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1 LIST OF FIGURES Figure. 2.1 Baffle spacers and tie rods 7 Figure 2.2 Fixed-tube plate (based on figures from BS 3274: 1960) 9 Figure 2.3 U-tube (based on figures from BS 3274: 1960) 9 Figure 2.4 Internal floating head without clamp ring (based on figures from BS 3274:1960) 10 Figure 2.5 Internal floating head with clamp ring (based on figures from BS 3274: 1960) 10 Figure 2.6 External floating head, packed gland (based on figures from BS 3274: 1960) 11 Figure 2.7 Kettle reboiler with U-tube bundle (based on figures from BS 3274: 1960) 11 Figure 3.1 Basic Design Procedure 12 Figure 4.1 Rating process for heat exchanger design 16 Figure 5.2.1. Idealized main stream flow 21 Figure 5.2.2. Shell-side leakage and by-pass paths 22 Figure 5.2.3 Heat-transfer factor for cross-flow tube banks 23 Figure 5.2.4 Tube row correction factor F n 24 Figure 5.2.5 Window correction factor 25 Figure 5.2.6 Bypass correction factor 26 Figure 5.2.7. Coefficient for F L , heat transfer 26 Figure 5.2.8 Friction factor for cross-flow tube banks 28 Figure 5.2.9. Bypass factor for pressure drop F b 29 Figure 5.2.10. Coefficient for F L , pressure drop 29 Figure 6.1.1 Business Example 33 Figure 6.1.2 Engineering Example 33 Figure 7.1.1 Selecting process fluid from dropdown menu 35 Figure 7.1.2 Preliminary manual inputs 36 Figure 7.1.3 Generating physical properties of the process fluids 36 Figure 7.1.4 Assuming the values for different parameters 37 Figure 7.1.5 Selecting the method of design 37
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LIST OF FIGURES

Figure. 2.1 Baffle spacers and tie rods 7

Figure 2.2 Fixed-tube plate (based on figures from BS 3274: 1960) 9

Figure 2.3 U-tube (based on figures from BS 3274: 1960) 9

Figure 2.4 Internal floating head without clamp ring

(based on figures from BS 3274:1960) 10

Figure 2.5 Internal floating head with clamp ring

(based on figures from BS 3274: 1960) 10

Figure 2.6 External floating head, packed gland (based on figures from BS 3274: 1960) 11

Figure 2.7 Kettle reboiler with U-tube bundle (based on figures from BS 3274: 1960) 11

Figure 3.1 Basic Design Procedure 12

Figure 4.1 Rating process for heat exchanger design 16

Figure 5.2.1. Idealized main stream flow 21

Figure 5.2.2. Shell-side leakage and by-pass paths 22

Figure 5.2.3 Heat-transfer factor for cross-flow tube banks 23

Figure 5.2.4 Tube row correction factor Fn 24

Figure 5.2.5 Window correction factor 25

Figure 5.2.6 Bypass correction factor 26

Figure 5.2.7. Coefficient for FL, heat transfer 26

Figure 5.2.8 Friction factor for cross-flow tube banks 28

Figure 5.2.9. Bypass factor for pressure drop F’b 29

Figure 5.2.10. Coefficient for F’L, pressure drop 29

Figure 6.1.1 Business Example 33

Figure 6.1.2 Engineering Example 33

Figure 7.1.1 Selecting process fluid from dropdown menu 35

Figure 7.1.2 Preliminary manual inputs 36

Figure 7.1.3 Generating physical properties of the process fluids 36

Figure 7.1.4 Assuming the values for different parameters 37

Figure 7.1.5 Selecting the method of design 37

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Figure 7.1.6 Results 37

Figure 8.1.1 Problem inputs 39

Figure 8.1.2 Physical properties of process fluids 39

Figure 8.1.3 Selecting no. of passes in tube side 40

Figure 8.1.4 Selecting the type of pitch 40

Figure 8.1.5 Selecting the type of head 40

Figure 8.1.6 Selecting the type of shell side & tube side fluid 40

Figure 8.1.7 Results obtained by kern’s method 41

Figure 8.1.8 Selecting other method of design 41

Figure 8.1.9 Results obtained by Bell-Delaware’s method 41

Figure 8.2.1 Selecting the other coolant for same problem 42

Figure 8.2.2 Physical properties of process fluids 42

Figure 8.2.3 Selecting no. of passes in tube side 43

Figure 8.2.4 Selecting the type of pitch 43

Figure 8.2.5 Selecting the type of head 43

Figure 8.2.6 Selecting the type of shell side & tube side fluid 43

Figure 8.2.7 Results obtained by kern’s method 44

Figure 8.2.8 Selecting other method of design 44

Figure 8.2.9 Results obtained by Bell-Delaware’s method 44

LIST OF TABLES

Table 7.2.1 Color code in spreadsheet 38

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UNIT-1

INTRODUCTION (1) The transfer of heat to and from process fluids is an essential part of most chemical

