1. Headbox, generally

The task of the headbox is to distribute the fibres evenly over the wire and give them a desired orientation.

At one time the headbox was really a box: an open box at the entrance of the paper machine.

Fig. 1. Old type of an open headbox.(13-001.tif)

Even if the headboxes used today look differently, the task is the same, to distribute the stock evenly over the wire.

To make this possible the stock jet must have an even thickness and concentration and it must stream out with the same velocity and direction in every position.

Fig. 2. The outflow of the stock jet on a Fourdrinier wire.(13-002.tif)

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Main reg Glossary < allery

The headbox, on this sack paper machine, can be seen as an example of a box type used for a long time.

Fig. 3. The headbox on a sack paper machine. (13-003.tif)

The stock comes into the flow spreader. This part distributes the stock across the machine.

Fig. 4. Headbox. Marking of the flow spreader. (13-004.tif)

The stock leaves the box through the headbox nozzle. In this part the jet is formed.

Fig. 5. Headbox. Marking of the outflow nozzle. (13-005.tif)

Between the flow spreader and the headbox nozzle there is an equalisation chamber.

It is, above all, the size of this chamber that separates the box type from the box mostly used today. Fig. 6. Headbox. Marking of the

equalisation chamber. (13-006.tif)

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2. Flow spreader

On old paper machines the stock was distributed to the box by branching off the feeder tube in a number of smaller tubes, ending on different spots across the headbox.

Fig. 7. Old type of flow spreader.(13-007.tif)

The flow spreaders used today look almost in the same way.

The stock is lead into the thick end of a tapered channel with rectangular, or sometimes circular cross-section.

Fig. 8. Modern type of a flow spreader.(13-008.tif)

The flow is then pressed through a thick plate with drilled holes or through a tube bank. In modern headboxes the diameter of the holes or tubes expands step by step. This type of tube is called ”step diffusor”.

Fig. 9. Tube parcel, a section cut out. (Seen obliquely from above.)(13-009.tif)

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The reason for the tapered form of the entrance channel is that the static pressure should be the same across the whole machine width.

Fig. 10. Illustration. A tapered flow spreader. The static pressure is the same in all positions. (13-010.tif)

If the flow spreader does not taper, the pressure would be lower in the outlet end.

Fig. 11. Illustration showing what would happen to the static pressure if the flow spreader did not taper. (13-011.tif)

The lower pressure in the outlet end would make the stock velocity slower.

Fig. 12. Illustration showing what would happen to the flow velocity through the tubes if the static pressure was lower in the end. (13-012.tif)

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It is not easy to get the same pressure over the whole width. If the flow to the flow spreader increases, the pressure increases the most in the outlet end.

Fig. 13. Illustration showing what happens to the static pressure when the stock flow increases. (13-013.tif)

To avoid the undisirable pressure difference there must be an overflow from the outlet of the flow spreader.

How much stock that has to pass through the outflow valve to even out the pressure difference, can be seen in the sight glass, a small piece of a glass tube, in the thin bypass pipe connecting the inlet and outlet ends of the flow spreader. What is to be done is to open the outflow valve until the flow in the sight glass stops.

Fig. 14. Illustration. The static pressure in the tapered flow spreader is kept equal by regulation of the outflow from the distributor’s outlet side. (13-014.tif)

Fig. 15. Illustration. Pressure check tube with a sight glass. (13-015.tif)

On modern machines the outflow valve is regulated by signals from pressure gauges mounted on the front and back parts of the flow spreader.

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In spite of all the static pressure at the entrance of the tubes will not be exactly the same in all positions. What now has to be done is to get the pressures after the tubes as equal as possible. Only when the pressures are the same, the stock speed out of the tubes will be the same all along the whole tube bank.

When the stock streams through the tubes a certain back-pressure will be formed. The higher the back-pressure is, the higher the pressure drop will become and the better the tubes will even out the inlet pressure differences in the stock before the tubes.

Earlier the tubes were totally straight, but they could be slightly conical, too.

Fig. 16.Illustration. Tube bank with totally straight tubes.(13-016.tif)

Fig. 17. ■ ■Illustration. I ITube bank with I Hconical tubes. I H

(13-017.tif) ■■^■■H

The reason for making the tubes conical was to increase the pressure drop over the tubes. The higher the pressure drop is, the more it will even out the pressure at the discharge side. The conical form made it possible to increase the pressure drop without, at the same time, increasing:

The velocity of the stock streaming Fd r

i go

.p

1 o8

v.e

Irll

su

t sr

ta

ria

gth

i ot

na

.n

Pd

r ce o

s sn

ui c

r ae l

out of the tubes. tubes. (13-018.tif) The distance between the mouths of the tubes.

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The lower the velocity is when the stock streams out, and the smaller the distance between the tubes, the smaller the cross flows on the discharge side of the tubes will be.

Fig. 19.(13-019.tif)

Fig. 20.(13-020.tif)

Figure set. Illustrations. Flow streams after straight and after conical tubes.

The tubes must not be too conical. If they are, the stock may no longer follow the tube wall. The distance between the flow streams leaving the tubes would increase and the undisired cross flows would be back again.

t / \ t M tiFig. 21. Illustration. Flow streams after conical tubes where the stock does not follow the tube wall. (13-021.tif)

In modern headboxes the tubes are longer and the risk for the stock to release the tube wall is not as great as in the older tube types.

Fig. 22. Illustration. Prolonged tubes. (13-022.tif)

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Another change is that the tube diameter in most cases is increasing step by step.

Fig. 23. Illustration. Tubes with a sudden increase of diameter.(13-023.tif)

When the stock passes such a step zone, small whirls, micro turbulence, will be formed.

Fig. 24. Illustration. Forming of micro turbulence in the tube where the diameter increases step by step.(13-024.tif)

The micro turbulence is meant to disperse the fibre flocs in the stock.

Fig. 26.(13-026.tif)

Figure set. Illustrations. Fibre flocs are broken down by micro turbulence.

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3. Equalisation chamber

The task of this chamber (sometimes called reception chamber, stilling pond, etc.) is to eliminate the pressure differences, if the pressures on the discharge side of the tubes still are not identical.

However, the pressure levelling out in this chamber is always achieved at the price of undesired cross streams. If these cross streams do not disappear, there may be problems with both the basis weight and the fibre orientation in the paper.

The velocity of stock jet has to follow the wire speed. In the first totally open headboxes only the force of gravity gave the stock jet the desired velocity. The higher the stock height in the chamber is, the higher the velocity will be.

If the wire speed increases, it is not enough to increase the stock height in a direct proportion to the increase in speed. If e. g. the wire speed is doubled, the stock height must be four times as high as before. The stock height increases with the square of the wire speed increase.

Already at a low machine speed, the limit is reached when the stock height is no longer large enough togive the jet the desired velocity.

Figure set. Illustration of the connection between the stock height and the velocity of the stock jet.

V = y ( 2 g h) * 60

V = m/ming = 9.81 m/sec2 h = m

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Main reg Glossary

The next development step was to close the boxes and to make them work under pressure.

Fig. 29. Illustration. Closed headbox.(13-029.tif)

It was not very easy to get a completely even stock flow to the headbox. Pressure pulses were easily formed after pumps and screens. The open chamber moderated the pulses and levelled out the flow. It was therefore desirable to keep the moderating effect of the air in the new closed boxes, too.

A way to make this possible is to keep a constant stock height and to work with a pressurised air cushion over the stock. This type of box was therefore called air cushion box.

The stock height in the chamber may be rather high, in most cases between 500 and 1000 mm.

Fig. 30. Illustration. Air cushion head-box. Normal stock height. (13-030.tif)

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The higher the stock height is, the more slowly the stock will stream through the chamber. The residence time will be longer.

The stock turbulence forces become weak and the fibres easily form flocs.

MM.Fig. 31. Illustration. Building up of a fibre floc. (13-031.tif)

To break down the developed fibre flocs, perforated rolls, so-called rectifier rolls, are used in the pond.

Fig. 32. Photo. Interior picture of a head-box with perforated rolls, rectifier rolls. (Photo, KMV) (13-032.tif)

Fig. 33.(13-033.tif)

The turbulence generated by the perforated rolls becomes coarse. Because of the coarse scale of the turbulence the fibre flocs are only partly broken down.

Fig. 34.(13-035.tif)

Figure set. Illustration. Fibre floc broken down by a coarse-scale turbulence.

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To prevent the fibres from getting stuck over the hole edges the rolls are slowly rotating.

Fig. 35. Illustration. Perforated rolls.(13-035.tif)

However, the primary reason for having rectifier rolls is not to break down flocs. When the stock streams through the holes, a pressure drop is generated. The pressure drop helps to even out the flow velocity in different positions across the box.

mmmmmmm

rmmmmmmttFig. 36. Illustration. The velocity of the stock flow in various positions across the headbox, before and after the perforated rolls. (13-036.tif)

The hole area of the rolls is in most cases smaller than half the total area. In other words the relative open area is less than 50%. Fig. 37. The surface of the rectifier

roll. (13-037.tif)

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When the stock streams through the holes, the velocity increases to more than the double. The flow also changes its direction. This creates whirls and cross streams; turbulence.

The turbulence downstream the rectifier roll is coarse. If it is not eliminated, it is to be found in the stock jet, when it is ejected to the wire.

Fig. 38. Illustration. Streams created when the stock passes through a hole in a perforated rectifier roll. (13-038.tif)

Thus, the perforated rectifier rolls may create streaks, local regions, where the basis weight and the fibre orientation in the paper become different. The higher the flow velocity is, the greater the problem will be. Another problem is the difficulty in preventing the sticking of the fibres over the hole edges. When the fibres then leave, hard flocs are formed, which may influence the paper quality or at worst give web breaks.

4. Headbox nozzle

The nozzle is the converging channel forming the stock jet.

Fig. 39. The nozzle part of a headbox with perforated rectifier rolls.(13-039.tif)

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The nozzle itself consists of a lower lip and an upper lip, sloping to create a converging channel.

Fig. 40. Illustration. Headbox nozzle. The top and the bottom lips are marked.(13-040.tif)

The pressure in the headbox determines the velocity of the stock jet when it leaves the nozzle. The height of the nozzle opening or the discharge opening, as often said, determines how much stock that will stream out.

V =/ (P) Q =

/ (P • h)

Fig. 41. Headbox. (13-041.tif)P = pressureV = the stock jet velocityh = discharge openingQ= stock flow(b= lip width)

The discharge opening is not the factor that determines the amount of fibres streaming out on the wire. That amount is already determined by the regulation of the flow from the machine chest.

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When the discharge opening increases, the flow area increases, too, and more stock immediately streams out on the wire.

Figure set. Illustration. Increased discharge opening increases the flow.

However, when the back-pressure in the nozzle drops, the pressure in the headbox drops, too, and the stock jet velocity decreases. The speed difference between the wire and the stock jet is changed.

44.044.tif)H Fig. \

(13-

7^_Fig. 45.(13-045.tif) EX.

Figure set. Illustration.Increased discharge opening ⇒ lowerback-pressure.Lower back-pressure ⇒ lower PLower P ⇒ lower V.

Sometimes, instead of difference, you talk about the jet to wire speed ratio or the discharge ratio.

The speed difference is small and even a small change of the difference may influence the paper formation. On most paper machines there is an automatic control system keeping the desired speed difference constant.

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When the box pressure decreases a pressure gauge in the headbox sends a signal to the fan pump. The rotation speed increases. More white water is added and the flow to the box increases.

Fig. 46. Short circulation. A signal from a pressure gauge in the headbox changes the pump rotation speed to achieve the correct headbox pressure. (13-046.tif)

When the flow increases, the flow resistance in the nozzle increases, too, and the pressure in the headbox is soon back on the same level as before. The stock jet recovers its former velocity.

Figure set. Illustration. Increased Qin ⇒ increased P ⇒increased V

When the rotation speed of the feeder pump increased, the stock from the machine chest was diluted with more white water and the concentration in the headbox decreased. Thus, by adjusting the discharge opening the desired headbox concentration was achieved.

If, on the other hand, the discharge opening was changed only locally? What would then happen?

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-KEP 2001-11-14 17


The outflow nozzle is equipped with a number of adjusting screws. With the help of these screws it is possible to adjust the discharge opening across the web.

Fig. 49. Adjusting screws for local adjustment of the discharge opening.(13-049.tif)

By locally changing the discharge opening it is possible to adjust how much stock that will stream out on the wire in that specific area and of course, the basis weight in that area will be influenced.

Fig. 50. Illustration. The stock jet streaming out from an outflow nozzle. (13-050.tif)

On big paper machines the top lip is stiff and therefore impossible to bend. Instead there is a thin, deformable metal strip at the edge of the top lip: the slice bar.

Fig. 51. Illustration. Marking of the slice bar. (13-051.tif)

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The discharge opening is regulated by bending the slice bar only.

Fig. 52. Illustration. A straight slice.(13-052.tif)

Adjustment of the discharge opening influences adjoining areas, too.

If the discharge opening is decreased, the stock streams towards the places where the discharge opening is larger and cross streams are formed. When dewatering on a Fourdrinier waves on the wire may be developed.

Fig. 53. Illustration. A local adjustment of the regulating lip causes cross streams on a Four-drinier wire: wave forming.(13-053.tif)

If these waves meet other similar waves, they may reinforce each other.

Fig. 54. Waves overlaying and reinforcing each other. (13-054.tif)

The cross streams make it difficult to adjust the correct basis weight profile. The cross streams also influence the fibre orientation in the sheet. The conclusion is that it is impossible to change the basis weight within an area without influencing the fibre orientation.

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The top lip can not be adjusted to increase and decrease the discharge opening, only. The top lip can be pushed forwards and backwards, too.

Fig. 55. Outflow from a headbox nozzle. The top lip can be horizontally adjusted. (13-055.tif)

The longer it is moved forwards, the more steeply the stock jet will go down.

Figure set. The angle of the stock jet against the wire. Influence at a horizontal displacement of the top lip.

However, it is not only the horizontal position of the top lip that determines the angle of the jet. The smaller the discharge opening is, the more the jet angle will be influenced at a certain horizontal adjustment.

Figure set. Illustration. Influence on the angle of the stock jet against the wire at a constant ”L”and a decreasing ”h”.

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The angle of the stock jet determines the location of the jet landing on the wire and by which force it will press against the wire. The more the angle is increased, the higher the force will be.

Fig. 60.(13-060.tif)

Fig. 61.(13-061.tif)

Figure set. Illustration. The yellow arrow describes the direction of the stock jet. Increased α ⇒ shorter Lh and higher F.

Thus, the discharge opening determines the flow from the headbox. If the opening is locally changed, the fibre orientation will be influenced, too.

If the discharge opening then is horizontally adjusted, the angle of the jet landing on wire will be influenced as well. The smaller the discharge opening is, the more the angle will be changed at a certain horizontal adjustment of the upper lip.

The discharge opening influences the stock jet in another way, too.

When the discharge opening decreases, the stock flow will contract even more, on its way through the headbox nozzle.

The more the stock flow contracts, the more the stock velocity will increase. The flow will accelerate.

When the stock accelerates, it will be drawn out in the stream direction. The flow elongates.

Fig. 62. Illustration. The acceleration of the stock in a headbox nozzle.(13-062.tif)

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The flow elongation is always in the machine direction, which makes the fibres orientate in the same direction.

Fig. 63. Illustration. Elongational flow in a headbox nozzle. At the flow acceleration the stock stretches out and the fibres orientate in the flow direction. (13-063.tif)

If there are fibre flocs in the stock, they are stretched. As a result of the stretching, the fibre flocs break down.

Figure set. Illustration. Fibre flocs are broken down when the stock elongates in the nozzle.

When the stock flow elongates, the turbulence whirls are also extended. As they do so, the energy in the whirl decreases. The turbulence declines.

Fig. 66. Illustration. A turbulence whirl stretching out in a contracting headbox nozzle. (13-066.tif)

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Thus, a small discharge opening is good, if it is desirable to orientate the fibres in the machine direction, disperse the fibre floc, and reduce the coarse scale turbulence in the stock.

However, it is not possible to reduce the discharge opening too much There is a limit for that.

A smaller discharge opening means that the pressure in the headbox will increase. Then the velocity of the jet leaving the discharge opening will increase.

Figure set. Illustration.A reduced discharge opening ⇒higher P ⇒ higher V

The velocity of jet has to follow the wire speed and must not be changed. Therefore, the signal from the pressure gauge will reduce the rotary speed of the fan pump. If the speed decreases, the amount of diluting white water decreases and the concentration in the stock to the headbox increases.

Fig. 69. If the velocity of the stock jet becomes too high, the rotary speed of the fan pump decreases until the pressure in the headbox is correct.(13-069.tif)

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However, an increased stock concentration means an increased risk for creation of new fibre flocs. Consequently, decreasing the discharge opening is always connected with an increased risk for distroying the formation.

Fig. 70.(13-070.tif)

Figure set. Photos. A well dispersed stock and a floced stock.

If the discharge opening is reduced, the fibre concentration will increase and the fibres will form flocs more easily. Indeed, the flocs are easier broken down again, but the most important is always to prevent fibres from forming flocs, that is to keep the fibre concentration as low as possible.

To find the optimal discharge opening is always a compromise between the risk of building flocs in the sheet and the need of reducing the coarse-scale turbulence.

5. Hydraulic headboxes

The development of headboxes has continued and today a new type of headbox is mostly used.

The equalisation chamber is much smaller and sometimes there is none. In most cases the box is completely filled with stock. These types are called hydraulic head-boxes. Fig. 72. An example of modern

hydraulic headboxes. (13-072.tif)

13-2.doc -KEP 2001-09-12 24


The hydraulic headbox is easier to place in small spaces and therefore it is suitable in such machines where the stock is dewatered between two wires.

Fig. 73. The placing of a hydraulic headbox in a modern twin wire former. (13-073.tif)

In this type of box there is no air cushion that can reduce the pressure variations of the stock.

The space between the flow distributor and the outflow nozzle is small and the time it takes for the stock passing through a hydraulic headbox is short.

Fig. 74. Exploded view. Hydraulic headbox.(13-074.tif)

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The desired turbulence is achieved when the stock streams through narrow tubes.

Fig. 75. Exploded view of the inside of a hydraulic headbox. Marking of the first tube package. (13-075.tif)

To create the desired fine-scale turbulence the tube diameter increases stepwise.

Fig. 76. Exploded view of the inside of a hydraulic headbox. The enlargement of the first step zone in the tube bank. (13-076.tif)

The small chamber levels out the pressure after the tubes and is therefore called equalisation chamber.

Fig. 77. Exploded view of the inside of a hydraulic headbox. The equalisation chamber. (13-077.tif)

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In some hydraulic headboxes there is no equalisation chamber.

Fig. 78. Example of a hydraulic headbox without any equalisation chamber. (Escher Wyss) (13-078.tif)

A small equalisation chamber, or sometimes no such chamber at all, reduces the possibility of evening out the pressure variations after the tube bank. Conseqently, the demand for equal pressure in all positions when the stock leaves the flow spreader is even more important.

After the chamber the flow must again be directed and the stock therefore passes through a secondary tube bank.

Fig. 79. Exploded view of the inner part of a hydraulic headbox. Secondary tube bank.(13-079.tif)

The inlet side of the tubes are round but later they become increasingly square. Square tubes can be positioned closer to each other than round ones.

Fig. 80. Exploded view of the inside of a hydraulic headbox. Enlarged picture of the finishing tube bank. (13-080.tif)

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When the stock streams out from the square holes, the jets come closer to each other and the cross flows become smaller.

Fig. 81. Illustration. Outflow from tube bank with finishing square flow channels. (13-081.tif)

In a hydraulic headbox the stock streams rapidly through the tubes. The turbulence becomes high and the narrow flow channels limit the whirl sizes. The turbulence becomes fine-scale.

Fig. 82. Illustration. Fine-scale turbulence. (13-082.tif)

Developing the right turbulence is very important. The contraction in the nozzle of a hydraulic headbox is much less pronounced than in the older air cushion box and therefore the stretch forces breaking down the fibre flocs become weaker.

Fig. 83. Exploded view. The outflow nozzle in a hydraulic head-box. (13-083.tif)

The low contraction in the nozzle of the hydraulic headbox makes the stretching forces, being able to break down the fibre flocs, weak. Consequently, the turbulence in the initial tubes must be so strong that the flocs are broken down, and the stock is well dispersed before entering the out flow nozzle.

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A way to prevent the fine-scale turbulence from getting weaker is to install thin partition walls in the outflow nozzle. The small contracting flow channels create turbulence and limit the size of the formed turbulence whirls.

i&VALMET

OptiFloAdjustable Turbulence Slice Section

Fig. 84. The outflow nozzle with thin partition walls. (13-084.tif)

However, the turbulence must not be too strong. If it is too strong it breaks up the stock jet and the paper easily becomes grainy. If the dewatering is done on a twin wire machine it may be difficult to orientate the fibres enough in the machine direction.

6. Hydraulic headboxes with air cushion

Working without air cushion is possible if the dewatering is done between two wires. On a single wire the absence of the cushion created a problem. The air cushion came back.

One way of getting back the pulsation moderating effect was to place an airfilled vessel on top of the headbox.

Fig. 85. Exploded view. A hydraulic headbox with a pulsation moderating pressure vessel.(13-085.tif)

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Another way is to place a tank before the headbox.

Fig. 86. Photo. Pressure levelling out tank before the flow spreader. (13-086.tif)

The aim of the perforated plate in the tank is to eliminate the coarse-scale turbulence.

Fig. 87. Exploded view. Pressure tank with a perforated plate. (13-087.tif)

The hole diameter increases step by step to increase the fine-scale turbulence.

Fig. 88. Exploded view. Perforated plate in a pressure tank. (13-088.tif)

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7. Hydraulic headboxes with local stock dilution

Adjusting the basis weight profile with the help of the slice was not an easy operation. Even if the profile finally was correct, the problem with the fibre orientation was still there. In the last developed headboxes there is no possibility to adjust the slice, but in return a totally new procedure is used.

A straight lip means that the stock flow can not be locally changed.

Fig. 89. Example of a headbox with a straight lip. (13-089.tif)

Instead of changing the stock flow the stock concentration is changed in a certain position across the machine.

Fig. 90. Photo. Dilution equipment for a local adjustment of the stock concentration. (13-090.tif)

The local stock concentration is changed by dilution with different quantities of white water in the tubes from the flow spreader.

The flow out of the tubes has to remain constant in all positions.

A = incoming stock flow B = white water flow C = outgoing stock flow C= A+B = constant Fig. 91. Exploded view. Dilution

arrangement for local adjutsment of the stock concentration. (13-091.tif)

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The stock is now distributed over the dewatering wire or wires. Now it is necessary to keep that distribution until the stock is dewatered and all the fibres are fixed in the created network.

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Mam reg Glossary Gallery


FORMING 4, FOURDRINIER SECTION

1. One-sided dewatering ....................................................................................................

2

2. Table rolls ......................................................................................................................

3

3. Forming board ...............................................................................................................

5

4. Landing of the stock jet on the wire ..............................................................................

7

5. Speed difference between stock jet and wire ................................................................ 9

6. Cross streams and their influence on fibre orientation ................................................

12

7. Foils .............................................................................................................................

157.1. Pressure and suction pulses ................................................................................. 167.2. Turbulence ........................................................................................................... 177.3. Pulse frequency .................................................................................................... 21

8. Wet suction boxes ....................................................................................................... 23

9. Dry suction boxes ........................................................................................................

24

10. Couch roll ..................................................................................................................

26

11. Dandy roll ..................................................................................................................

26

12. Forming wire .............................................................................................................

27

^>

^>

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The forming process is not finished until the water is drained from the stock and a wet web is formed. Thus, it is the dewatering of the stock, in the correct manner, that determines the success of the forming process.

The dewatering can be either one-sided or two-sided. On a Fourdrinier the sheet is always dewatered in the one-sided fashion.

1. One-sided dewatering

One-sided de-watering. This is how it once started in the manual paper mill.

Fig. 1. Manual paper production. (14-001.tif)

It was even from the beginning quite obvious that there had to be a certain movement in the stock mix, to prevent the fibres from forming flocs. This experience was then used at the more modern production on paper machines.

Fig. 2. Paper machine with a Fourdrinier.(14-002.tif)

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01-11-08 2


When the stock is de-watered through a forming wire, a fibre network starts forming closest to the wire. As the water in the fibre suspension is removed fibres are successively deposited flat on a wet web. Above the formed web the stock still has the same low concentration as in the jet from the headbox. When all free stock has been

Fig. 3.(14-003.tif)

Figure set. Illustration. One-sided dewatering. The fibre concentration in the undewatered part of the stock stays constant.

dewatered, the remaining dewatering takes place as a thickening process, which means a compression of the fibre network.

2. Table rolls

From the beginning the driving force for the dewatering was the gravitation only. However, as the machine speed gradually increased the rotating wire support rolls, the table rolls, got a growing influence on the dewatering. Fig. 5. Old Fourdrinier section

with table rolls. (14-005.tif)

When the wire entered a table roll, a pressure pulse was generated. Later, when leaving the roll, instead a suction pulse was generated. The suction drew the wire towards the roll.

The pressure difference caused turbulence in the stock on the wire and to some degree improved the final formation. The suction after the roll contributed to the dewatering of the stock.

Fig. 6. Illustration. Pressure and suction pulse over a table roll.(14-006.tif)

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01-09-13 3


The machine speed increased and so the strength of the pressure pulses. Soon the machine speed had passed a critical limit. The pulses began breaking up the fibre net.

When these fragments of the broken up fibre net rejoined the net, they disturbed the forming process.

Figure set. Illustration. Fibre network broken up by a pressure pulse.

Not only the formation became worse when the machine speed increased. The retention was influenced, too.

The fibre network is considerably more fine meshed than the wire. The tighter the net, the easier it catches the fine material in the undrained stock.

Fig. 9. Illustration. Wire, covered by a fine-meshed fibre net. (14-009.tif)

If the pressure pulses break up the fibre net, the fine material could pass through more easily. The retention will decrease.

Fig. 10. Illustration. A broken fibre network. (14-010.tif)

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01-11-08 4


The table rolls made it difficult to increase the speed of the paper machine, without destroying the paper formation and decreasing the wire retention. Consequently, the table rolls were replaced and the wire passed over fixed bars only.

3. Forming board

The first element supporting the wire is called the forming board.

The forming board is built up of a number of bars or blades.

Fig. 11. Illustration. Forming board.(14-011.tif)

One of the reasons for installing a forming board is to prevent the wire from bending down when it is hit by the force of the stock jet.

Fig. 12. Illustration. The stock jet lands on the wire. (14-012.tif)

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01-09-12 5


The wire, dewatering the stock, can have various mesh sizes. Even if the wire meshes are small the wire is so open that it does not really prevent the dewatering of the stock.

Fig. 13. Illustration of the difference in size between the wire meshes and the overlaying fibres.(14-013.tif)

The other purpose of the forming board is to prevent too quick a dewatering.

The dewatering speed on the forming board can be regulated by the blade length and the distance between the blades. The forming board can have different grades of openness.

Fig. 14. Forming board. (Huyck)(14-014.tif)

Normally a small part of the stock jet meets the wire just in front of the first blade. The reason is to get rid of the air in the wire meshes. If the air follows the wire and comes in between the stock and the forming board, the formation of the fibre net may deteriorate.

Fig. 15. Illustration. A small part of the stock jet lands just before the forming board. (14-015.tif)

14-1.doc 01-11-16 6


4. Landing of the stock jet on the wire

Not only the position of the forming board and its openness influence the dewatering. The angle between the stock jet and the wire is very important, too.

The stock jet is ejected with a small angle towards the wire.

The oblique angle makes the stock jet move both forward and downward, both horizontally and vertically.

Fig. 16. The velocity of the stock jet in horizontal and vertical direction.(14-016.tif)

The steeper the jet, the greater the velocity in the vertical direction will be. The force of the jet, pressing against the wire, increases and the dewatering in the point of impact increases.

Fig. 17. Calculation of the vertical force of the stock jet against the wire. (14-017.tif)

The vertical force of the jet must not be too high. Parts of the stock jet may then bounce on the wire and be thrown up again, stock jumping. Another effect of a too high jet pressure is a slower dewatering over the remaining part of the wire section.

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01-11-16 7


The wire is woven of round threads and the upside opening between the meshes almost has the form of a funnel.

Fig. 18. Illustration showing the narrowing form of wire channels, where the water has to stream through.(14-018.tif)

If the fibres are pushed too powerfully into the wire, they will penetrate and plug the funnels and decrease the openings. The result of this is a slow continued dewatering.

Fig. 19. Illustration. Fibres penetrating into and decreasing the mesh openings.(14-019.tif)

If some fibres are pressed too hardinto the narrowing channels, theymay be difficult to pull out again. Asa consequence, instead somefibres will be pulled out of the webwhen it is lifted on the coach roll. Fig. 20. The web lifted from

the wire over the couch roll.(14-020.tif)

Even if that does not happen, an inprint of the wire pattern may sometimes be seen in the finished paper. The wire is said to mark.

The angle of the stock jet determines where on the wire it will land and how the velocity will be divided between a vertical and a horizontal direction. The velocity in the vertical direction will influence the amount of dewatering in the point of impact.

Not only the velocity in the vertical direction is important. The velocity in the horizontal direction is important as well.

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01-09-12 8


5. Speed difference between stock jet and wire

Normally the stock jet velocity differs from the wire speed. In most cases the jet velocity is somewhat higher than the wire speed, but it may be lower, too.

Fig. 21. Illustration. The stock jet velocity related to the wire speed. (14-021.tif)

When the jet meets the wire, the stock closest to the wire will, quicker than the rest of the jet, approach the wire speed. Because of the speed difference the stock will be sheared.

Fig. 22. Illustration. Cross-section of a stock jet. The velocity in different positions when it is slowed down by the wire. (14-022.tif)

The shear forces are the same forces that are achieved when using pressure generating dewatering elements or when a wire shakes on a paper machine. The intention is the same; to break down the fibre flocs in the stock.

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01-09-12 9


One thing distinguishing the shear fields of the jet speed difference from those created by other means. All shear fields, related to velocity differences within the stock jet, have the same direction; the machine direction. Fig. 23. Illustration. The size of the shear

forces and their capability to break down the fibre flocs at different positions. (14-023.tif)

The greater the speed difference between the jet and the wire is, the greater the shear forces become and the easier the fibre flocs are dispersed.

The fact that the shear fields have the same direction makes the fibres orientate in the shear direction.

The closer to the wire, the greater the shear becomes, andthe more the fibres orientate in the machine direction.

Figure set. Shear fields directed in the machine direction orientate the fibres in the same direction.

14-1.doc

01-11-07 10

am reg Glossary iallery

Thus, one effect of various intensity of the shear fields is that the fibres on the two sides of the paper will be differently orientated in the machine direction. The paper becomes unequal-sided.

Fig. 26. Illustration. Paper where the fibres of the paper’s two sides are differently orientated in the machine direction. (14-026.tif)

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01-09-12

11


6. Cross streams and their influence on the fibre orientation

The fact that the shear forces are directed only in the machine direction presumes that there are no cross streams in the stock jet leaving the head box nozzle. However, such cross streams are developed for instance when the lip opening is locally changed. How will then the cross streams influence the fibre orientation?

The speed differences between the jet and the wire make the fibres move in relation to the wire.

If the stock jet velocity is greater than the wire speed the movement is forward along the wire and the fibres are orientated in the same direction.

Fig. 27. Illustration. Stock on the Fourdrinier with an enlarged cut. The stock moves forward in relation to the wire. (14-027.tif)

If there are cross streams in the stock jet, the fibres will also move across the wire and that movement will orientate the fibres as well.

Fig 28. Illustration. Cross streams make the stock move in the cross direction, in relation to the wire.(14-028.tif)

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01-09-12 12


Thus, the fibres move at the same time both in the machine and in the cross directions. The real movement in relation to the wire follows the white arrow on the picture.

The direction of the shear forces follows the real movement and the fibres orientate in the same direction. Fig. 29. Illustration. The white arrow

marks the stock’s real movement in relation to the wire. (14-029.tif)

If the jet velocity is instead lower than the wire speed, the fibres will move in the opposite direction, i. e. backwards in relation to the wire.

Fig. 30. Illustration. The stock moves backwards in relation to the wire.(14-030.tif)

The crosswise movement is the same as before.

Fig. 31. Cross streams make the stock move crosswise in relation to the wire.(14-031.tif)

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01-09-12 13


The white arrow shows the real movement, determining the fibre orientation. It now is pointing in the opposite direction.

Fig. 32. Illustration. The white arrow marks the stock’s real movement related to the wire. (14-032.tif)

Thus, how the cross streams influence the fibre orientation does not depend only on the direction of the stock jet. The speed difference between the jet and the wire is also of importance. The less the difference is, the more the fibres orientate crosswise when there are cross streams in the stock jet.

Dewatering the stock on the forming board is easy. The jet pressure is high. The fibre network is thin and the drainage resistance low. The shear forces can break down the fibre flocs.

During the subsequent dewatering it is different. The shear forces decline and the gravity force is not strong enough to give an acceptable drainage rate.

A way to strengthen the forces, which dewater and shear the stock, is to use foils.

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01-09-12 14


7. Foils

A foil is a bar or a strip, in most cases with a width between 45 and 75 mm. The bar has an upper blade surface that first is plane and later slightly sloping.

The slope of the blade is in most cases between one and three degrees. Fig. 33. Foil. (14-033.tif)

When the waterlogged wire runs over the sloping blade surface it is influenced by a force wanting to lift the wire and split the water film. The water film resists and the water in the wire is sucked downwards towards the bladesurface.

Fig. 34. Illustration. The path of the wire over a foil. (14-034.tif)

The water sucked away follows the underside of the wire and is then scraped off towards the front edge of the next strip.

Fig. 35. Illustration. Dewatering over foils. (14-035.tif)

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01-09-12 15


7.1. Pressure and suction pulses

The strength of the suction pulse depends on the blade angle. The larger the angle is, the stronger the suction pulse becomes.

Fig. 36. Illustration. Pressure and suction pulse formed when the wire runs over the blade surface. (14-036.tif)

The distance the wire runs over the blade surface is also of importance. When the blade-length increases, the suction becomes higher and it works during a longer time.

Fig. 37. Illustration. The suction pulse increases by the increase of the blade length. (14-037.tif)

However, the foil blades do not only give suction pulses.

When the wire enters the plane blade surface, short pressure pulses are generated.

The small pressure pulses loosens up the fibre layer and the subsequent dewatering will be easier. Fig. 38. Illustration. The pressure pulse

over the front blade edge loosens the fibre net. (14-038.tif)

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01-09-12 16


The pulse strength does not depend only on the design of the foils. It also depends on the wire speed. If the wire speed is doubled, the pulses will be four times stronger. The pulses increase with the square of the wire speed.

7.2. TurbulenceSucking out the water of the stock might be done in different ways. Using foils is a way to create not only suction pulses, but the necessary turbulence in the stock, too.

Fig. 39. Forming of turbulence over the foils. (14-039.tif)

When the wire runs over the sloping blade it has to move downwards. Before the next blade is reached it has to move upwards again.

The repeated down and up movements of the wire create shear forces breaking down the fibre flocs in the stock. Fig. 40. Illustration. Shear forces

breaking down fibre flocs.(14-040.tif)

When the wire runs over the sloping blade surfaces, turbulence is created in the undrained fibre suspension. The faster the wire runs over the blade, the greater the turbulence becomes.

The created shear forces break down the fibre flocs. It is obvious that the size of the shear forces is important. What will then determine the strength of the shear forces and how will they get the right size?

The importance of the machine speed is manifest, but how important is the influence of the foil itself?

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01-09-12 17


In the first example the running distance, along the slope of the blade, is doubled.

Figure set. Foils with different blade lengths.

A doubling in the blade length means a doubling in the horizontal distance (Sh). At a

constant angle (α) the vertical movement of the wire will be doubled, too.