processes. The most commonly used type of heat-transfer equipment is the ubiquitous shell

and tube heat exchanger; the design of which is the main subject of this report.

The word “exchanger” really applies to all types of equipment in which heat is exchanged

but is often used specifically to denote equipment in which heat is exchanged between two

process streams.

Exchangers in which a process fluid is heated or cooled by a plant service stream are referred

to as heaters and coolers.

If the process stream is vaporized the exchanger is called a vaporizer if the stream is

essentially completely vaporized; a reboiler if associated with a distillation column; and an

evaporator if used to concentrate a solution.

The term fired exchanger is used for exchangers heated by combustion gases, such as boilers;

other exchangers are referred to as “unfired exchangers”.

The principal types of heat exchanger used in the chemical process and allied industries,

1. Double-pipe exchanger: the simplest type, used for cooling and heating.

2. Shell and tube exchangers: used for all applications.

3. Plate and frame exchangers (plate heat exchangers): used for heating and cooling.

4. Plate-fin exchangers.

5. Spiral heat exchangers.

6. Air cooled: coolers and condensers.

7. Direct contact: cooling and quenching.

8. Agitated vessels.

9. Fired heaters.

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1.1 Heat Exchanger Type Heat transfer equipment is usually specified both by type of construction and by service.

A heat exchanger is a specialized device that assists in the transfer of heat from one fluid to

the other. In some cases, a solid wall may separate the fluids and prevent them from mixing.

In other designs, the fluids may be in direct contact with each other. In the most efficient heat

exchangers, the surface area of the wall between the fluids is maximized while

simultaneously minimizing the fluid flow resistance. Fins or corrugations are sometimes used

with the wall in order to increase the surface area and to induce turbulence.

In heat exchanger design, there are three types of flow arrangements: counter-flow, parallel-

flow, and cross-flow. In the counter-flow heat exchanger, both fluids entered the exchanger

from opposite sides. In the parallel-flow heat exchanger, the fluids come in from the same

end and move parallel to each other as they flow to the other side. The cross-flow heat

exchanger moves the fluids in a perpendicular fashion. Compare to other flow arrangements

counter –flow is the most efficient design because it transfers the greatest amount of heat.

There are two major different designs of heat exchangers: shell and tube, and plate heat

exchanger. The most typical type of heat exchanger is the shell and tube design. This heat

exchanger can be design with bare tube or finned tubes. One of the fluids runs through the

tubes while the other fluid runs over them, causing it to be heated or cooled. In the plate heat

exchanger, the fluid flows through baffles. This causes the fluids to be separated by plates

with a large surface area. This type of heat exchanger is typically more efficient than the

shell and tube design.

1.1.1 Shell & Tube Exchanger A shell and tube heat exchanger is a class of heat exchanger designs. It is the most common

type of heat exchanger in oil refineries and other large chemical processes, and is suited for

higher-pressure applications. It consists of a tube bundle enclosed in a cylindrical casing

called a shell. One fluid runs through the tubes, and another fluid flows over the tubes

(through the shell) to transfer heat between the two fluids.

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Two fluids, of different starting temperatures, flow through the heat exchanger. One flows

through the tubes (the tube side) and the other flows outside the tubes but inside the shell (the

shell side). Heat is transferred from one fluid to the other through the tube walls, either from

tube side to shell side or vice versa. The fluids can be either liquids or gases on either the

shell or the tube side. In order to transfer heat efficiently, a large heat transfer area should be

used, so there are many tubes. In this way, waste heat can be put to use. This is a great way to

conserve energy.

Typically, the ends of each tube are connected to plenums through holes in tube sheets.