Fig. 43. Ex. 1. Doubling the blade length. Vertical motion.(14-043.tif)

The time (2tv) for the wire to move the vertical distance (2Sv) will, at the same time, be twice as long.

th

atv= th

2-th a

| 2-tv

Fig. 44. Ex. 1. Doubling the blade length. Time-consumption at vertical motion. (14-044.tif)

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When the distance (Sv) and the time (tv) increase proportionally the vertical speed (Vv) remains the same as before and the shear forces will not be appreciably influenced by the change.

Fig. 45. Ex.1. Doubling the blade length. The velocity at the vertical movement. (14-045.tif)

In another example, instead the angle (α) increases from 1o to 2o. Now, the wire moves vertically a distance twice (= 2Sv) as long as before.1

Sh

a Sv

Sh

2•a2-Sv

Fig. 46. Ex. 2. Doubling the blade angle. The length of vertical movement. (14-046.tif)

The time (tv) of the motion (2Sv) will still be the same.

th

a tv=th

th

2•a

lth=tv

Fig. 47. Ex. 2. Doubling the blade angle. The time-consumption at vertical motion.(14-047.tif)

The exactness is enough at such small angles.

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The vertical velocity now becomes twice as high and the shear forces increase.

Vh

a Vv

Vh

2 • a

2-Vv

Fig. 48. Ex. 2. Doubling the blade angle. The velocity at vertical motion.(14-048.tif)

A large blade angle is to prefer if it is desirable to produce great shear forces, but attention is necessary. If the angles become too large, the fibre network will break up.

Fig. 49. Illustration. Fibre net broken up by too strong shear forces.(14-049.tif)

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Different foils give differently powerful turbulence and therefore the choice of the foil must be very careful. However, it is not only a question of the foil design. The number of foils is also important.

7.3. Pulse frequency

The number of foils determines how many times the stock is influenced by the vertical wire movements.

Instead of selecting a large blade angle it might be suitable to decrease the angle and increase the number of foils.

Fig. 50. Foils battery. (14-050.tif)

A smaller blade angle and a greater number of blades decrease the vertical movement over each blade and increase the number of pulses. The amount of energy generated in each pulse decreases and instead the pulse frequency increases.

Turbulence energy =

f (pulse strength • pulse freq.)

A high pulse frequency means that the stock turbulence will mainly remain in the areas between the foil blades and that there is no need to put in a high amount of energy replacing lost turbulence energy.

The reduced energy input in each pulse decreases the risk of breaking up the fibre network.

That risk is also connected to the type of fibres, and the number of fibres in the created network. The more fibres and the longer, the stronger the network will be.

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When describing what is happening in the stock on the wire the word used is ”activity”. A certain activity is the visible proof of a stock turbulence.

Fig. 51. Stock turbulence: activity.(14-051.tif)

Normally, small, short waves in the free stock are desirable.

Fig. 52. Example of a suitable grade of acitvity. (14-052.tif)

If the activity instead grows to a degree causing free drops to leave the stock it is most probably a sign that the turbulence is too high.

Fig. 53. An activity that, most probably, is too high. Free water drops leave the stock. (14-053.tif)

Thus, by studying the activity the degree of turbulence in the undrained fibre suspension could be estimated, making it possible to choose the right conditions. The longer the fibres in the stock, the greater the turbulence must be to avoid harmfull fibre flocs.

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8. Wet suction boxes

The more the fibre network grows, the more difficult it will be to drain the stock.

Fig. 54. Illustration. Fibre network. Now, the larger part of the fibres are fixed. (14-054.tif)

A way to overcome the increasing drainage resistance is to gather the foils in closed boxes and to let them work in a vaccum.

Fig. 55. Illustration. Wet suction box.(14-055.tif)

The underpressure between the foils makes the suction pulses longer and more water is sucked through the fibre net.

Fig. 56. Suction pulses over wet suction boxes. (14-056.tif)

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The vaccum in the suction boxes is increased keeping step with the growth of the fibre network.

Fig. 57. Normal vaccum in wet suction boxes (metre columns of water ). (14-057.tif)

When the wire has passed the last suction box, the free stock is in most cases gone and the water mirror on the wire has disappeared. This point is called the wet line. All fibres are now fixed in the fibre network and the forming is in principle finished.

Fig. 58. The last wet suction box. The wet line is visible. (14-058.tif)

9. Dry suction boxes

The vacuum level in the dry suction boxes is higher than in the wet ones. In most cases the levels are between 10 and 30 kPa (1 -3 metres columns of water).

Fig. 59. Illustration. Dry suction boxes. (14-059.tif)

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The water can be removed only at a limited rate.

If the suction increases too much, the wire is pressed harder against the blades of the suction box, and the friction becomes greater.

Fig. 60. Illustration. The counter force F strongly increases by the increase of the pressure P. (14-060.tif)

The increased friction causes the wire and the blades of the suction box to get unnecessarily worn and the force moving the wire strongly increases.

Fig. 61. Fourdrinier section with wet and dry suction boxes. (14-061.tif)

The blades of the suction boxes are quite plane. Now the fibres are totally fixed and it is not necessary to create turbulence.

Fig. 62. Dry suction boxes with quite plane blades. (14-062.tif)

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10. Couch roll

The last dewatering on the wire section takes place when the web runs over a perforated roll, the couch roll.

In the couch roll there is a suction box, its suction force is greater than in the former suction boxes.

Fig. 63. Couch roll on a liner machine.(14-063.tif)

11. Dandy roll

The importance of shear forces in the undrained fibre suspension to avoid harmfull fibre flocs has been known since the production of the very first paper sheet. However, the way to achieve the shear forces has varied.

In the wire section on a Fourdrinier machine producing fine paper a dandy roll was used for a long time.

The dandy roll is a wire covered drum placed on the wire with a carefully adjusted pressure. The roll is rotating with the wire or driven with a somewhat higher speed than that of the wire. It is located in the middle of the suction box section just before the wet line. Fig. 64. The dandy roll on a fine paper

machine. (14-064.tif)

The reason for having the dandy roll is not only to disperse un-desired fibre flocs. It also presses down the fibre net and levels out the top side of paper, which otherwise has a tendency to become uneven.

At high machine speed, it is difficult to avoid splash from the drum. Consequently, on the machines of today the dandy roll is no longer used.

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12. Forming wire

The wire has to be as plane as possible on the paper side and the mesh openings small.

Fig. 65. Illustration. The top side of a forming wire. (14-065.tif)

As most of the fibres are orientated in the machine direction it is an advantage that the surface threads are orientated crosswise.

The fibres then more easily bridge the wire openings without penetrating and blocking the water flow.

Fig. 66. Illustration. Fibres drawn against the top side of a forming wire. (14-066.tif)

The backside shall give the wire the mechanical stability and the durability.

Fig. 67. Illustration. The underside of a forming wire. (14-067.tif)

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A way to meet the various demands on the wire is to use different thread layers.

A wire of several layers has a fine paper side and coarse backside.

Fig. 68. Illustration. Cross-section of a forming wire with several layers. (14-068.tif)

On the Fourdrinier section the dewatering always takes place in one direction, only. The sheet easily becomes unevenly sided and the dewatering is comparatively slow. The fact that the stock on the topside is facing the air makes it difficult to avoid waves and splashing.

A way to avoid these weaknesses is to dewater between two wires. The next part is about just that technique.

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FORMING 5, TWIN WIRE FORMER

1. Two-sided dewatering ..................................................................................................... 22. Hybrid former .................................................................................................................. 6

2.1. Generally .............................................................................................................. 62.2. Sym-Former from Valmet .................................................................................... 82.3. Duoformer D from Voith .................................................................................... 132.4. SymFormer MB from Valmet ............................................................................ 16

3. Twin wire formers ......................................................................................................... 173.1. Roll former ................................................................................................................. 18

3.1.1. Speed-Former from Valmet ............................................................................. 183.1.2. Roll formers for tissue paper ........................................................................... 23

4. Blade formers ................................................................................................................ 244.1. Bel Baie 2 from Beloit ........................................................................................ 24

5. Blade-roll formers and roll-blade formers ..................................................................... 275.1. Blade-roll former ................................................................................................ 285.1.1. Bel Baie 4 from Beloit ..................................................................................... 285.1.2. Bel Baie 3 from Beloit ..................................................................................... 30

6. Roll-blade formers ......................................................................................................... 326.1. Speed-Former HS from Valmet .......................................................................... 326.2. Speed-Former SC from Valmet .......................................................................... 346.3. Speed-Former HHS from Valmet ....................................................................... 356.4. OptiFormer with forming shoe from Valmet ..................................................... 366.5. Duoformer CFD from Voith ............................................................................... 386.6. OptiFormer with loadable blades from Valmet .................................................. 40

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1. Two-sided dewatering

The Fourdrinier machine works well if the speed is not too high. However, when the speed exceeds about 1100 metres per minute the friction between the free stock and the air is growing to a level causing problems with waves and splashing. It becomes increasingly difficult to control the formning process.

Fig. 1. Dewatering on a Four-drinier section. (15-001.tif)

Besides, at a one-sided dewatering, the fine material is unevenly distributed in the Z direction of the paper. The paper becomes unequal-sided.

The difficulties increasing the machine speed and the constantly increasing demand on paper quality have led to the use of formers dewatering in two directions; twin wire formers.

Fig. 2. Dewatering in a twin wire former. (15-002.tif)

Dewatering in two directions means that the fibre net, which the water has to cross, is just half as thick as when dewatering in only one direction.

When dewatering through a fibre net half as thick the flow resistance, becomes only half as high as well.

Figure set. Illustration. The thickness of the fibre net at one- and at two-sided dewatering.

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The flow penetratinga single fibre net will alsochange. When the stock isdewatered in twodirections, the amount ofwater streaming in eachdirection becomes half asmuch as when dewatering in onlyone direction. This, too, meansthat the flow resistance is halved.

Figure set. Illustration. Flow amounts in each direction at one-sided and two-sided dewatering, respectively.

Thus, if the flow resistance is halved twice, it is decreased to one fourth compared with dewatering in one direction. This makes it possible to dewater considerably faster.

But how fast, depends not only on the flow resistance.

When dewatering on a Fourdrinier machine only gravity and suction forces are at work.

Fig. 7. Suction of water on a Fourdrinier section. (15-007.tif)

If the dewatering is instead done between two wires the water can also be pressed out of the stock.

Fig. 8. A combined pressure and suction dewatering in a hybrid former. (15-008.tif)

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The pressure in the dewatering zone can be constant.

Figure set. Illustration. Dewatering at a constant pressure.

It could also be divided into a number of single pressure pulses.

Figure set. Illustration. Dewatering at pulsating pressures.

The pressure pulses make the undrained fibre suspension alternately slow down and speed up. The created stretch and shear forces, orientate the fibres in the machine direction and break down the fibre flocs in the stock.

Figure set. Illustration. Stretch and shear forces in the undrained stock break down fibre flocs and orientate fibres in the machine direction.

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The pressure pulses must not be too strong. Pulses that are too strong make the shear forces so violent that they break up the already formed fibre net. Then, the ability of the fibre net to catch the fine material is reduced. The retention becomes low.

Fig. 15. Illustration. Too strong a pressure pulse breaks up the already formed fibre net.(15-015.tif)

Dewatering the stock between two wires goes quickly and the time it takes to form the sheet becomes short. The shorter that time is, the fewer flocs the fibres manage to form before they are fixed in the formed fibre net.

However, the time is in most cases not so short that the fibres do not manage to form any flocs at all. Consequently, the way to avoid getting flocs is just to create pressure pulses during the stock dewatering.

How strong the pressure pulses have to be is always a balance between the wish to achieve a good formation and a high retention. The formers develops continuosly to meet these conflicting demands.

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2. Hybrid former 2.1.

Generally

The stock is not always dewatered in the two-sided manner from the very beginning. Some Fourdrinier machines are rebuilt in a way making only the later part of the dewatering two-sided. Such machines are called hybrid formers.

Fig. 16. Hybrid former in a sack paper machine. (15-016.tif)

The fact that the initial dewatering is more slow when dewatering on a Fourdrinier wire could sometimes be an advantage. Dewatering in a gap former means a very short drainage time. The water has to stream so quickly through the fibre net that small channels are easily formed.

In the paper the small channels are then to be seen like small holes, pinholes.

Fig. 17. Twin wire former in a newsprint machine.(15-017.tif)

The risk of forming pinholes increases the tighter the fibre net is. It becomes particularly tight when the fibres are soft and formable; well beaten chemical fibres. The thickness of the fibre net is also important. The thinner the fibre net is, the weaker it becomes and the easier it breaks up.

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In the hybrid former the first dewatering to about 2% dryness takes place on a conventional Fourdrinier wire. Compared to a pure twin wire former (a gap former), the drainage area is larger and the dewatering is less intensive.

Fig. 18. The dewatering on a Four-drinier wire before the forming roll.(15-018.tif)

Besides, in a hybrid former a rather stable fibre net has been formed before it enters the nip between the two wires. The power of the network prevents too many pinholes from being formed.

Fig. 19. Illustration. Forming of fibre net at a one-sided dewatering.(15-019.tif)

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2.2. Sym-Former from Valmet

A Fourdrinier machine can, rather easily, be rebuilt to a hybrid former.

The headbox and the main part of the wire section can be the same.

Fig. 20. Sym-Former R. (15-020.tif)

The picture shows a Fourdrinier machine being rebuilt to a hybrid former.

Fig. 21. Sym-Former R is a concept suitable particularly when rebuilding an existing Fourdrinier section. (15-021.tif)

A new model of machine is similar to the rebuilt machine.

Fig. 22. Sym-Former. (15-022.tif)

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The dewatering over the top wire normally corresponds to between 20 and 40% of the total dewatering.

Fig. 23. The top wire section in a Sym-Former.(15-023.tif)

The velocity of the stock entering the nip between the wires is the same as the speed of the two wires. In the nip a certain hydraulic pressure is created, and results in a deceleration of the undrained fibre suspension.

Fig. 24. Illustration. The retarding of the "free" stock in the nip before the forming roll.(15-024.tif)

The fact that the undrained stock and the fibre net get different velocities creates shear forces in the undrained fibre suspension, breaking down the fibre flocs and orientating the fibres in the machine direction.

Figure set. Illustration. Shear forces break down the fibre flocs and orientate the fibres in the machine direction.

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The forming roll is open and covered with a wire cloth.

Fig. 27. Forming roll, covered with a wire cloth. (15-027.tif)

The dewatering over the forming roll takes place at a constant pressure. How high the pressure becomes depends on two things; the radius of the forming roll and the tension in the bottom wire.

P = dewatering pressureT = wire tension, bottom wireR = the radius of the forming roll

P = f (T/R)

Fig. 28. Components determining the dewatering pressure over the forming roll. (15-028.tif)

The stock will mainly be dewatered upwards, towards the forming roll. During the one-sided dewatering on the Fourdrinier wire a fibre net, preventing the dewatering through the bottom wire, has already been formed.

Fig, 29.(15-29.tif)

t t t t t tFig. 30.(15-030.tif)

Figure set. Illustration. The fibre net, built on the bottom wire, prevents the dewatering and influences the direction of the dewatering.

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The water, pressed out, mainly enters the cavities in the forming roll. After the nip this water is thrown out.

Fig. 31. Illustration. The water leaving the forming roll enters a special flow channel.(15-031.tif)

After the forming roll there is a slightly curved zone, with parallel blades, the forming shoe.

The reason why the forming shoe is curved is to increase the dewatering pressure between the wires.

However, in this case, the curvature is small and as a consequence the constant pressure becomes low.

Between the blades the forming shoe has a certain amount of vaccum. The reason for this vaccum is to suck down the wire between the blades.

Fig. 32. Illustration. Dewatering over the forming shoe. (15-032.tif)

Fig. 33.(15-033.tif)

Figure set. Illustration. The vaccum between the blades determines how much the wire will bend over the blade edges.

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The higher the vaccum is, the more the wire bends over the blade edges and the stronger the pressure pulses become.

Fig. 35. Illustration. (15-036.tif)

When the web enters the blade zone, the dryness is already so high that the majority of the fibres are fixed in the fibre net. Consequently, the pressure pulses aimed to break down the fibre flocs and improve the formation are not now as useful as when the stock had more fibres left in the un-drained fibre suspension.

Fig. 36. Illustration. The majority of the fibres are already fixed in the fibre net when the web enters the blade zone. (15-036.tif)

This machine concept is very suitable for production of printing paper containing mechanical as well as a mixture of long and short chemical fibres. However, when producing papers with high basis weights or papers with a high amount of long fibre a new former type, with blades positioned on opposite wire sides, can develop an improved formation.

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2.3. Duoformer D from Voith

In Duoformer D there is no longer any predewatering over the forming roll.

Fig. 37. Duoformer D. Note the distance ”h” between the forming roll and the bottom wire. (15-037.tif)

The forming roll is now quite smooth and it is placed a bit above the bottom wire.

Fig. 38. The bottom wire before the forming roll. (15-038.tif)

In this concept the driving force dewatering the stock and creating the necessary shear forces is the pressure pulses produced by loadable blades pressing the wires against each other.

Fig. 39. The blade part in a Duoformer. The blade pressure can be individually regulated.(15-039.tif)

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The blades in the lower row are mobile and can be adjusted in the way making the stock movement in the nip more or less ”zig zag-formed”.

Fig. 40. Illustration. The blade pressure controls the path of the web through the blade zone. (15-040.tif)

How much the web bends over the blade edges determines how strong the pressure pulses will become.

Fig. 41. The size of the pressure pulses depends on how much the wire bends before and after the blade edges.(15-041.tif)

Under each blade in the lower row there is a small rubber tube filled with air.

Figure set. Illustration. When the pressure over the blade increases, the rubber tube is pressed together. How much depends on the air pressure selected for the tube.

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If the dewatering resistance increases, the tubes are pressed together and the distance between the opposite blade rows increases.

The path, the web has to follow through the blade zone, remains the same and as a consequence the pressure pulses over the blade edges remain the same.

Figure set. Illustration. The path the web has to follow is independent of the dewatering pressure and therefore the dewatering pulses remain constant.