The tubes may be straight or bent in the shape of a U, called U-tubes. Most shell-and-tube

heat exchangers are 1, 2, or 4 pass designs on the tube side. This refers to the number of

times the fluid in the tubes passes through the fluid in the shell. In a single pass heat

exchanger, the fluid goes in one end of each tube and out the other.

There are two basic types of shell-and-tube exchangers. The first is the fixed tube sheet unit,

in which both tube sheets are fastened to the shell and the tube bundle is not removable. The

second type of shell-and-tube unit has one restrained tube sheet, called the stationary tube

sheet, located at the channel end. Differential expansion problems are avoided by use of a

freely riding floating tube sheet at the other end or the use of U tubes.

This design may be used for single or multiple pass exchangers. The tube bundle is

removable from the channel end, for maintenance and mechanical cleaning.

There are often baffles directing flow through the shell side so the fluid does not take a short

cut through the shell side leaving ineffective low flow volumes.

Counter current heat exchangers are most efficient because they allow the highest log mean

temperature difference between the hot and cold streams. Many companies however do not

use single pass heat exchangers because they can break easily in addition to being more

expensive to build. Often multiple heat exchangers can be used to simulate the counter

current flow of a single large exchanger.

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Shell-and-tube exchangers are designed and fabricated according to the standards of the

Tubular Exchanger Manufacturers Association (TEMA).

1.1.2 Plate Heat Exchangers Plate and frame heat exchanger for general refinery service can be referring as gasketed plate

heat exchangers. The plate heat exchanger consists of a frame, which consists of a head,

follower, column, carrying bar, guiding bar, and a number of clamping bolts. In between

head and follower a varying number of pressed plates are clamped together. Each plate is

supplied with a gasket, so that the plates form a closed system of parallel flow channels,

through which the Medias flow alternatively at every second interval.

The gaskets are glued on the plates, securing tightness between Medias and the atmosphere.

Between the different Medias there are double gaskets, which have intermediate drain areas,

meaning that mixing of the two Medias is impossible. Every second plate in the stack has to

turn 180°, so that the plates form a closed system of parallel flow channels, through which

the Medias flow alternatively at every second interval.

The advantage of the gasketed plate heat exchanger:

(i) High thermal efficiency due to high film efficiency of heat transfer for fluids, no

bypassing and leakage streams, and counter-current operation.

(ii) Plate design is feasible with size, chevrons angles and pass arrangements.

(iii) Easy maintenance that the plate can be easily disassembled for cleaning.

(iv) The plates of the unit can be rearranged, added or removed from the plate rack to suit for

difference of service condition.

(v) Have very wide range of total surface area up to 15,000 ft2.

(vi) Low fouling is encountered due to high turbulence create by plate and the fluid low

residence in plate.

The disadvantage,

(i) Have limitations in service temperature and pressure. Maximum service temperature is

450oF and pressure is 335 psig.

(ii) The gaskets impose restrictions on the nature of the fluids which can be handled.

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UNIT- 2

SHELL AND TUBE EXCHANGERS: CONSTRUCTION

DETAILS (1) The shell and tube exchanger is by far the most commonly used type of heat-transfer

equipment used in the chemical and allied industries. The advantages of this type are:

1. The configuration gives a large surface area in a small volume.

2. Good mechanical layout: a good shape for pressure operation.

3. Uses well-established fabrication techniques.

4. Can be constructed from a wide range of materials.

5. Easily cleaned.

6. Well-established design procedures.

Essentially, a shell and tube exchanger consists of a bundle of tubes enclosed in a cylindrical

shell. The ends of the tubes are fitted into tube sheets, which separate the shell-side and tube-

side fluids. Baffles are provided in the shell to direct the fluid flow and support the tubes.

The assembly of baffles and tubes is held together by support rods and spacers, Figure 2.1.

Figure. 2.1 Baffle spacers and tie rods

2.1 Exchanger types The principal types of shell and tube exchanger are shown in Figures 2.2 to 2.7.

Diagrams of other types and full details of their construction can be found in the heat

exchanger standards (see Section 2.5.1.). The standard nomenclature used for shell and tube

exchangers is given below; the numbers refer to the features shown in Figures 2.2 to 2.7.