Between the blades on the over side row a vaccum is applied.

That the fibre net is thinner on the upper side increases the possibility of influencing the dewatering in that direction.

Fig. 46. The underpressure between the blades in the upper blade row can be used to control the dewatering direction. (15-046.tif)

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The dewatering of the stock made before the blade zone is now lower than in the earlier described concept where the dewatering took place already over the forming roll.

Thus, the amount of fibre remaining in the un-drained fibre suspension is higher than before. The created stretch and shear forces can now diperse the fixed in the fibre net. The ability increases.

Fig. 47. Illustration. Without pre-dewatering over the forming roll.(15-047.tif)

Fig. 48. Illustration. With predewatering over the forming roll.(15-048.tif)

Figure set. Comparison between the amounts of still undrained stock suspension.

fibre flocs before they are of improving the formation

2.4. SymFormer MB from Valmet

SymFormer MB is a former with a construction similar to the one described above.

The former is in the first place used at the production of paper with high basis weight or with a high amount of long fibres in the stock. Fig. 49. SymFormer MB. (15-049.tif)

Generating pulses over straight blade units brings the advantage that the strength of the pressure pulses is not influenced by the wire tension in the same way as when dewatering over a roll or a curved forming shoe.

The ability of creating controllable dewatering pulses make the formers particularly suitable for a stock with a high amount of long fibres.

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3. Twin wire formers

The hybrid formers often run somewhat more than 1100 metres per minute. However, when the speed is too high, the predewatering in the Fourdrinier section becomes a problem. So on very fast machines, pure twin wire formers, called gap formers are used.

Pure twin wire formers were used already in the beginning of the sixties. The main purpose was to increase the speed at the production of newsprint.

From the beginning two main types were developed. One of them can be regarded as a pure roll former and the other one as a pure blade former.

The difference between the two models was the way of pressing the water out of the stock.

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3.1. Roll former

3.1.1. Speed-Former from Valmet

Speed-Former is an example of a pure roll former being mainly used at the production of newsprint.

Fig. 52. Speed-Former. (15-052.tif)

The roll former has two wires surrounding a large, open forming roll.

Fig. 53. Principle sketch. Roll former.(15-053.tif)

The wrap angle of the surrounding wires is high, up to 120o.

Fig. 54. Illustration. Forming roll.(15-054.tif)

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The centrifugal force drives the water towards the outer wire.

Fig. 55. Photo. Dewatering over the forming roll. (15-055.tif)

In the forming roll there is a broad suction box.

The vaccum in the suction box has to create a suction force as strong as the centrifugal force.

Normally, it is desirable to get as much water leaving the web in the both directions.

Fig. 56. Illustration. Dewatering over the forming roll. (15-056.tif)

It is neither the centrifugal nor the suction force that determines how fast the dewatering takes place. What determines this is the pressure between the wires.

P = dewatering pressureR = the radius of the forming rollT = the tensile tension of the wire

P = f (T/R)

Fig. 57. Illustration. Factors determining the dewatering pressure over the forming roll. (15-057.tif)

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In a roll former the pressure in the dewatering zone is almost constant during the dewatering.

The shorter the radius of the forming roll is, and the higher the wire tension is, the higher the pressure and the quicker the dewatering takes place.

Fig. 58. Illustration. Dewatering over a forming roll. The pressure ”P” is constant through the whole dewatering zone. (15-058.tif)

Looking back on a Fourdrinier machine, it is the angle between stock jet and the wire that determines how strong the dewatering will be in the point of impact.

The shear forces developt in the stock depend on the speed difference between the stock yet and the wire. Fig. 59. Illustration. The direction and

the speed of the stock jet on a Fourdrinier machine. (15-059.tif)

When dewatering in a nip between two wires results become the same. However, before evaluating the influence of the speed difference the following fact has to be considered:

Fig. 60. Illustration. The direction of the stock jet against the forming wire in a roll former. (15-060.tif)

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In a roll former the dewatering zone is short. To compensate for that, the nip pressure must be high.

Fig. 61. Illustration. Dewatering over a forming roll. The pressure ”P” is high. (15-061.tif)

The higher the nip pressure is, the higher the pressure in the stock jet must be. The velocity of jet must be high.

Fig. 62. Photo. The high dewatering pressure requires a high velocity of the stock jet. (15-062.tif)

When the stock jet comes into the nip between the wires it is retarded by the hydraulic pressure necessary to dewater the stock.

Fig. 63. Illustration. Dewatering overa forming roll. (15-063.tif)V1 = The original velocity of the stock

jet.V2 = The velocity of the stock jet after

retardation.

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When dewatering on a twin wire former it is the speed difference between the retarded stock jet and the wires that creates the shear forces.

Fig. 64. Illustration. Dewatering over a forming roll. The speed difference between Vstock and Vwire creates shear

forces. (15-064.tif)

Shear forces break down the

fibreflocs in the fibre suspension.

Here, the shear forces are directed only in the machine direction. Consequently, they orientate the fibres in that direction as well.

Fig. 65. Illustration. Dewatering over a forming roll. Shear forces break down flocs and orientate the fibres in the machine direction. (15-065.tif)

The shear forces created in the dewatering zone in the roll former influence the stock only during the very first part of the dewatering. As a consequence, the effect of the shear forces is low.

The loss of pressure pulses during the continued dewatering decreases the stress on the formed fibre net, but that the fibres in the still undrained fibre suspension can easily form flocs again.

Roll formers have been used on some of the fastest newsprint machines. They give a high retention, but the formation is never as good as it ought to be.

Pure roll formers are not very common today, but there is one important exception; paper machines for tissue production.

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3.1.2. Roll formers for tissue paper

Roll formers are frequently used at the production of tissue paper.

Fig. 66. Tissue paper machine. (Photo Valmet) (15-066.tif)

The forming roll, used on the tissue paper machine, is in most cases quite smooth and the dewatering takes place in only one direction.

The low basis weight gives a low dewatering resistance and therefore the dewatering is very fast. Fig. 67. Illustration. Dewatering

over the forming roll on a tissue paper machine. (15-067.tif)

The formeron thepicturediffers fromother typesas the pressfelt is a partof the

Fig. 68. Tissue paper machine. Crescent former. (15-068.tif)

formingunit. The sheet is formed between the forming wire and the formingfelt. The felt transfers the sheet to a press nip between a roll and thehot yankee cylinder.

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4. Blade formers

A way to avoid flocs, thus improving the formation, is to create pressure pulses during the stock dewatering. Even here, the pressure pulses are achieved with special blades. This type of former is called a blade former.

Fig. 69. Twin wire former. Bel Baie 2. (15-069.tif)

4.1. Bel Baie 2 from Beloit

Bel Baie 2 can be seen as an example of a pure blade former.

In the blade former there is no dewatering at all over the forming roll.

Fig. 70. Principle sketch, Bel Baie 2. (15-070.tif)

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In the drainage zone there is a slightly curved forming shoe.

The curvature of the forming shoe creates a small constant pressure between the wires.

Fig. 71. Illustration. Forming shoe. (15-071.tif)

The pressure pulses are then formed when the wires bend over the blade edges in the forming shoe.

Fig. 72. Illustration. Pressure pulse over blade bars.(15-072.tif)

The stretch and shear forces over the forming shoe in a blade former influence the fibres until they are completely fixed in the formed fibre net. Compared with dewatering over the forming roll in a roll former, the fibre suspension are now more well dispersed and the fibres are more orientated in the machine direction.

Even now, the angle between the stock jet and the forming wire at the point of impact determines how much the stock will be dewatered in the different directions.

Just as is the case on the forming board of the Fourdrinier machine, it is desirable to have a certain de-watering before the forming shoe. The reason is to get rid of the air following the wire.

Fig. 73. Illustration. The angle between the stock and the forming wire. (15-073.tif)

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The curvature of the wire over the forming shoe is much smaller than over the forming roll in a roll former, and because of that the pressure in the nip in a blade former will be less high.

Fig. 75.(15-075.tif)

Figure set. Illustration. The bending of the wires in the dewatering zone. Comparison between a blade and a roll former.

That the pressure in the nip is lower means that the pressure from the stock jet does not have to be as high as in a roll former.

Consequently, the velocity of the jet in a blade former can be lower than in a roll former.

Fig. 76.(15-076.tif)

XFig. 77.(15-077.tif)

Figure set. Illustration. Comparison between the pressure in a blade and in a roll former.

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5. Blade-roll formers and roll-blade formers

A blade former made it possible to produce a well formed paper, but the pressure pulses easily became a bit too strong. The problem with a blade former was therefore the low retention of fine material.

In the machines of today the dewatering is done over both blades and a roll, but the order of succession may change.

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5.1. Blade-roll former

5.1.1. Bel Baie 4 from Beloit

Bel Baie 4 is the latest design in the row of pure twin wire formers from Beloit and is an example of a blade-roll former. There are alternative designs. The type described here is equipped with blades positioned on opposite sides of the wires.

In the blade-roll former the dewatering is done first over blades and then over a roll.

Fig. 80. Principle sketch. Bel Baie 4. (15-080.tif)

Compared with the older models Bel Baie 2 and 3, the first forming shoe is now placed closer to the headbox nozzle. The shorter the distance between the forming roll and the forming shoe is, the more stable is the wire run.

Another advantage is that the length of the free stock jet gets shorter. The shorter it is, the less is the risk to break up the free stock jet.

Fig. 81. Principle sketch. Bel Baie 4. The forming roll and the first forming shoe. (15-081.tif)

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The stock jet meets the inner wire just in front of the first blade.

Fig. 82. Illustration. The stock jet angle and the point if impact against the forming wire.(15-082.tif)

The other forming shoe is now placed on the opposite side. Here, in contrast to the earlier described blade formers, a vaccum is applied between the blades.

The fact that the blades are oppositely placed makes the pressure pulses to influence the stock from the two sides.

How strong the pressure pulses will be, depends on how high the blade load is.

The increasing number of pressure pulses may make it possible to lower the strength in each single pulse.

Fig. 83. Principle sketch. The second forming shoe in a Bel Baie 4. (15-083.tif)

Figure set. Blade load determines the size of the pressure pulses.

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Another advantage with oppositely directed pulses is that it is easier to distribute the fine material evenly in the Z direction of the sheet. The main advantage to dewater first over blades and then over a roll is that the pressure pulses can work while many fibres still remain in the free undewatered part of the stock. Thus, the stretch and shear forces can from the very beginning prevent fibres from forming harmfull flocs. Fig. 86. Illustration. The

pressure pulse over the blade creates stretch and shear forces preventing the fibres from forming flocs. (15-086.tif)

The subsequent dewatering over the suction roll takes place under constant pressure. However, the main part of the fibres are now already fixed in the formed network, and there is no longer use of pressure pulses.

Fig. 87. Principle sketch. Dewatering over the suction roll. (15-087.tif)The length of the wires

surroundingthe couch roll is higher than in theprecurser, Bel Baie 2. Thus, in this former the dewatering overthe suction roll is higher and the need to dewater over the formingshoe becomes lower. The pressure pulses may be softer and therisk of breaking up the formed fibre net is decreased. The formationand the retention develops better.

5.1.2. Bel Baie 3 from Beloit

Bel Baie 3 was a precursor to Bel Baie 4, and the first former using the combination of blade and roll dewatering. A difference from the precursor, Bel Baie 2, is the ability of regulating the dewatering direction and the strength of the pressure pulses with the help of vaccum in the forming shoe.

Fig. 88. Photo. Bel Baie 3.(15-088.tif)

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Comparison between Bel Baie 3 and the earlier described Bel Baie 4

On Bel Baie 3 the distance between the forming roll and the forming shoe is longer.

On Bel Baie 3 there are

Fig. 89. Principle sketch Bel Baie 3.(15-089.tif)

no opposite blades being able to generate directed pressure pulses.

The main difference from the pure blade former is that on the blade-roll former there is more of dewatering over the roll, so the need of dewatering over the blade part is lower. The less water to remove over the blade, the smaller the pressure pulses can be. The risk to break up the formed fibre net decreases. The formation gets better and the retention does not decrease as earlier.

If counter-directed pressure pulses then are formed, it is easier to distribute the fine material in the Z direction.

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6. Roll-blade formers

The aim of the roll-blade former is the same as the blade-roll former; to decrease the amount of water needed to be removed over the blade zone.

In the roll-blade former it is the pre-dewatering over the forming roll that makes it possible to reduce the dewatering over the blades.

6.1. Speed-Former HS from Valmet

Speed-Former HScan be regarded as an example of a roll-blade former.

Fig. 90. Speed-Former HS. (15-090.tif)

The stock jet is injected tangentially towards the forming roll and meets the outer wire just after the breast roll.

The length of a wire wrapping the forming roll is much smaller than on the earlier described pure roll former.

Fig. 91. Dewatering over the forming roll.(15-091.tif)

Now the dewatering over the forming roll is going so quickly that the fibres do not have time enough to form flocs disturbing the forming process.

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On the old roll former the stock was dewatered and the fibre firmly fixed already after the forming roll. However, in the roll-blade former they remain to a great extent in the undewatered stock when entering over the forming shoe.

The forming shoe is slightly curved. The reason for that is to increase the constant dewatering pressure between the two wires.

Fig. 92. The dewatering shoe on a Speed-Former HS. (15-092.tif)

The size of the pressure pulses is even now regulated by using vaccum between the blades.

Figure set. Illustration. Regulation of the pressure pulses by adjustment of the underpressure between the blades.

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The pressure pulses must not be too strong. Strong pulses can break up the fibre net formed over the forming roll.

Fig. 95. Illustration. Too strong a pressure pulse breaking up the formed fibre net. (15-095.tif)

6.2. Speed-Former SC from Valmet

The former type, Speed Former HS, is generally used at the production of newsprint.

However, when magazine paper is produced there has to be a larger amount of filler in the stock. When using a filler it is most important to retain and correctly distribute the filler in the Z direction of the paper. Only when this is achieved the desired quality properties of the paper can be reached.

SPEED-FORMER HS(FOR SC GRADES)

Outer wire

Hi VALMET Paper Machinery

Fig. 96. Speed-Former SC. (15-096.tif)

In order to meet that specific quality demand better the former has got a somewhat different design.

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In contrast to the earlier described former, the forming roll is now placed in the outer wire and the forming shoe is in the inner wire.

Fig. 97. Speed-Former SC(15-097.tif)

The dewatering over the forming roll makes the top side well closed and the filler will be more evenly distributed in the Z-direction.

Compared to Speed-Former HS the wire wrapping on the couch roll is longer and more water is removed.

Fig. 98. Speed-Former HS(15-098.tif)

Figure set. Comparison between Speed-Former HS and SC.

The increased dewatering over the couch roll decreases the demand of dewatering over the forming shoe. The pressure pulses may be weaker and consequently the risk for breaking up the formed fibre net decreases. The retention becomes higher and the formation better.

6.3. Speed-Former HHS from Valmet

When rebuilding of a paper machine with a Fourdrinier wire it may be both difficult and expensive to replace the forming section with a twin wire former in the vertical designs described before. Consequently, special designs suitable for rebuilding have been developed. The following formers equate technically with the type described before, Speed Former SC.

The picture describes a pure Fourdrinier machine rebuilt to a twin wire former.

Fig. 99. Example of rebuilding of a Four-drinier former to a roll-blade former.(15-099.tif)

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The rebuilding shown on the picture, the starting-point was instead a hybrid former from which the pre-dewatering part has been removed.

Fig. 100. Example of the rebuilding of a hybrid former to a roll-blade former.(15-100.tif)

6.4. OptiFormer with forming shoe from Valmet

The machine speed at the production of printing paper is today almost 2000 metres per minute and the formers are adjusted to meet the new demands.

Fig. 101. OptiFormer is an example of a modern roll-blade former. (15-101.tif)

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An importent difference between OptiFormer and the earlier described Speed-Former concept is that the distance between the forming roll and the couch roll is shorter. The shorter this distance is, the more stable the basic construction and the risk of vibrations will be lower.

Forming Section for SC and LWC Paper OptiFormer with OptiFlo

Fig. 102. OptiFormer. (15-102.tif)j^valmet

An OptiFormer with a forming shoe is used primarily in production of magazine paper.

The length of thewire wrapping thecouch roll is high,just like in Speed-Former SC.

Another difference between the concepts is that the forming shoe in OptiFormer is closer to the forming roll.

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6.5. Duoformer CFD from Voith

Duoformer CFD is another example of a modern roll-blade former. This former is distinguished from the earlier described types by the number of blades and how they are positioned.

Fig. 105. Principle sketch. Duoformer CFD. (15-105.tif)

The initial dewatering is done over the forming roll.

Fig. 106. Photo. The forming roll and parts of the blade section.(15-106.tif)

The fact that the radius of the forming roll is smaller than in the earlier described roll formers causes the pressure to be higher and the dewatering to be faster.

Fig. 107. Illustration. The forming roll and parts of the blade section.(15-107.tif)

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In the same way as in the hybrid former, Duoformer D, the wires then run between two rows with individually adjustable blades.

Thus, pulses influence the stock alternately from both sides.

Fig. 108. Illustration. Dewatering over the blade section part. (15-108.tif)

The frequent pulses give enough stretch and shear forces in the stock, in spite of the low strength in the individual pulses.

Fig. 109. Illustration. The path of the web through the blade zone. (15-109.tif)

The continued dewatering then takes place over the firm suction boxes and the final couch roll.

Fig. 110. Illustration. Dewatering over firm bars and over the couch roll. (15-110.tif)

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6.6. OptiFormer with loadable blades from Valmet

Stock containing high amounts of long fibres demands high turbulence to prevent the fibres from forming flocs.

In machines mainly producing fine paper, but also newsprint, the forming shoe may be replaced by loadable blades.

Fig. 111. OptiFormer with loadable blades.(15-111.tif)

The short dewateringtime over the formingroll and the decreaseddistance between theforming roll and theblade section reduces thetime when the stock isnot influenced by shearforces. The followingpressure pulses come

Fig. 112. (15-112.tif)

more frequently than inearlier designs. The concept allows the necessary turbulence in thestock to be kept up during the whole dewatering process and the risk ofthe fibres forming flocs is minimised.