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Nomenclature

Part number

1. Shell 2. Shell cover

3. Floating-head cover 4. Floating-tube plate

5. Clamp ring 6. Fixed-tube sheet (tube plate)

7. Channel (end-box or header) 8. Channel cover

9. Branch (nozzle) 10. Tie rod and spacer

11. Cross baffle or tube-support plate 12. Impingement baffles

13. Longitudinal baffle 14. Support bracket

15. Floating-head support 16. Weir

17. Split ring 18. Tube

19. Tube bundle 20. Pass partition

21. Floating-head gland 22. Floating-head gland ring (packed gland)

23. Vent connection 24. Drain connection

25. Test connection 26. Expansion bellows

27. Lifting ring

The simplest and cheapest type of shell and tube exchanger is the fixed tube sheet design

shown in Figure 2.2. The main disadvantages of this type are that the tube bundle cannot be

removed for cleaning and there is no provision for differential expansion of the shell and

tubes. As the shell and tubes will be at different temperatures, and may be of different

materials, the differential expansion can be considerable and the use of this type is limited to

temperature differences up to about 80°C. Some provision for expansion can be made by

including an expansion loop in the shell (shown dotted on Figure 12.3) but their use is

limited to low shell pressure; up to about 8 bar. In the other types, only one end of the tubes

is fixed and the bundle can expand freely.

The U-tube (U-bundle) type shown in Figure 2.3 requires only one tube sheet and is cheaper

than the floating-head types; but is limited in use to relatively clean fluids as the tubes and

bundle are difficult to clean. It is also more difficult to replace a tube in this type.

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Figure 2.2 Fixed-tube plate (based on figures from BS 3274: 1960)

Figure 2.3 U-tube (based on figures from BS 3274: 1960)

Exchangers with an internal floating head, Figures 2.4 and 2.5 are more versatile than fixed

head and U-tube exchangers. They are suitable for high-temperature differentials and, as the

tubes can be rodded from end to end and the bundle removed, are easier to clean and can be

used for fouling liquids. A disadvantage of the pull-through design,

Figure 2.4, is that the clearance between the outermost tubes in the bundle and the shell must

be made greater than in the fixed and U-tube designs to accommodate the floating head

flange, allowing fluid to bypass the tubes. The clamp ring (split flange design),

Figure 2.5, is used to reduce the clearance needed. There will always be a danger of leakage

occurring from the internal flanges in these floating head designs.

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In the external floating head designs, Figure 2.6, the floating-head joint is located outside the

shell, and the shell sealed with a sliding gland joint employing a stuffing box. Because of the

danger of leaks through the gland, the shell-side pressure in this type is usually limited to

about 20 bars, and flammable or toxic materials should not be used on the shell side.

Figure 2.4 Internal floating head without clamp ring (based on figures from BS 3274:1960)

Figure 2.5 Internal floating head with clamp ring (based on figures from BS 3274: 1960)

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Figure 2.6 External floating head, packed gland (based on figures from BS 3274: 1960)

Figure 2.7 Kettle reboiler with U-tube bundle (based on figures from BS 3274: 1960)

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UNIT-3

BASIC DESIGN PROCEDURE FOR A SHELL & TUBE HEAT

EXCHANGER A lot has been written about designing heat exchangers, and specifically, shell-and-tube heat

exchangers. For example, the book by Kern (2) published in 1950 details basic design

procedures for a variety of heat exchangers. An article in 1979 by Taborek (3) outlines how

heat exchanger design techniques evolved over the years since the appearance of the book by

Kern. More recent developments are discussed in numerous articles in the magazine

“Chemical Engineering.”.

From here on, references to page numbers, table numbers, and equation numbers are from

Coulson & Richardson’s Chemical Engg., vol.6, “Chemical Engineering Design”(1).

3.1 Basic Design Procedure Route

Figure 3.1 Basic Design Procedure

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Usually, the flow rates and the physical properties of the two streams involved are specified,

and the temperatures at which the fluids are available are known. If the outgoing temperature

of one of the streams is not specified, usually a constraint (e.g. the temperature of the cooling

water cannot exceed 99◦C) is given. Then, by an energy balance, the outgoing temperature of

the second stream can be calculated along with the heat duty.

1. Select the stream that should be placed on the tube side. The tube side is used for the fluid

that is more likely to foul the walls, more toxic or more corrosive, or for the fluid with the

higher pressure. Cleaning of the inside of the tubes is easier than cleaning the outside. When

a gas or vapor is used as a heat exchange fluid, it is typically introduced on the shell side.

Also, high viscosity liquids, for which the pressure drop for flow through the tubes might be

prohibitively large, can be introduced on the shell side.

2. The heat duty Q is usually fixed by the required service. The selected heat exchanger has

to meet or exceed this requirement.

Heat load of a heat exchanger can be estimated from heat balance:-

Q= (m Cp Δt) = (m Cp ΔT)

Where Δt is the temperature difference in the tube side fluid & ΔT is the temperature

difference in the shell side fluid.

If three of the temperatures are given, the fourth can be calculated using the above equation.

The above equation assumes no phase change in any of the fluids.

3. Make an approximate estimate of the size of the heat exchanger by using a reasonable

guess for the overall heat transfer coefficient.

For typical shell-and-tube heat exchangers in a chemical process or a refinery, Table 12.1 can

be used as a starting point for the estimate. Using this estimate, calculate the heat transfer

area Aο.

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Where

• Ao Outside tube surface area

• q Heat duty – heat exchange between tube and shell side

• Uo Overall heat transfer coefficient

• F Correction factor, F=1.0 for cross flow heat exchanger

• Tm True mean temperature, Tm = F Tlm

• Tlm Log means temperature difference.

This will give you an idea of the approximate size of the heat exchanger, and therefore its

cost.

4. The next step is to determine the approximate number of tubes Nt needed to do the job.

Because we have an idea of the approximate heat transfer area, we can write

Aο = Nt (Π D◦) L

Where D◦ is the OD of a tube and L is its length. Both of these are only available in discrete

increments. For example, the length is selected as 8, 10, 12, 16, or 20 feet. Likewise, the OD

is specified as ¼, 3/8, 1/2, 5/8, 3/4, 1, 1 1/4, 1 1/2 inch.

The tubes are typically specified to be 14 BWG.

The most common tube lengths are 16 and 20 feet and the most common tube OD values are

3/4 and 1 inch. So, selecting one of the values in each set will get you started in estimating

the approximate number of tubes.

Check the velocity through a single tube; it should not exceed roughly 1 to 2 m/s for liquids,

to keep the pressure drop under reasonable constraints, but it should be at least 4 m/s (the

specific choice depends on the viscosity as well) to maintain turbulent flow, and minimize

fouling.

If necessary, adjust the number of tube passes to get the velocity to fall in this range.

You can learn more about tubes and the tube-side construction from Section 12.5.2.

5. Determine the shell size. To do this, once the number of tubes is known, select a pitch and

the number of passes.

Typical initial guesses are 1 or 2 tube passes. A square pitch is chosen for reasons of

convenience in cleaning the outside of the tubes; when the tubes are in-line, cleaning is

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relatively straightforward. The standard choice is a pitch equal to 1.25 inches for 1-inch OD

tubes, and a pitch of 1 inch for 3/4 –inch OD tubes. Tubes on a triangular pitch cannot be

cleaned by tools, but rather by passing a chemical solution through on the shell-side. Because

triangular pitches allow for the packing of more tubes into a given space, they are more

common when cleaning the outside is not a major issue. Rectangular pitches are uncommon.

Knowing the number of tubes to be used and the number of passes, you can select the

required shell size. For this, you need to know about the clearance that must be allowed

between the tube bundle and the shell inside diameter.

A 1-pass shell is the most common in use, but sometimes a 2-pass shell can be specified to

improve thermal effectiveness. Shells are made from commercial steel pipes up to an outside

diameter of 24 inches. Shells with a larger OD are made by rolling steel plate and welding.

6. You need to estimate the number of baffles to be used and the spacing among them. You

can read about baffles from 12.5.7.

Normally, baffles are equally spaced. The minimum baffle spacing is one-fifth of the shell

diameter, but not less than 2 inches, and the maximum is determined by considerations

involving supporting the tube bundle.

7. Now, we are ready to check the thermal performance of the selected heat exchanger.

Calculate the tube-side and shell-side heat transfer coefficients, the tube wall contribution to

the resistance, and the appropriate fouling resistances.

See if the calculated Uº matches the required Uº that you used for estimating the heat transfer

area. If it is too small, start all over again! If it is too large, then the heat exchanger is over-

specified for the required thermal duty.

If the calculated Uº is too small, you need to examine whether the tube-side or the shell-side

resistance is controlling (sometimes they are comparable).