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When producing fine paper it is desirable that the fibres are orientated as much as possible in all directions. However, the pressure pulses over the blades have a tendency to orientate the fibres in the machine direction.

A way to prevent the fibres from orientating too much in the machine direction is to create an extra high micro turbulence in the stock jet. One way to achieve this high turbulence is to use headboxes with special blades in the outflow nozzle.

Fig. 113. Headbox with special blades in the out flow nozzle is used in the Opti-Former concept at production of fine paper. (15-113.tif)

Formers should give a good formation without decreasing the retention. Besides, it is necessary to control how much the fibres orientate in the machine direction and how the fine material is distributed in the Z direction.

It is important to select the right strength of the pressure pulses during the whole dewatering course. If the pressure pulses are too weak the paper develops a bad formation. If the pressure pulses are too strong the retention is decreased. If they are too strong at the end of the formning process the formation is destroyed as well.

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WET PRESSING

1. Wet Pressing, Introduction .......................................................................

2

2. Counter-forces in a press nip ................................................................... 6

3. Wet pressing and paper properties ........................................................ 11

4. Single or double felted press nip ........................................................... 16

5. Press rolls and press felts ...................................................................... 21

6. Various ways to extend the press nip .................................................... 25

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1. Wet Pressing, Introduction

It is always cheaper to press the water out than to dry it out. Another advantage is that with less drying, an increased production on the paper machine will often turn out to be possible.

The design of the press section is a question of the type of rolls that are used, how they are placed and how the press felts run. The press result is also influenced by the nip pressure, the nip residence time, and by the press felts.

The following pages will mainly inform about the fibre properties, their influence on the conditions in the press nip and about the final paper properties.

When the paper web enters the presses it has a dry solids content of about 20%.

In the press section, the web is then pressed between one or two felts in a number of press nips and the dry solids content sometimes increases to more than 50%.

Fig. 1. Press section in a liner machine. (16-001.tif)

The function of a the press felt is to support the sheet and maximise the water removal.

Fig. 2. Double felted press nip.(16-002.tif)

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When the web is pressed together the water between the fibres and in the lumen is squeezed out.

Fig. 3. Illustration. Fibres in the wet web. (16-003.tif)

There is water in the fibre wall, too.

Fig. 4. Illustration. Enlarged cut of a fibre wall. (16-004.tif)

Particularly much water is to be found in a chemical fibre, beaten so that the fibre wall is split up.

The water in the fine pores is more difficult to squeeze out than the water between the fibres. Consequently, the water in the fibre wall disappears only partly when the web runs through the press section.

Fig. 5. Illustration. Enlarged cut of a swollen fibre wall.(16-005.tif)

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With rising temperature, the viscosity of the water drops. The higher the web temperature is, the easier the web is dewatered.

Fig. 6. The viscosity of water at different temperatures. (16-006.tif)

Sometimes the web is heated by steam boxes located ahead of the press section.

Fig. 7. Steam box, placed at the end of the wire section on a sack paper machine. (16-007.tif)

Normally, the steam boxes are placed in the press section. By placing the steam box over the suction box in a suction roll, the cooling, that otherwise takes place when cold air passes through the web, is prevented.

Fig. 8 and 9. Press section with steam box in a newsprint machine.

The water in the fibre wall is not only difficult to squeeze out. The water also makes the fibre soft and formable which increases the flow resistance when the water streams between the fibres and out of the web.

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Thus, what happens in the wet pressing process depends on the webproperties, but of course also on how the pressing is done.The press nip in a paper machine can be designed in many differentways.

The result of the pressing depends on the pressure in the press nip and on the nip length.

Fig. 10. Press nip. (16-010.tif)

The pressure in the nip is not the same all along through the nip length. The amount of water pressed out depends on the average nip pressure.

Fig. 11. The division of the nip pressure in a press nip.(16-011.tif)

If the average nip pressure is multiplied with the time it takes for the web to pass the press nip, the nip residence time, the so called press impulse is achieved. The larger the press impulse is, the more water is pressed out.

Fig. 12.The press impulse = P • t.(16-012.tif)

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The press impulse can never be so large that enough water can be pressed out in a single press nip. As a consequence a press section always has more than one press nip.

Fig. 13. Press section in a fine paper machine. (16-013.tif)

2. Counter-forces in a press nip

When the pressing is done in several nips, the sum of the press impulses in the single nips determines the total press impulse.

However, the web solids content after the presses does not depend on the press impulse alone. The way the water has to stream out of the wet paper web is also important.

If the web is pressed between a smooth roll and a felt it is called single-felted pressing.

Fig. 14. Illustration. Single-felted pressing. (16-014.tif)

The water squeezed out streams from the web and into the felt.

Fig. 15. Illustration. The flow direction of the water in a single-felted press nip.(16-015.tif)

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When pressing the water out of the wet paper web there are two counter forces to overcome.

The first force is the mechanical counter-force arising when the fibres as such are compressed.

Fig. 16. Illustration. Cross-section of fibres in a web.(16-016.tif)

The second counter-force is the force arising when the water is forced to stream out of the fibre network.

Fig. 17. Illustration. Fibres in a wet web pressed together.(16-017.tif)

That the flow meets a resistance creates an internal fluid pressure in the paper web. The higher the flow resistance is, the higher this pressure becomes; the hydraulic pressure increases.

Fig. 18. Illustration. A pressure increasing (P) in the wet web when it is pressed together.(16-018.tif)

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The mechanical counter-forces in the fibre network can be compared with the counter-force formed when a spring is pressed together. It is in fact often called the spring force.

Fig. 19. Illustration. The mechanical counter-force in the web can be compared with a compressed spring. (16-019.tif)

The hydraulic pressure corresponds in the same way to the counter-force in a shock absorber.

Fig. 20. Illustration. The hydraulic counter-force in the web can be compared with the counter-force created when pressing together a shock absorber. (16-020.tif)

Thus, the press force added when pressing together and driving out the water of the wet web is absorbed by two counter-forces:

- The mechanical spring force.- The hydraulic force originationg from the resistance when the

water is forced to stream through the web and into the felt.

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If the fibres are stiff and hard, the spring force is in most cases the dominating counter-force. Overcoming the spring force it holds true that a higher pressure in the nip gives a more compressed fibre net.

Fig. 21.Illustration. Cross-section of fibres in a web.(16-021.tif)

Thus, if the mechanical counter-force dominates, it is just to increase the press load in order to compress the web more and to increase the solids content in the web after the press nip.

If the fibres instead have more water in the fibre wall, making them soft and pliable, the flow channels will be increasingly narrow. The resistance, when the water streams through the fibre network, increases, and so does the hydraulic pressure.

Fig. 22. Illustration. Cross-section of soft and formable fibres in a web pressed together. (16-022.tif)

It is not only the properties of the fibres that determines which of the two counter-forces will be the greatest, the thickness of the web entering the press nip is also of importance.

Fig. 23. Illustration. Single-felted press nip. The web thickness is marked.(16-023.tif)

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The longer the distance the water has to flow through a fibre network, the higher the flow resistance will be; the hydraulic counter-force will increase. Consequently, the hydraulic pressure is always highest at the beginning of the flow channel and decreases as the flow approaches the outflow.

Fig. 24. Illustration. The pressure P is highest in the beginning of the flow channel and decreases as the flow approaches the outflow.(16-024.tif)

The flow resistance arising when the water streams out of the paper web does not only depend on the width and the length of the flow channels. The resistance also depends on how fast the water streams in the channels.

The velocity of the water streaming in the channels is connected to how quickly the paper web is compressed in the press nip or, in other words, if everything else is constant, how high the applied press load is.

If the press load gets too high, the hydraulic pressure in the web increases so much that the fibre network is damaged. The web is said to be crushed.

If the flow resistance limits how fast the water can be pressed out, it is not as simple as to increase the press load in order to increase the solids content after the press nip.

Fig. 25. Illustration of the idea of sheet crushing. (16-025.tif)

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Thus, how much water feasible to press out depends on the applied average nip pressure multiplied with the nip residence time; this is the press impulse.

In most cases, it is not feasible just to increase the press load without running the risk of crushing the web. Then, the other alternative has to be selected; increase the press time.

Fig. 26.(16-026.tif)

Fig. 26 and 27. Press nip curves illustrating the increase of the press impulse by the increase of the press time.

3. Wet pressing and paper properties

Which of the two counter-forces is dominating, the spring force or the force originating from the hydraulic pressure, determines what is happening in the press nip during the dewatering of a web.

The counter-forces do not only influence the web in the press nip. The properties of the finished paper will be influenced as well.

If the mechanical force dominates, the web will re-expand immediately after the press nip.

Fig. 28.(16-029.tif)

Fig. 29.(16-029.tif)

Fig. 30.(16-030.tif)

Fig. 28, 29 and 30. Illustrations showing what happens in a web with stiff fibres passing through a press nip.

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That the fibres take back their original form means that the conditions in the press nip only marginally influence achieved paper properties.

Fig. 31. Reeling up of finished paper. (16-031.tif)

Fig. 32. Microscope photo. TMP-fibres.(16-032.tif)

Mechanical fibres or unbeaten chemical fibres are examples of fibres giving the web the re-expanding properties.

Fig. 33.Microscope photo, unbeaten softwood sulphate fibres.(16-033.tif)

Well beaten chemical fibres, on the other hand, swell a lot and are therefore soft and formable.

Fig. 34. Microscope photo, well beaten chemical long fibres.(16-034.tif)

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If a paper web consisting of such chemical fibres is too quickly compressed, the flow resistance develops so high that a large part of the applied press load is balanced by the hydraulic pressure created in the fibre network.

Fig. 35. Illustration. The hydraulic pressure ”P” is strongly related to the velocity of the outstreaming water. (16-035.tif)

To avoid crushing the fibre net the press load must be low. However, a low pressure in the press nip means a low compression of the web and consequently an insufficient dewatering.

Fig. 36. Press nip curve.(16-036.tif)

A way to get around this, and still be able to increase the compression of the paper web, is to prolong the nip residence time. The velocity of the water flowing through the fibre net becomes lower and so the flow resistance. The hydraulic pressure decreases.

Fig. 37. Press nip curve.(16-037.tif)

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Because of the reduced hydraulic pressure the web will be more compressed. The area of direct contact between the fibres increases.

The fact that beating increases the swelling makes the fibre wall soft and pliable. The fibres remain pressed together even after the nip.

Fig. 38. Illustration. Soft strongly compressed fibres in web. (16-038.tif)

When such a web is dried, many hydrogen bonds can be formed between the fibres.

Fig. 39. Illustration. OH groups on the fibre surfaces bind to each other with hydrogen bonds.(16-039.tif)

Thus, compressing the wet paper web in a press nip is a way to get a paper with a high strength.

Fig. 40. Measuring of tensile strength. (16-040.tif)

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However, pressing together fibres in the web does not give only advantages. The high paper density gives a low bending stiffness.

Fig. 41. Measuring of the bending stiffness. (16-041.tif)

The optical paper properties are influenced as well. The low amount of free, non bound, fibre surfaces inside the paper sheet decreases the capacity to reflect the light. The paper develops a low opacity.

Fig. 42. Low capacity of reflecting light means that printing ink is seen on the paper’s reverse. (16-042.tif)

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4. Single or double felted press nip

The longer distance the water pressed out has to stream through the web, the stronger the hydraulic pressure in the web always becomes. Felts in the press nip are used to reduce the flow distance of the water that is pressed out.

The press felts are manufactured in two steps.

In the first step a basic fabric is produced, the base weave. In the second the batt fibre is laid on the base weave. The fibres are mechanically bound into the base weave by barbed felting needles.

Fig. 43. Press felt. Batt layer fastened by needling. (Nordiska filt) (16-043.tif)

The felt has a much more open structure than the web has.

Fig. 44. Illustration. Web pressed between a roll and a felt surface.(16-044.tif)

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The flow channels in the felt are coarser than in the web, thus the hydraulic pressure becomes less. From the web the water streams out and into the felt. Since, the main water flow is straight through the web, the distance becomes the shortest possible.

Fig. 45. Illustration. The water streams out from the web and into the felt. (16-045.tif)

A press nip without felts means that the water squeezed out must stream along the web.

Fig. 46. Illustration. Flow directions in an unfelted press nip.(16-046.tif)

Thus, in an unfelted press nip the water has to stream a long distance and the hydraulic pressure reaches a critical level even at a very low speed.

Fig. 47. Illustration. Web burst into pieces. Too high hydraulic pressure.(16-047.tif)

The fact that the applied press load is balanced by the sum of the mechanical force counteracting the compression, the spring force and the force depending on the hydraulic pressure does not apply only on the web as a whole. The same thing is true for each single point in the Z direction of the web.

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If the dewatering is done in a singled felted press nip the flow resistance will diminish as the water streams through the fibre network. The hydraulic pressure decreases.

Fig. 48. Illustration. The pressure P is high in the beginning of the flow channel and diminishes as the water stream approaches the channel mouths. (16-048.tif)

As the hydraulic pressure on the felt side is lower than on the roll side the compression resistance is lower closest to the felt. The fibres in the web are therefore more compressed on the felt side than on the roll side and the area of direct contact between the fibres is larger.

As a result, the fibres on the side of the web running against the felt bind stronger to each other than the fibres on the opposite side.

Fig. 49. Illustration. Fibres in a web more pressed together on the felt side than on the roll side.(16-049.tif)

The roll surface is smoother than the felt surface and the surface of paper running against the roll is therefore smoother than the opposite side running against the felt.

Thus, when dewatering in a single felted press nip the web will always get different properties on each side.

In most cases you prefer to have the two sides of the paper as equal as possible: the paper should be equal-sided.

There are different ways to avoid getting unequal-sided paper.

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Fig. 50.(16-050.tif)One way to avoid unequal sided paper

is to change between felt and roll pressing on the two sides.

Fig. 50 and 51. Illustrations. Change between felt and roll pressing in single felted press nips.

Another way is to press the web between two felts by using a double felted press nip.

Fig. 52. Double felted press nip.(16-052.tif)

In a double felted press nip the water will stream from the web into the two felts.

Fig. 53. The flow of the water in a double felted press nip. (16-053.tif)

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When the water streams from the middle of the sheet and outwards the pressure conditions will be equal in the two directions. The result will be a more symmetrically compressed web.

Fig. 54. Illustration. Fibres in a double felted press nip are equally pressed together on the two sides of the web. (16-054.tif)

However, the main reason to use a double felted press nip is not to achieve a more symmetric compression of the paper web. Instead this concept is used when large amounts of water are to be pressed out or when the web is difficult to dewater: In the first press nip at the production of e.g. liner and board.

Fig. 55. Press section in a board machine. (16-055.tif)

If the dewatering is done in two directions, the distance the water has to flow through the web is only half as long as when dewatering in only one direction; the flow resistance will be half as high.

The amount of water flowing through the fibre network and the felts is halved and this halves the flow resistance as well.

Fig. 56. Illustration. Water flow in a double felted press nip.(16-056.tif)

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A twice halved flow resistance makes a higher nip load possible without risk of crushing the sheet.

Fig. 57. Illustration. An increased press load is possible when the dewatering is done in two directions. (16-057.tif)

5. Press rolls and press felts

The conditions in the press nip are not only influenced by running one or two felts in the nip. The type of press rolls and felts used is of great importance, too.

If the amount of water streaming out of the paper web increases excessivly, the flow resistance in the felt prevents further water transport. The hydraulic pressure in the web reaches a level that will cause sheet crushing.

Fig. 58.(16-058.tif)

Fig. 059.(16-059.tif)

Fig. 58 and 59. Illustrations. The hydraulic pressure crushes the web; a break will happen.

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If the water runs a shorter distance through the felt in the press nip, the flow resistance decreases. The shortest way is always just across the felt.

A way to make this possible is to use a press roll having a surface with channels or holes big enough to provide room for the water, leaving the web.

Fig. 60. Illustration. The flow of the water in a single felted press nip with a blind drilled bottom roll.(16-060.tif)

After the press nip the water might return from the felt and stream back into the web, this is known as rewetting.

To minimise this rewetting it is important to separate felt and web directly after the press nip.

Fig. 61. Illustration. Rewetting of the web after the press nip. (16-061.tif)

The type of felt used is important. The felt surface is much coarser than the paper surface.

Fig. 62. Picture. Enlarged part of the felt surface. (16-062.tif)

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KEP 2001-09-13 22


In the press nip all the voids in the felt are filled with water.

Fig. 63. Illustration. Water filled felt in a press nip. (16-063.tif)

After the press nip, when the press felt is separated from the web a great deal of the water closest to the felt surface will follow the paper web.

Fig. 64. Illustration. Separation between web and press felt after the press nip.(16-064.tif)

Thus, using a felt with a surface as smooth as possible is important if it is desirable to produce a paper with a smooth surface but also to decrease the rewetting. However, there are also other factors to consider when selecting the press felt.

If the flow resistance in the web is low the nip pressure can be high and the nip residence time short.

In such a case the dewatering of the web proceeds rapidly and the flow velocity in the felt becomes so high that the water spurts out of the nip. Fig. 65. Illustration. A

rapid dewatering makes the water spurt out after the press nip.(16-065.tif)

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KEP 2001-11-19 23


When the water spurts out of the nip, the press felt needs not absorb all the squeezed water but can be thin and compact.

Fig. 66. Cross-section of a thin compact felt. (16-066.tif)

If the web is difficult to dewater the nip pressure must be lower and the nip residence time longer. The water streams slowly through the felt and remains in the felt after the nip.

Fig. 67. Illustration. Press nip with a long press time and a low nip pressure. (16-067.tif)

The felt in such a press nip must be able to carry all the water streaming out of the web.

To be able to do this the felt must have voids large enough even when the felt is compressed.

The base weave in the press felt must be heavier and coarser than in the felts mentioned earlier.

Fig. 68. Cross-section of a heavy and coarse press felt.(16-068.tif)

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KEP 2001-11-19 24


The water in the press felt is then sucked away when the felt passes suction boxes.

Fig. 69. Felt suction box. (16-069.tif)

Press felts must always be designed to the web and to the press roll types.

Advances in felt technology have been directed toward improved uniformity and reduced rewetting. The improved uniformity makes water removal more efficient and the sheet quality higher by causing less ”felt marking”.

6. Various ways to extend the press nip

The facts that the nip pressure must not be too high and the nip residence time must be long when the web is difficult to dewater are often called attention to. But what can be done to prolong the pressing time?

The nip residence time is often short; in most cases not more than a few milliseconds.

Fig. 70. The press impulse.(t = 1-2 m sec.) (16-070.tif)

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KEP 2001-09-13 25


Usually the rolls are covered with rubber or other ”elastomers” having varying elasticity. The softer the rubber is, the longer the nip, and residence time becomes.

Fig. 71. Single felted press nip with rubber covered rolls.(16-071.tif)

If there are two felts in the nip, the residence time is prolonged even more.

Fig. 72. Double felted press nip with rubber covered rolls.(16-072.tif)

Another way to prolong the nip residence time is to use rolls with very large diameters. However, such rolls are expensive to produce and the weight makes them difficult to handle.

During recent years a totally new type of press has been used, a so called shoe press.

Fig. 73. Shoe press. (16-073.tif)

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KEP 2001-09-13 26


The reason why the press is called shoe press is that one of the press rolls is replaced by a fixed shoe.

SYM-BELT S, SHOE UNIT ^*i^

z?Z^'*^fflmju rv^4^/ .!p\

^1Fig. 74. Exploded view. Shoe press.(16-074.tif)

With this technique the nip length can be increased up to about 25 cm.

Such a nip length makes it possible to increase the press impulse considerably without increasing the maximal nip pressure.

Fig. 75. Exploded view. Shoe press.(16-075.tif)

The graph, showing the nip pressure, becomes different. A quickly decreasing nip pressure at the end is another advantage with this roll. The rewetting will be reduced.

Fig. 76. Typical press nip curve in a shoe press. (16-076.tif)

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KEP 2001-09-13 27

Main reg C^ Glossary

The design of the press section in a paper machine can vary. In each machine there is an optimal design of the presses and the felts to get the best runability and paper quality.

Fig. 77. A modern press section in a sack paper machine. (16-077.tif)

What determines the design of a press section is how much water there is to squeeze out of the web and how fast it can be done.

However, there is one important thing to remember. How the water is pressed out always influences the fibres in the web and the final paper properties as well.

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KEP 2001-09-13 28


DRYING

1. Introduction..............................................................................................................- ^^S

2. The capability of the fibre wall to absorb water.......................................................3 \^^M3. Forces affecting external and internal fibre surfaces................................................9 \^^M4. Fibre shrinkage.......................................................................................................12 1

5. Shrinkage of the paper web....................................................................................14 \^^M6. Drying and fibre properties.....................................................................................15 \^^M7. Shrinkage and drying section conditions................................................................17 KM

8. Shrinkage and fibre orientation in the web.............................................................18 j

9. Shrinkage and internal sheet tension......................................................................19 ^^M

10. Drying and additives.............................................................................................22 MJ

11. Evaporation process..............................................................................................24 ^^B

12. Multicylinder machines........................................................................................30 ^S

13. Drying in a multicylinder machine.......................................................................33]

14. Final paper drying.................................................................................................38 ^^B


KEP 01-09-13 1


1. Introduction

Paper drying is an expensive method to remove water. The energy consumption is high and the investment cost for the drying section is considerable. As a result it is mostly the high investment cost for the drying section that limits the production on a paper machine.

In the wire section and later in the press section, large amounts of water are mechanically removed. The dry solids content of the web, when it enters the dryer section, is normally somewhere between 35 and 52%.

Fig. 1.(17-001.tif)

Fig. 2.(17-002.tif)

Fig. 1 and 2. Wire and press sections in a sack paper machine.

The dry solids content in the finished paper is in most cases between 90 and 96%.

Fig. 3. Measuring instrument for basis weight and moisture content. (17-003.tif)

Water, impossible to remove mechanically, must be thermally removed; evaporated.

Fig. 4. Drying cylinders in a multicylinder drying section.(17-004.tif)


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Even if the amount of water to be evaporated is less than 1% of the amount removed during the forming and the press operation, there is still a lot of water left to evaporate. Counted in tons it is about as much as the produced paper. Fig. 5. Dried sack paper.

(17-005.tif)

Drying a paper web means that the water leaves the paper machine’s drying section as vapour, going to the atmosphere.

Fig. 6. The vapour leaving the drying section. (17-006.tif)

2. The capability of the fibre wall to absorb water

Most of the water between the fibres in the web is removed in the previous press section. The same thing happens to the water on the outer fibre surfaces and in the lumen.

But there is water inside the fibre wall, too, and most of that water is still there.

Fig. 7. Illustration. Cross-section of two partly collapsed fibres.(17-007.tif)


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Most water is to be found in the fibre wall from a well beaten chemical fibre. The split up of the fibre wall, inner fibrillation, is enhancing the water penetration.


There is another factor facilitating water penetration into the fibre wall; the pores formed during the cooking process when the lignin, and to some extent also the hemicellulose, in the fibre wall are dissolved.

Fig. 9 and 10. Illustrations. Strongly enlarged cut of a fibre wall. In fig. 10 the pores in the dark parts, formed when the lignin is dissolved, are seen.

The cellulose in the fibre wall is built up of a large number of cellulose molecules binding to each other with hydrogen bonds.

Fig. 11 and 12. Illustrations. Enlarged cut of the fibre wall. It can be seen how the cellulose molecules bind to each other by hydrogen bonds.


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A number of cellulose chains form a micro-fibril.

Fig. 13. Illustration. A number of cellulose molecules has formed a micro-fibril.(17-013.tif)

A number of micro-fibrils joining form fibrils.

Normally the distance between the cellulose molecules is so small that a water drop can not penetrate between them.

Fig. 14. Illustration. Cellulose fibrils: orientated and closely packed. (17-014.tif)

However, in some zones the cellulose molecules are not so strictly arranged.

Fig. 15. Illustration. A part in the fibre wall with disorientated cellulose molecules.(17-015.tif)


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In disordered areas the distance between the cellulose chains is larger than the size of a water molecule, and water can penetrate.

Fig. 16. Illustration. Water has penetrated between the cellulose chains. (17-016.tif)

When the water penetrates the cellulose chains, the hydrogen bonds are partly split up. The distance between the chains increases; The micro fibrill swells.

0---H H--- O

H 0 O---H H— o

n H

H O 0- H H— 0

HFig. 17. Water molecules break up the hydrogen bonds. (17-017.tif)

The hemicellulose in the fibre wall is located between the cellulose fibrils; partly in thin pure layers, partly in mixed layers of hemicellulose and lignin.

Fig. 18. Illustration. Enlarged cut of the fibre wall showing the location of the hemicellulose.(17-018.tif)

The hemicellulose consists of branched molecule chains.

Fig. 19. Illustration. Branched hemicellulose chain. (17-019.tif)


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The molecule chains in the hemicellulose are not as strongly orientated in the fibre wall as the cellulose chains are. This, together with the fact that the chains are branched, makes the hemi-cellulose chains less tightly packed than the cellulose chains. Fig. 20. Illustration. Loosely

packed hemicellulose molecules.(17-020.tif)

Water can penetrate between the molecule chains and the hemicellulose layer will swell greatly.

Fig. 21. Illustration. Water has penetrated between the hemi-cellulose chains. (17-021.tif)

The lignin in the fibre wall is naturally hydrophobic. Even if that property can be modified during the pulp production process, the lignin will decrease the amount of water that may be absorbed by the fibre wall. The swelling will be low.

Fig. 22. Illustration. The location of the lignin in a fibre wall. (17-022.tif)


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Water localised in the fibre wall is always difficult to remove. One reason is the very narrow pores which makes it difficult for the water to leave the fibre wall.


But it is not only by reason of physics the water is difficult to remove. There is also a chemical reason.

The OH groups on the cellulose and the hemicellulose chains attract adjacent water molecules. The closer to the OH groups the water molecules are located, the greater the attractive forces become. As a consequence, the last water remaining in the fibre wall is especially difficult to remove.

Fig. 24. Illustration. Water molecules attracted by the OH groups on the fibre surface.(17-024.tif)

Not only OH groups attract water molecules. If there are also charged chemical groups, e. g. carboxyl acid groups, the attraction force becomes so strong that some of the water molecules even become impossible to remove.

Fig. 25. Illustration. The carboxyl acid groups on the fibre surface are marked. (17-025.tif)


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The water molecules are not only attracted by the internal chemical groups, localised on the lamellae inside the fibre wall. The same charged groups can be found on the outer side of the fibre wall. These external groups attract water molecules as well.

Fig. 26. Illustration. Enlarged cut of a fibre surface with many OH groups. (17-026.tif)

3. Forces affecting external and internal fibre surfaces

The fact that the water disappears is not the only thing happening to the web in the drying section. The fibres as such will be influenced. When this happens the fibres get other properties which affect the properties of the finished paper.

When the web enters the drying section, forces of the surface tension, keep the fibres together.

Fig. 27. Illustration. Two fibres kept together by the common water layer. (17-027.tif)


KEP 01-09-13 9


When the water in the lumen disappears the same forces will affect the inside of the fibre wall, too. The attraction makes the lumen disappear. Fibres will more and more look like bands. The fibres collapse. The close contact between the fibres in a web increases. Fig. 28. Illustration. A fibre is

drawn together when the water in the lumen disappears. (17-028.tif)

How close the fibre surfaces come depends on how formable the fibres are.

The more water the fibre wall absorbs, the softer and the more pliable it will be. The area of close contact increases.

Well beaten chemical fibres are examples of fibres that are particularly formable.

Fig. 29. Illustration. Collapsed fibres with large area of close contact. (17-029.tif)

In liquid water there are forces attracting all molecules to each other; hydrogen bonds. During the drying process it is necessary to add energy, enough to overcome these attractions.

When this happens, the kinetic energy of the water molecules is high enough to enable them to move freely. Liquid water is transformed to a gas; vapour.

Fig. 30.1. Illustration. Attraction forces between the water molecules. (17-30-1.tif)


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The closer to the fibre surfaces the water molecules are located, the greater the attraction between the molecules.

The water molecules are attracted to the OH groups on the fibre surfaces with a force greater than the one prevalent between the water molecules, themselves. The water molecules directly attracted to the OH groups are said to be bound; bound water.

Fig. 30.2. Illustration of the concept ”bound water”. The magnitude of the attraction between the OH groups on the fibre surface and the water molecules is higher than the attraction between the water molecules. (17-30-2.tif)

When the bound water gradually disappears hydrogen bonds are formed, direct between the OH groups on adjoining fibre surfaces.

It is only when such direct bonds are formed that the hydrogen bonds reach their maximal strength.

Fig. 31. Illustration. OH-groups binding direct to each other.(17-031.tif)

The more the paper web dries, the more hydrogen bonds can be formed and the stronger the paper web becomes.

Fig. 32. Paper web in a multi-cylinder dryer. (17-032.tif)

Naturally, there is a direct connection between the amount of hydrogen bonds that can be formed and the number of OH groups on the fibre surface.


Mam reg Gallery

The number of OH groups on the outer fibre surface depends on the type of chemical pulp that is used. Most OH-groups are to be found on a chemical fibre beaten to a high degree of outer fibrillation.

Fig. 33. Illustration. Beaten chemical fibre with many OH groups on the surface.(17-033.tif)

On a mechanical fibre partly covered with lignin the OH groups are considerably fewer.

Fig. 34. Illustration. Mechanical fibre with few OH groups on the surface.(17-034.tif)

4. Fibre shrinkage

When the water on the fibre surfaces is removed the water in the pores inside the fibre wall starts disappearing. When this happens the fibres begin to shrink crosswise.

Fig. 35 and 36. Illustrations showing that fibre walls shrink crosswise when the water inside the wall dries away.


KEP 01-11-19 12

Main reg ^ Glossary Gallery

The more water there is in the fibre wall, the more the fibre will always shrink. As a consequence of this:

• Well beaten fibres always shrink more than unbeaten ones.

Fig. 37 and 38. Microscope photos. Beaten and unbeaten chemical softwood fibres. (STFI)

• Mechanical pulps always shrink less than well beaten chemical pulps.

Fig. 39. Microscope photo. Fibres in a TMP pulp. (STFI)(17-039.tif)


am reg Glossary Gallery

5. Shrinkage of the paper web

The fibres shrink almost only crosswise, but when the paper web is dried the shrinkage will also affect the length of the fibres.

When the fibres shrink crosswise, in the cross points they will compress the fibres lengthwise. So called micro compressions are achieved.

Fig. 40. Microscope photo showing how an overlying fibre must be compressed lengthwise when the underlying fibre shrinks crosswise. (STFI) (17-040.tif)

As a result the whole fibre network diminish; the web will shrink.

Fig. 41. Illustration. Fibres in a paper sheet. (17-041.tif)

On a paper machine, the shrinkage could be seen as a narrowing of the web.

Fig. 42. Paper web in a multi-cylinder dryer. (17-042.tif)


KEP 01-09-13 14


The shrinkage is especially high at 65 to 80% dryness, the shrinkage zone. The shrinkage is due to the fact that at this point the water in the fibre walls starts to evaporate.

Fig. 43. The paper shrinkage at dryness increase.(17-043.tif)

6. Drying and fibre properties

When the water disappears the lamellae in the fibre wall draw closer to each other. The fibre wall develops more compact.

Fig. 44. Illustration. Enlarged cut of the fibre wall.(17-044.tif)

When the lamellae come in contact with each other hydrogen bonds are, as said before, formed also between these inner fibre surfaces.

Fig. 45. Illustration. OH groups on the fibre lamellae bind directly to each other by hydrogen bonds. (17-045.tif)


KEP 01-09-13 15


The fibre wall develops more compact and the fibre gets harder and stiffer.

Fig. 46. Illustration. Beaten chemical fibre. (17-046.tif)

The changes taking place when the fibres dry make it more difficult for water to penetrate into the fibre wall if the paper is slushed again.

Fig. 47. Broke rolls before slushing. (17-047.tif)

Actually, that a fibre swells less if it formerly has been dried means that the fibres in a dried pulp will swell less than the fibres in a never dried pulp.

Fig. 48. Dried paper pulp.(17-048.tif)

Recycled fibres can be dried several times. The more times, the less the fibres will swell when they are slushed.

Fig. 49. Corrugated board bales. (17-049.tif)


KEP 01-09-13 16

Main reg ^

The less a fibre can swell, the less it can shrink, too. Thus, if it is desirable to get a paper changing its dimensions as little as possible when the humidity in the surrounding atmosphere is changed, fibres with a limited capacity to absorb water should be used. Consequently, mechanical fibres with high content of lignin, or fibres that have been dried are selected. The more times a fibre has been dried, the higher its dimensional stability will be.

7. Shrinkage and drying section conditions

When the fibres shrink during drying, the whole web will shrink. But how much it will shrink is strongly connected to the conditions in the drying section.

If the shrinkage of the paper web is prevented by supporting dryer fabrics and a high web tension, the fibres will be straightened when the paper dries.

When loading the paper sheet all the fibres are sharing the load evenly. The paper develops strong and stiff characteristics.

Fig. 50. Illustration. Straightened fibres in a paper sheet. (17-050.tif)

The opposite way of drying is to let it shrink freely within that part of the drying section where the paper web shrinks the most. In a paper that shrinks freely the fibres will not be straightened. Such a paper will elongate greatly when it is loaded. The paper becomes tough. Fig. 51. Illustration.

Unstretched fibres in a paper allowed to shrink freely.(17-051.tif)


KEP 01-11-19 17


The tensile strength and the elongation of a paper sheet is to a great extent connected to how much the sheet is allowed to shrink freely during the drying. Low shrinkage means a paper with a high tensile strength. High shrinkage on the other hand means a paper with a high elongation but a lower tensile strength.

8. Shrinkage and fibre orientation in the web

The fibre orientation in the web strongly influences the paper web during the drying.

If most fibres are orientated in the machine direction the paper will shrink mainly in the cross direction.

Fig. 52. Illustration. Paper with most of the fibres orientated in the machine direction. (17-052.tif)

The drying tension is not equal all across the web. It is always lower close to the web edges.

That the tension is lower at the edges makes the paper edges shrink more than the rest of the web. The higher shrinkage causes the basis weight near the edges to become higher. Fig. 53. Paper web in

a multicylinder dryer.(17-053.tif)


KEP 01-09-13 18


To compensate for the increased basis weight the head box nozzle opening has to be smaller at the edges.

But if the opening is unequal cross streams are generated. Such cross streams always influence the fibre orientation.

_^>1__Fig. 54. Headbox with locally adjustable nozzle openings.(17-054.tif)Different fibre orientation at the edges of the paper web is rather

usual. The different fibre orientation is one reason why just the edge rolls may cause problems at the use of the paper.

At the modern headboxes with local stock dilution, the nozzle opening is constant across the machine and therefore no cross streams are generated. To the regulate the grammage local stock concentration is instead changed.

Fig. 55. Headbox with a fixed nozzle and locally variable stock concentration.(17-055.tif)

9. Shrinkage and internal sheet tension

Drying paper is a complicated and delicate operation. The dryness must be equal both in the machine direction and in the cross direction.

It is important to dry uniformly in the Z direction, too. The two web sides must normally have the same dryness and the centre should not differ too much.


KEP 01-09-13 19


If a paper with a high basis weight is produced, the sheet may have different dryness on the two sides.

Fig. 56. Board. Microscope photo. (17-056.tif)

When the difference in dryness is later levelled out such a paper will shrink or expand differently on the two sides. The result will be a curled paper.

Fig. 57. Edge cut of a sheet of paper. Illustration of curl.(17-057.tif)

Thus, if a paper curls or not does not only depend on the fact that the fibres are unequally orientated on the paper’s two sides. Different dryness along the paper’s Z direction may cause the same problem.

But not only the dryness in the Z direction of the finished paper creates curl. That the paper dries at the same rate on the two sides is of importance, too.