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UNIT-4

RATING OF THE HEAT EXCHANGER DESIGN (4)

Rating an exchanger means to evaluate the thermo-hydraulic performance of a fully specified

exchanger.

v Input to the rating process is heat exchanger geometry (constructional design parameters),

process conditions (flow rate, temperature, pressure) and material/fluid properties

(density, thermal conductivity)

Ø First output from the rating process is either the outlet temperature for fixed tube

length or the tube length itself to meet the outlet temperature requirement.

Ø Second output from the rating process is the pressure drop for both fluid streams

hence the pumping energy requirements and size.

Figure 4.1 Rating process for heat exchanger design

v If the output of the rating analysis is not acceptable, a geometrical modification should be

made

Ø If the required amount of heat cannot be transferred to satisfy specific outlet

temperature, one should find a way to increase the heat transfer coefficient or increase

exchanger surface area

• One can increase the tube side heat transfer coefficient by increasing the fluid

velocity - Increase number of tube passes.

• One can increase the shell side heat transfer coefficient by decreasing baffle spacing

and/or baffle cut.

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• One can increase the surface area by,

§ Increasing the heat exchanger length

§ Increasing the shell diameter

§ Multiple shells in series

v If the pressure drop on the tube side is greater than the allowable pressure drop, then

§ the number of tube passes can be decreased or

§ the tube diameter can be increased which may result to

• decrease the tube length – (Same surface area)

• increase the shell diameter and the number of tubes

Ø If the shell side pressure drop is greater than the allowable pressure drop then baffle

spacing, tube pitch, and baffle cut can be increased or one can change the baffle type.

THERE IS ALWAYS A TRADE-OFF BETWEEN THERMAL & PRESSURE

DROP RATINGS.

Different approaches for Shell Side heat transfer Coefficients v There are three rating methods to calculate the shell side heat transfer coefficient:

Ø Kern method is a simplified approach suitable for shell side flow without baffles

Ø Taborek method

Ø Bell Delaware method is the most complex but accurate way of rating a heat

exchanger with baffles.

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UNIT-5

DIFFERENT DESIGN APPROACHES FOR SHELL & TUBE

HEAT EXCHANGER (1) 1. Kern’s method

This method was based on experimental work on commercial exchangers with standard

tolerances and will give a reasonably satisfactory prediction of the heat-transfer coefficient

for standard designs. The prediction of pressure drop is less satisfactory, as pressure drop is

more affected by leakage and bypassing than heat transfer. The shell-side heat transfer and

friction factors are correlated in a similar manner to those for tube-side flow by using a

hypothetical shell velocity and shell diameter. As the cross-sectional area for flow will vary

across the shell diameter, the linear and mass velocities are based on the maximum area for

cross-flow: that at the shell equator. The shell equivalent diameter is calculated using the

flow area between the tubes taken in the axial direction (parallel to the tubes) and the wetted

perimeter of the tubes.

The procedure for calculating the shell-side heat-transfer coefficient and pressure drop for a

single shell pass exchanger is given below:

1. Calculate the area for cross-flow As for the hypothetical row of tubes at the shell

equator, given by:

Where pt = tube pitch,

do = tube outside diameter,

Ds = shell inside diameter, m,

lB = baffle spacing, m.

The term (pt –do)/pt is the ratio of the clearance between tubes and the total

distance between tube centers.

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2. Calculate the shell-side mass velocity Gs and the linear velocity us:

Where Ws = fluid flow-rate on the shell-side, kg/s,

ƍ = shell-side fluid density, kg/m3.

3. Calculate the shell-side equivalent diameter (hydraulic diameter), For a square pitch

arrangement:

For an equilateral triangular pitch arrangement:

Where de = equivalent diameter, m.

4. Calculate the shell-side Reynolds number, given by:

5. For the calculated Reynolds number, read the value of jh for the selected baffle cut

and tube arrangement, and calculate the shell-side heat transfer coefficient hs from:

The tube wall temperature can be estimated using the method given for the tube-side.

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6. For the calculated shell-side Reynolds number, read the friction factor and calculate

the shell-side pressure drop from:

Where L = tube length,

lB = baffle spacing.

The term (L/lB) is the number of times the flow crosses the tube bundle = (Nb +1)

Where Nb is the number of baffles.

2. Bell’s method

In Bell’s method the heat-transfer coefficient and pressure drop are estimated from

correlations for flow over ideal tube-banks, and the effects of leakage, bypassing and flow in

the window zone are allowed for by applying correction factors.