KEP 01-09-13 20


If the web dries at the same rate on the two sides, the fibres in the sheet shrink uniformly and the paper lies flat.

Fig. 58. Illustration. Edge cut of a sheet of paper.(17-058.tif)

On the other hand, if one side dries faster than the other, that side also shrinks earlier.

Fig. 59. Illustration showing what happens when the bottom side of the paper shrinks earlier than the top side.(17-059.tif)

When the other side then shrinks the fibres on the dry side are not as soft as when they were wet. The shrinkage resistance in the dry part has increased. As a result the tension will be different on the sheet’s two sides. This is another reason for developing curl.

Fig. 60. Illustration. The picture shows what happens when the upper side of the paper begins to shrink only when the fibres in the under side have become stiff and incompressible.(17-060.tif)


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10. Drying and additives

How the paper is dried does not influence only the fibres in the paper, but some of the additives, too. Particularly important is the drying temperature when the paper is sized.

The heat during the drying makes the size particle soften, spread and cover the surface.

Fig. 61.(17-061.tif)

Fig. 62.(17-062.tif)

Fig 61 and 62. Illustrations. Size particles spread and cover the surface.

A rosin size particle becomes fully hydrophobic only when the resin acid has reacted with an aluminium atom giving an aluminium resinate. Then, when the temperature is high enough, the resinate molecules start orientating. The molecules turn their most hydrophobic side away from the fibre surface. The hydrophobicity will increase to the maximal level.

Fig. 63. Aluminium resinate molecules after the heat treatment in the dryer. (17-063.tif)


KEP 01-09-13 22

Main reg Gallery

Even the AKD and ASA sizes need a certain temperature before being active.

However, the two sizes, as here AKD, react chemically with the OH groups of the cellulose molecules.

Fig. 64 and 65. Illustrations. AKD molecules reacting with the cellulose in the fibre wall.

The necessary reactions can not take place until in the drying section.

Fig. 66. Sack paper machine.(17-066.tif)

High temperature in the drying section is a must not only for sizing chemicals. If for example a wet strength agent is added in the stock the necessary reaction can not even now take place until the drying section.


KEP 01-11-19 23


11. Evaporation process

The previous part of this lesson was about how the drying process affects the properties of the finished paper. Before entering the subject, what will happen when a paper dries and what determines how fast it will dry, we will study the evaporation process from a theoretical point of view. A water drop serves as a model.

All molecules in water are attracting each other.

The water molecules, like molecules in all substances, are constantly moving. The higher the temperature is, the faster they move.

Fig. 67. Illustration. Attraction forces between water molecules.(17-067.tif)

When the temperature in a water drop gets high enough the kinetic energy of some water molecules will be stronger than the forces holding the molecules together, and free water molecules will leave the drop as vapour.

Fig. 68 and 69. Illustrations. Free water molecules leave the drop as vapour.


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The energy needed to release the water molecules from the drop equals the added energy. As a consequence, the drop temperature will remain constant.

Fig. 70. Illustration. Free water molecules leaving a water drop, as vapour. (17-070.tif)

When free water molecules leave the drop the water is said to evaporate. The released water molecules have formed vapour. The energy necessary to form vapour is called the vaporisation heat. The amount of energy added determines how fast the water evaporates. The temperature in the drop always remains constant.

It is not only the amount of added energy that determines how fast the water evaporates. The conditions in the air receiving the vapour is also of importance.

The air receiving the vapour molecules has a determined pressure, the air pressure ”P”.

Fig. 71. Illustration. The air above the water surface has a determined pressure, P.(17-071.tif)


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Dry air is a gas containing molecules, mainly nitrogen and oxygen. Molecules in motion will perpetually bounce against each other. Because of the continuous movement of the molecules, they exert a certain amount of pressure

Fig. 72. Illustration. (Blue) nitrogen and (red) oxygen molecules in air. (17-072.tif)

Each molecule in the air contributes to the total air pressure. The more molecules of a certain kind, the higher the contribution from just that sort of molecule.

8

Fig. 73. Illustration. The nitrogen and oxygen molecules contribute to the total air pressure. (17-073.tif)

P air = nitrogen + P oxygen

Vapour is also a gas and

■

Pbutes to the total pressure.

«fc

wet air =

P N2+ P O2 + P H20

Fig. 74. Illustration. The vapour-molecules part of the total pressure, is called the vapour-pressure. (17-074.tif)


KEP 01-11-02 26


With more vapour molecules in the air, a larger part of the total pressure originates from the these molecules. The partial pressure for the vapour, the vapour pressure, increases.

Fig. 75. Illustration. When the number of vapour molecules increases the vapour pressure increases, too. (17-075.tif)

The lower the vapour pressure is from the beginning, the more vapour molecules the surrounding air can receive.

Fig. 76 and 77. Illustrations. Vapour molecules leave a water surface.

When enough water molecules have left the drop the vapour pressure in the air has finally become so high that the evaporation stops. The air is saturated with vapour.

Fig. 78. Illustration. The maximal amount of vapour molecules which can exist above a water surface.(17-078.tif)


KEP 01-11-02 27


Now, the added energy is instead used to increase the temperature in the water drop.

Fig. 79. When the air is saturated with vapour the temperature in the water drop increases. (17-079.tif)

The evaporation stops because the air can not receive more vapour molecules. The air is saturated with vapour. How much vapour the air can receive before it is saturated depends on the air temperature.

The higher the air temperature is, the quicker the vapour molecules move and the closer they can come to each other without being caught in each other’s field of force. So, warm air can hold more vapour than cold air.

M~H

A A

AT AFig. 80. Illustration. Vapour molecules. (17-080.tif)


KEP 01-11-02 28


If the air temperature falls the kinetic energy decreases and the vapour molecules rebind to each other. The vapour has formed liquid water again; condensed.

Fig. 81. Illustration. The temperature falls. The vapour molecules bind together and form a water drop.(17-081.tif)

When the vapour molecules are caught up by the hydrogen bonds their kinetic energy decreases and that difference in kinetic energy is transformed to heat. The temperature in the water drop increases.

Fig. 82. Illustration. The water drop temperature increases when the new vapour molecules are caught up.(17-082.tif)

Thus, how many vapour molecules there are in the air surrounding a water drop depends on the temperature. When the air contains the maximal numbers of molecules at a certain temperature the air is said to be saturated. When the number becomes too high the vapour molecules form liquid water again. The vapour condenses and the heat being generated is called condensation heat.


KEP 01-11-02 29


12. Multicylinder machines

In the manual paper mill the paper was hanged up and dried in the air. How fast the paper dried depended on the air temperature and on how well the drying room was ventilated.

Fig. 83. Manual manufacture of paper. (17-083.tif)

For a paper machine the same rules apply, but here the drying must be done very fast. To succeed, large amounts of heat must be added and the air surrounding the paper must be exchanged very fast.

How the drying is done can be seen by following the course on the most common type of drying section, the multicylinder type.

Heat is added alternately to the web’s two sides and the web temperature starts to increase.

Fig. 84. Multicylinder dryer. (17-084.tif)


KEP 01-11-02 30


On the first cylinder there is almost no evaporation of water. Evaporation starts when the web temperature has increased.

The temperature on the first cylinders is low. Otherwise there is a risk to burn the web.

(The web on the previous picture runs outside the wire in the bottom cylinder row. This configuration makes it possible to get the dryer fabric to support the web even between the cylinders. The risk for web flutter in the open draw is eliminated).

Fig. 85. The first drying cylinder in a multicylinder drying section. (17-085.tif)

On later dryers the cylinder surface can be higher than 100°C.

The web temperature always stays below 100°C. The water is not boiled away.

Fig. 86. Paper web on a multi-cylinder dryer. (17-086.tif)

What determines how much water that can be evaporated is the difference between the vapour pressure in the paper web and in the surrounding air. A way to increase that difference is to increase the web temperature so that the vapour pressure in the web will increase, too.

P H20Fig. 87. Paper web. Marking of the vapour pressure in the paper closest to the evaporation surface and in the surrounding air. (17-087.tif)


KEP 01-11-19 31


Another way is to keep the vapour pressure in the surrounding air as low as possible.

Fig. 88. Paper web. Marking of the vapour pressure in the paper closest to the evaporation surface and in the surrounding air. (17-088.tif)

If the vapour pressure in the surrounding air is too high, the air becomes saturated and the evaporation stops. Then, the added heat only contributes to increase the web temperature.

To avoid saturation the moist air must be exhausted and new, drier and preferably warmer, air must be added.

By special blow boxes the hot, dry air is added at the right place and with the correct flow.

The wires between the dryers also help to ventilate the dryer pockets.

Fig. 89. Cylinder dryer. The blow box is marked. (17-089.tif)


KEP 01-11-02 32


To be able to exchange the moist air a lot of hot, dry air must be added.

The weight of the air leaving the drying section is in fact in most cases ten times higher than the paper production.

Fig. 90. (17-090.tif)

It is important to arrange the ventilation in the very best way. If the ventilation is not high enough the drying capacity falls and the cost of drying the paper increases.

If the ventilation is uneven the paper dryness becomes uneven, too. The paper quality gets worse.

13. Drying in a multicylinder machine

To get the contact as good as possible the dryer fabrics press the paper against the cylinders. It is tempting to believe that heat is transmitted between the cylinder surface and the paper web by conduction only.

However, the high heat transfer can not be explained only by heat conduction. There must be a strong element of convection, too.


KEP 01-11-02 33


The paper web is a complicated element where both water and vapour can move in the paper’s Z direction, thus straight through the web.

Fig. 91. Cross-section of the paper web. (17-091.tif)

When the paper surface next to the drying cylinder has reached a certain temperature the emerging vapour will penetrate the water filled channels between the fibres.

Fig. 93.(17-093.tif)

Fig. 92 and 93. Illustrations. Cross-section of the paper web. Vapour is formed closest to the cylinder surface.

As the vapour penetrates the inner part of the web it cools down and condenses.

Fig. 94. Illustration. Cross-section of the outer part of a paper web. The vapour is cooled down when it penetrates deeper into the web.(17-094.tif)


KEP 01-11-02 34


When the vapour condenses, the vaporization heat is set free. This heat is now taken up by the paper web. The temperature in the web increases.

Fig. 95. Illustration. Cross-section of the outer part of a paper web. The temperature T increases when the vapour condenses. (17-095.tif)

In the open draw between the drying cylinders the vapour on the web surface and in the adjacent internal channels will leave the web.

(17-097.tif)Fig. 96, 97 and 98. Illustrations. Cross-sections of the outer part of a paper web. In the draw between the cylinders the vapour leaves the paper web.


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Capillary suction brings the condensate back towards the warm paper surface again.

Fig. 99. Illustration. Paper web in the free draw between drying cylinders. (17-099.tif)

As long as the vapour pressure at the web surface is higher than in the surrounding air, the more water will evaporate and new water will stream in the empty channels towards the surface of the paper web.


The heat needed to evaporate the water is taken from the web itself. As a result the web temperature will decrease.


When the paper web enters the following drying cylinder the process will repeat itself.


KEP 01-11-02 36


The water between the fibres vaporises. This time the vapour will penetrate a bit longer into the web before it cools down and condenses again.

Fig. 102 and 103. Illustrations. Paper web and drying cylinder.

In the following draw the free vapour leaves the web and the condensate flows back towards the warm paper surface.


This is repeated as long as there is water left in the channels to be vaporised.

Fig. 105. Multicylinder dryers.(17-105.tif)


KEP 01-11-02 37

Mam reg&j Glossary Gallery

The web temperature falls slightly in the open draws between the cylinders. However, as long as the water manages to flow against the warm cylinder surface in the same rate as it evaporates, the amount of heat needed to evaporate the water will be as much as the amount generated when the vapour condenses. The web temperature will remain constant.

14. Final paper drying

The lower the amount of water remaining in the web, the more difficult it will be to transfer heat from the cylinders and into the evaporation zones.

The declining heat transfer is one of the reasons why the evaporation rate always decreases in the end, but there are other reasons, too.

When the water in the channels between the fibres is removed the vapour pressure inside the fibre walls will be higher than outside the walls.

(17-107.tif)

Fig. 106 and 107. Illustrations. Cross-section of the paper web with enlarged cut.



It is the difference in vapour pressure that makes the water in the fibre walls evaporate.

P H20 in the fibre wall > P H20 in the channels between the fibres.

Fig. 108. Illustration. Fibre walls with intermediate channel.(17-108.tif)

The pores in the fibre wall itself are much smaller than the channels between the fibres.

Fig. 109. Illustration. Lamellae in the fibre wall. (17-109.tif)

The fact that the pores are that narrow makes the water stream more slowly to the evaporation zones; the evaporation slows down.

Fig. 110. Illustration. Vapour penetration through the fibre wall.(17-110.tif)


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The amount of heat necessary to evaporate the water now becomes lower than the amount transferred from the cylinders. The web temperature increases.

Fig. 111. Paper web on a multi-cylinder dryer. (17-111.tif)

The last water molecules bind chemically on the cellulose and hemicellulose. These strong bonds are another reason why the evaporation proceeds so slowly at the end of a drying section.

Fig. 112. Illustration. Water molecules attracted by OH and COO- groups on the fibre surface. (17-112.tif)

To remove the chemically bound water it is not enough with the vaporisation heat. What is called desorption heat is required, too. To remove the bound water is not only a slow process, it is also expensive.

All water can not be removed from a paper web. Some water molecules bind so strongly to cellulose and hemicellulose that they are impossible to release.

A filler in the paper does not bind the water molecules in the same way. Consequently, the more filler there is in a paper, the less energy is needed to dry the paper.


KEP 01-11-02 40


When producing thick paper, i.e. board, it is very important that the web does not dry too fast.

Fig. 113. Microscope photo. (STFI). Sheet of board. (17-113.tif)

If the evaporation becomes too strong the fibre surfaces next to the paper surfaces can dry and start binding to each other and close the pores, preventing the vapour from leaving the wet web.

As a result the sheet centre, in the Z-direction, becomes too moist and the layers closest to the surface become too dry.

Fig. 114. Illustration. Apaper web that has driedtoo quickly. (17-114.tif)

The dryness in the finished paper must not only be the same straight through the sheet. The dryness must also be correct, which means that the paper will absorb or desorb as little moisture as possible.

How high the dryness must be depends on how easily the moisture can penetrate into the fibre wall. The more the outer fibre walls are broken up and the more the inner fibre wall is split up, the easier the moisture penetrates.

The amount of water absorbed in the fibre wall is a question of the chemical composition of the fibre wall. If the hemicellulose content in the fibre wall is high the water absorption becomes high, too. The fibre wall softens. In the finished paper it is above all the toughness that increases and the stiffness that decreases. The paper becomes less brittle.

If the air is very moist the bonds between the fibres are influenced, too. The hydrogen bonds break up and the paper sheet weakens.


KEP 01-11-20 41


The filler does not bind water to the same extent as the fibres do. The more filler there is in the paper, the higher the dryness in the paper must be to prevent the water content in the fibre wall from being too high.


KEP 01-11-02 42

<^

Main reg >allery


FORMING 6, MULTI-LAYER

1. Forming of layers - separately or simultaneously ..........................................................

2

2. Layer adhesion ...............................................................................................................

3

3. Separate forming ...........................................................................................................

6

3.1. Conventional cylinder (vat) machines ................................................................. 63.2. Modified cylinder machines ................................................................................ 83.3. Fourdrinier machine with a separate top wire; top former .................................. 93.4. Fourdrinier machine with a separate former of a Fourdrinier type ................... 123.5. Fourdrinier machine with separate top formers of the roll former type ............ 163.6. Modified Fourdrinier machine with separate top formers of the roll- blade

former type ........................................................................................................ 17

3.7. Headboxes ......................................................................................................... 19

4. Simultaneous multi-layer forming ............................................................................... 21

Headbox with stratified jet .............................................................................................. 21

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1. Forming of layers - separately or simultaneously

Board is an example of a product built up from several layers. At the production of board, normally, separate layers are formed and pressed against each other; they are couched together.

One reason of forming separate layers at board production is the difficulty to form a single layer with an enough high basis weight.

The fact that each layer usually has its own furnish is another reason to form separate layers.

Fig. 1. Board machine with separate forming units.(FS-001.tif)

Giving the paper the desired properties at the lowest cost has led to a development of producing paper with several layers at low basis weights as well. The applied technique differs from the conventional ones used in a board machine and the paper produced is sometimes defined as a multilayer paper.

Instead of forming each stock layer separately, a multi-channel headbox is used. Each stock jet has its specified furnish and the forming is done in a single or more often a twin wire former.

Fig. 2. Stratified stock jet from a sectioned headbox, known as a multi-layer box. (Valmet) (FS-002.tif)

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2. Layer adhesion

Developing a sheet in several layers gives a good possibility to optimise the sheet properties but causes some problems, too. A multi-layer sheet is more complicated to produce. Different shrinkage properties make it extra difficult to develop a plane sheet, which remains plane even when the moisture in the surrounding air changes.

An important board property, being direct related to the forming process, is how strongly the different layers will bind to each other .

How strongly the layers will bind to each other in the finished board is called layer adhesion.

Fig. 3. Laboratory method for measuring of the strength in the Z direction of a sheet. (FS-003.tif)

It is the number of hydrogen bonds formed between adjoining fibre surfaces which determines how strongly the fibres will bind to each other.

Fig. 4. Microscope picture. Corner cut of a board sheet. (FS-004.tif)

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The number of hydrogen bonds depends on the number of OH groups available on the fibre surfaces and on the size of the direct contact area between the adjoining fibre surfaces.

Fig. 5. Illustration. Hydrogen bonds between two adjoining fibre surfaces.(FS-005.tif)

In a multi-layer construction, the direct contact area between the fibre surfaces in the adjoining layers never becomes as large as within the layers as such. As a result the hydrogen bonds, formed in the border zone, becomes fewer. In other words, the bonds between two separate formed layers do not ever become as strong as the bonds within the layers. Fig. 6. Illustration. Board produced

from three separate stock layers.(FS-006.tif)

One thing that determines the strength of the bonds between the layers is the dry substance content of the different layers when meeting each other.The lower the dry substance content is, the better the chance of getting a water film covering the whole fibre surfaces. The importance of the water film depends on the surface tension in the water. It is the surface tension that attracts the fibre surfaces to each other in the wet fibre network.

Fig. 7. Illustration. Two fibres with a joint water layer attract each other.(FS-007.tif)

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When the water on the fibre surfaces gradually evaporates in the dryer section, the forces attracting the remaining water molecules increase. This increased attraction between the water molecules makes the fibre to attract each other more and more strongly. When most water molecules have left the fibre surfaces, the adjoning fibre surfaces are so close that hydrogen bonds can be formed directly between the OH groups.

Figure set. Illustration. Drying progression. Water molecules broken off and OH groups bind directly to each other.

The softness and formability of the fibre wall are of course important. The more the fibre wall swells, the more it will soften and the closer the fibre surfaces can come to each other.

Fig. 10. Illustration. Soft fibres with a large direct contact surface. (FS-010.tif)

Well beaten chemical fibres therefore always bind more strongly to each other than mechanical fibres do.

Fig. 11. Well beaten chemical fibres.(FS-011.tif)

A more pliable fibre does not only mean that the fibres bind more strongly to each other within the layers. The direct contact surface between the adjoining layers gets larger, too. More hydrogen bonds can be formed between the layers.

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A soft and formable fibre develops a high layer adhesion. However, it also makes the sheet easier to compress. The sheet thickness decreases and the bending stiffness becomes lower.

Another way to strengthen the forces keeping the layers together is to have as high an amount as possible of fine material in the interface.

The fine material fills up the cavities between the fibre layers and thus helps to increase the direct contact surface between the layers. The larger contact surfaces, the more hydrogen bonds can be created.

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Fig. 13. Illustration. Fine material fills out and creates bridges between the fibre layers. (FS-013.tif)

3. Separate forming

3.1. Conventional cylinder (vat) machines

The cylinder machine, is a former used since a long time to produce board.

Fig. 14. Old cylinder mould and vat.(FS-014.tif)

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The stock comes into a trough and is dewatered through a wire covered drum rotating in the trough.

The fibre layer formed on the drum can be rather thick, up to 100 grams per m2.

Over the drum a felt is running.

Fig. 15. Principle sketch. Cylinder mould and vat. (FS-015.tif)

The strong capillary forces between the felt and the fibre net make the fibre net follow the felt rather than the wire. The layer is couched off against the felt.

Fig. 16. Illustration. Couching of a fibre layer against the top felt.(FS-016.tif)

The number of forming units in a board machine depends on the basis weight of the product being produced, but also on the desired properties.

If a heavy board grade is produced, there may be up to five formers.

The cylinder machine had a simple construction but it was not easy to get an even basis weight across the web. The speed difference between the drum and the stock in the trough gave shear forces orientating the fibres too much in the machine direction. The speed of the original cylinder board machine hardly reached more than 100 metres per minute.

Flersk4 01-09-13 7

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ossary ■aiiery

3.2. Modified cylinder machines

The next step in the development of the cylinder machine was to distribute the stock directly on the cylinder.

Fig. 17. Modified Cylinder former. (FS-017.tif)

Now it was possible to regulate the stock flow across the machine, which gave a more even basis weight. Another advantage was less fibre orientation in the machine direction than in the original type of cylinder former (mould and vat).

Fig. 18. Sketch. Head box (BRDA-former). (FS-018.tif)

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3.3. Fourdrinier machine with a separate top wire; top former

The machine speed on the fastest cylinder machines could come up to about 250 metres per minute. However, this speed was of course still too low. New board machines therefore look quite different.

In this new construction a base layer is often formed on a conventional Fourdrinier machine and layer after layer is then added.

Fig. 19. Fourdrinier with four separate headboxes and five Bel-Bond units. (Beloit) (FS-019.tif)

One way to form the added layers was to inject the stock in the already formed Fourdrinier layer and then dewater it through a top wire.

Fig. 20. Fourdrinier with a separate headbox and a following Bel-Bond unit. (Beloit)(FS-020.tif)

The forming board is curved to let the constant pressure in the dewatering zone to increase. It has a number of blades.

Fig. 21. Top former, Bel-Bond. (Beloit)(FS-021.tif)

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9

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When the wire bends over the blade edges, pressure pulses are formed. The aim of pressure pulses is to create shear forces improving the formation.

Fig. 22. Illustration. Pressure pulse before a blade.(FS-022.tif)

The stock is mainly dewatered up towards the forming board. The one-sided dewatering on the Fourdrinier wire means that a fibre bed preventing the dewatering towards the bottom wire already has been formed. Consequently, the stock is mainly dewatered upwards into the forming board.

Fig. 23. Illustration. Fibre net formed during the dewatering on the Fourdrinier wire.(FS-023.tif)

Between the blades in the forming board there is a vaccum enhancing the dewatering through the top wire.

Fig. 24. Bel-Bond, Suction chamber over the forming board. (FS-024.tif)

The vaccum also helps to adjust the size of the pressure pulses. The higher the vaccum is, the more the wire bends over the blade edges and the stronger the pressure pulses become.

Fig. 25. Illustration. The bending of the wire over the blade edges is adjusted by changing the vaccum in the suction chamber.(FS-025.tif)

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The task of the suction box in the bottom wire is to prevent the new layer from following the top wire.

Fig. 26. Bel-Bond. The suction box in the bottom wire is marked. (Beloit) (FS-026.tif)

One thing that distinguishes the described former from the conventional cylinder type is that no already formed fibre layers are being couched together. As a consequence the layer adhesion gets higher.

A problem with this type of former is the risk for destroying the already formed fibre net when it is hit by the stock jet from a following head-box. The dry substance content of the already formed web must not be too low.

An advantage is that the machine speed may be considerably higher than in the previous cylinder former.

However, the speed of such a board machine will not be as high as when producing paper on a paper machine and the pressure pulses formed when the wire runs over the blade edges are weak. The low shear forces makes the formation fall short.

Flersk4 01-09-13 11

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3.4. Fourdrinier machine with a separate former of a Fourdrinier type

The stock in the surface layer often contains chemical fibres, only. As the surface of the board is mostly printed the demand for a good colour reproduction is high. Actually, this often means that the fibres have to be bleached and that the cost for the furnish as a consequence becomes relatively high.

One way to decrease the production cost is to keep the grammage of the surface layer as low as possible. To make this possible the formation and the opacity of this layer must be the very best.

A good formation is achieved by working with a low concentration in the stock jet and by creating and maintaining shear forces in the undrained fibre suspension during the whole forming process.

The high opacity is achieved by adding a filler. To develop the desired effect the wire retention must be high.

A former giving both a good formation and a high retention is the Fourdrinier former. But the speed must not be too high.

The upper critical speed limit for a Fourdrinier former is soon above 1 000 metres per minute and that speed is normally not reached at board production.

Fig. 27. Fourdrinier machine.(FS-027.tif)

An increasingly common way is therefore to form the individual layers on separate Fourdrinier formers and couch them together.

Fig. 28. Board machine with separate Fourdrinier formers. (Escher Wyss) (FS-028.tif)

Flersk4 01-09-13 12

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If it is desirable to increase the dewatering over the Fourdrinier wire and at the same time increase the shear forces, it is possible; just like on a hybrid former, to add a top wire.

Fig. 29. Fourdrinier former with a top wire. (Duoformer Voith). (FS-029.tif)

On the overlying Fourdrinier unit in a board machine the outer wire sometimes wraps the turn roll in the bottom wire and follows the web towards the couch roll.

(The lower machine height is a practical advantage facilitating when rebuilding an old board machine.)

Fig. 30. Fourdrinier wire with a top wire. (Duoformer D/K Voith.) (FS-030.tif)

The strength of the formation improving pressure pulses can be controlled by adjustment of the positions of the counter-directed blades in the dewatering zone.

Fig. 31. Illustration. Blade section in a Duformer D, D/K. (Voith) (FS-031.tif)

Flersk4 01-09-13 13

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How much of the fine material that will be localised in the contact surfaces between the layers depends on how much fine material there is in the stock from the beginning and how that material then is directed in the thickness direction of the formed fibre net.

Fig. 32.(FS-032.tif)

The fibre net formed on a Fourdrinier wire is tighter than the wire itself, so the fibre net catches the fine material in the stock more easily. The more the net grows, the better it catches the fine material. Consequently, the content of fine material is higher on the top side than on the wire side of the formed web.

Figure set. The growth of the fibre net at one-sided dewatering.

Thus, to get as strong bonds as possible between the layers it is desirable to couch two top sides together. However, couching two top sides is possible only when there are not more than two layers in the board.

2

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/T•ps"Sffi affii 7T

1

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* nQQQQJH!

Fig. 34. Principle sketch. Part of a board machine. Two top sides will be couched against each other in the first couch nip. (FS-034.tif)

When there are more than two layers, one wire side and one top side will always meet. Fig. 35. Principle sketch. In the 2:nd and 3:rd couch nip a top and a

wire side will be couched against each other. (FS-035.tif)

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A way toincrease the finematerial contentin the contactzone betweenthe layers is torefine a small

Fig. 36. Principle sketch. A complete board machine. A highly refined chemical pulp is put on the base web before the second and the third couch nip. (FS-036.tif)

amount of stockto a very high degree, and add it on the base web just before the nip wherethe webs are couched together. An alternative to a highly refined stockcould be to spray an uncooked starch slurry on the stock.

If the different layers are formed on separate Fourdrinier wires, large amounts of water can be dewatered and the concentration in the headbox can be kept low. The concentration can be especially low when there are top wires reinforcing the dewatering. The lower the concentration is, the better the conditions for developing a good formation.

If there are top wires with blades they also give pressure pulses which improve the formation.

Flersk4 01-09-13 15

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ossary >allery

3.5. Fourdrinier machine with separate top formers of the roll former type

Fourdrinier formers make it possible to form a sheet with a good formation. The disadvantage is the high investment cost and the large space requirements.

A way to increase the dewatering and further improve the formation on a Fourdrinier wire is to have a top wire making the later part of the dewatering two-sided.

Another way becoming more and more common is to use two-sided dewatering from the very beginning. A former early used, is the roll former.

Nowadays, the same development can be seen on a board machine.

A way of producing board in several layers is to complete the Fourdrinier machine with a top former of a roll former type.

Fig. 37. TWINTOP former N. (Escher Wyss) (FS-037.tif)

A weak point with a pure roll former is that the shear forces, formed when the stock jet meets the forming wire, influence the stock only during the very first part of the forming process.

The lack of pressure pulses during the dewatering in a roll fomer means that the fibres easily form flocs. Fig. 38. Illustration. Dewatering in a

roll former. (FS-038.tif)

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Main reg >allery

One disadvantage with a roll former is that the formation is not the very best.

Another disadvantage related to the two-sided dewatering in the top former is the fact that the fine material content becomes higher in the middle of the sheet than close to the surfaces.

As long as there is only one top former the layer formed on the top wire will be couched against the top side of the base layer. The amount of fine material in the contact zone is high enough to make the two layers bind strongly to each other.

However, as soon as there are more top formers it will no longer be possible to couch together two top sides. As a result, it will be difficult to get the layers to bind strongly enough to each other.

3.6. Modified Fourdrinier machine with separate top formers of the roll-blade former type

A technique used in ordinary paper production was to combine roll and blade dewatering.

This technique to improve the formation on the top wire was previously used also in a board machine.

The stock is dewatered in the two-sided manner from the very beginning in a roll-blade former.

After the two-sided de-watering the wire runs as in a conventional Fourdrinier machine.

Fig. 39. Board machine with two-sided dewatering of a base layer. (DUOFORMER BASE, Voith).(FS-039.tif)

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Main reg >allery

The stock is ejected into the nip between two wires. The initial dewatering takes place at a constant pressure over the forming roll.

The continued dewatering then takes place over the slightly curved blade section. The strength of the pressure pulses and the direction of the dewatering is controlled by adjusting the vaccum between the blades.

Fig. 40. The dewatering zone in a roll blade former of the type (DUOFORMER BASE, Voith) (FS-040.tif)

In the following top formers the stock is in principle dewatered in the same way as in the base former.

Fig. 41. Top former with two-sided dewatering. (DUOFORMER TOP, Voith). (FS-041.tif)

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Mam reg Gl

ossary ■allery

The dewatering pulses over the forming blades do not only help to improve the formation. The pressure pulses also loosen up the formed fibre net. In that way the fine material can be more evenly distributed in the Z direction of the layer. As a consequence the formers should not cause the same problem with the layer adhesion as the former roll top former did. The effect of the pressure pulses over the forming blades is not only to improve the formation. Compared with a conventional roll former, the fine material now is more evenly distributed in the Z direction of the formed layer which contributes to a higher layer adhesion.

3.7. Headboxes

Hydraulic headbox or perforated roll box

The headboxes used in a board machine are of the same type as those on a Fourdrinier machine. In new machines there is often a hydraulic headbox.

Fig. 42. Hydraulic headbox. (Escher Wyss)(FS-042.tif)

The effect of the turbulence generator is connected to the flow through the pipe tubes.

If the flow becomes too low, the effect becomes low and the fibres easily floc.

Fig. 43. Illustration. Turbulence generating pipe tubes with sudden increases in diameter. (FS-043.tif)

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If the flow, on the other hand, becomes too high, large scale whirls could be generated in the stock when leaving the pipe tubes.

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Fig. 44. Illustration. If the flow becomes too high the stock will no longer follow the wall of the pipe. The distance between the flow streams increases and large scale whirls are formed after the pipes.(FS-044.tif)

The fact that the flow through the headboxes often varies at the production of multi-layer grade, makes hydraulic headboxes a good choise even on new board machines.

Fig. 45. Modern hydraulic headbox. (Valmet)(FS-045.tif)

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>allery

4. Simultaneous multi-layer forming

Headbox with stratified jet

When producing board the different layers are normally formed separately and then they are laid upon each other one after one.

But there is also another way making it possible to use separate furnishes without forming each layer separately; simultaneous multi-layer forming.

The process is to use a multi-channel headbox. From a single head box, a stratified jet is ejected with different furnish in the layers.

Fig. 46. Modern sectioned hydraulic headbox with a stock jet in three layers. (FS-046.tif)

Pipe tubes are not always used. There are concepts where the stocks are separated by thin plates in the discharge nozzle.

Fig. 47. Hydraulic headbox with a three layers stratified jet. The jets are separated by thin plates in the discharge nozzle. (Beloit Strataflow) (FS-047.tif)

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A disadvantage is that the stock layers can not be kept totally separated from each other in the finished sheet.

The stronger the stock turbulence is during the forming, the higher becomes the risk to mix the different stocks.

Fig. 48. Illustration. High turbulence in the stock jet increases the risk to mix the stocks. (Valmet) (FS-048.tif)

Of course, the greatest risk for mixing the stock is in the zone closest to the borderline between the layers. The thicker each layer is, the smaller the part of the total stock to be mixed together and the less the problem to keep the layers separated. As a consequence there is a limitation when using a multi-layer box. The paper formed may not have a too low basis weight.

The turbulence during the dewatering serves to prevent the fibres from forming flocs, deteriorating the formation. Decreasing the turbulence to prevent the different stocks from being mixed together is therefore not possible.

Figure set. Small whirls, micro turbulence, break down the fibre flocs in the stock.

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In a roll-former the turbulence is low during the forming and the dewatering takes place so quickly that the amount of the stock being mixed in the borderline zone between the layers becomes low. The quicker the dewatering takes place, the smaller the risk for mixing the layers.

Fig. 51. Illustration. Dewatering over the forming roll in a roll former. Low turbulence gives a low degree of mixing of the stratified stock jets. (FS-051.tif)

Roll formers are mainly used when producing tissue paper. The low basis weight and the low refining can give a high machine speed. The dewatering is very fast.

Fig. 52. Illustration. Dewatering over the forming roll in a tissue machine.(FS-052.tif)

The low risk of mixing the stocks is one reason for using head-boxes with stratified stock jets at the production of tissue paper.

Fig. 53. Headbox with three separate stock jets. (Valmet) (FS-053.tif)

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The extremely fast dewatering and the low turbulence in a roll former producing tissue paper makes it possible to use a multi-layer headbox in spite of the low basis weight.

An example of another grade produced in two, or sometimes three, layers and where headboxes of this type are used is liner.

In the machine shown on thepicture there is a headbox with

two separate stock jets.If a third layer is desired you

Fig. 54. Paper machine with a headbox with two layers. (Escher Wyss) (FS-054.tif)

can choose to increase thenumber of separate stock jets from the headbox or to prolong theFourdrinier wire and form the third layer in a separate forming unit.

An advantage of simultaneous multi-layer forming is that the investment cost for the forming part is lower, than for a machine with separate formers. Another advantage is the good layer adhesion.

A disadvantage, however, is that it is more difficult to keep the stocks separated, than when separate layers are formed.

Another weakness is that the white water is mixed in the wire pit, making it impossible to prevent one stock from influencing both the stock purity and the chemical environment in another stock.

To provide a multi-layer box with different stock there must be a separate line for each stock. Such a system will be a costly investment.

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A way to avoid the high cost for the extra stock lines, still making it possible to utilise the advantages of the multilayer box, is to use the same grade of fibre in all stocks and only vary the addition of chemicals and fillers.

Fig. 55. Headbox fed with the same grade of fibre but with different amounts of chemicals and fillers in the three stock layers. (FS-055.tif)

When producing a paper or board with a high basis weight it is necessary to form separately by pure manufacture considerations. The main disadvantage with this procedure is, the high investment cost for the forming units.

The other reason to form a sheet with several layers is that the desired properties for the finished product can be reached at a lower furnish cost. Thus, producing a sheet of separate furnishes is an advantage at the production of paper grades with low basis weights as well.The technique to form simultaneously (and not separately) is a way to decrease the investment cost for the forming unit as such. The cost for the separate stock lines can however not be avoided, if the technique is to be fully used. That cost has so far been an obstacle for a wider application of the new forming principle.

The method to form several layers simultaneously is however most interesting and in the future more paper grades will probably be developt of stock layers with various properties.

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CompletPresentationA.an

Documents

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