This approach will give more satisfactory predictions of the heat-transfer coefficient and

pressure drop than Kern’s method; and, as it takes into account the effects of leakage and

bypassing, can be used to investigate the effects of constructional tolerances and the use of

sealing strips. The procedure in a simplified and modified form to that given by

Bell (1963) is outlined below.

The method is not recommended when the by-pass flow area is greater than 30% of the

cross-flow area, unless sealing strips are used.

2.1. Flow pattern The flow pattern in the shell of a segmentally baffled heat exchanger is complex, and this

makes the prediction of the shell-side heat-transfer coefficient and pressure drop very much

more difficult than for the tube-side. Though the baffles are installed to direct the flow across

the tubes, the actual flow of the main stream of fluid will be a mixture of cross flow between

the baffles, coupled with axial (parallel) flow in the baffle windows; as shown in Figure

5.2.1. Not all the fluid flow follows the path shown in Figure 5.2.1; some will leak through

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gaps formed by the clearances that have to be allowed for fabrication and assembly of the

exchanger. These leakage and bypass streams are shown in Figure 5.2.2., which is based on

the flow model proposed by Tinker (1951, 1958). In Figure 5.2.2., Tinker’s nomenclature is

used to identify the various streams, as follows:

§ Stream A is the tube-to-baffle leakage stream. The fluid flowing through the

clearance between the tube outside diameter and the tube hole in the baffle.

Figure 5.2.1. Idealized main stream flow

§ Stream B is the actual cross-flow stream.

§ Stream C is the bundle-to-shell bypass stream. The fluid flowing in the clearance area

between the outer tubes in the bundle (bundle diameter) and the shell.

§ Stream E is the baffle-to-shell leakage stream. The fluid flowing through the

clearance between the edge of a baffle and the shell wall.

§ Stream F is the pass-partition stream. The fluid flowing through the gap in the tube

arrangement due to the pass partition plates. Where the gap is vertical it will provide a

low-pressure drop path for fluid flow.

Note. There is no stream D.

The fluid in streams C, E and F bypasses the tubes, which reduces the effective heat transfer

area.

Stream C is the main bypass stream and will be particularly significant in pull-through

bundle exchangers, where the clearance between the shell and bundle is of necessity large.

Stream C can be considerably reduced by using sealing strips; horizontal strips that block the

gap between the bundle and the shell.

Dummy tubes are also sometimes used to block the pass-partition leakage stream F.

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The tube-to-baffle leakages stream A does not bypass the tubes and their main effects are on

pressure drop rather than heat transfer.

Figure 5.2.2. Shell-side leakage and by-pass paths

2.2. Heat-transfer coefficient The shell-side heat transfer coefficient is given by:

hs = hoc Fn Fw Fb FL

Where hoc = heat transfer coefficient calculated for cross-flow over an ideal tube bank, no

leakage or bypassing.

Fn = correction factor to allow for the effect of the number of vertical tube rows,

Fw = window effect correction factor,

Fb = bypass stream correction factor,

FL = leakage correction factor.

The total correction will vary from 0.6 for a poorly designed exchanger with large clearances

to 0.9 for a well-designed exchanger.

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hoc, ideal cross-flow coefficient The heat-transfer coefficient for an ideal cross-flow tube bank can be calculated using the

heat transfer factors jh given in Figure 5.2.3. Figure 5.2.3 has been adapted from a similar

figure given by Mueller (1973). Mueller includes values for more tube arrangements than are

shown in Figure 5.2.3. As an alternative to Figure 5.2.3, the comprehensive data given in the

Engineering Sciences Data Unit Design Guide on heat transfer during cross-flow of fluids

over tube banks, ESDU 73031 (1973), can be used; see Butterworth (1977).

Figure 5.2.3 Heat-transfer factor for cross-flow tube banks

The Reynolds number for cross-flow through a tube bank is given by:

Where Gs = mass flow rate per unit area, based on the total flow and free area at the

bundle equator.

This is the same as Gs calculated for Kern’s method,

do = tube outside diameter.

The heat-transfer coefficient is given by:

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Fn, tube row correction factor The mean heat-transfer coefficient will depend on the number of tubes crossed.

Figure 5.2.3 is based on data for ten rows of tubes. For turbulent flow the correction factor Fn is close to 1.0. In laminar flow the heat-transfer coefficient may decrease with increasing

rows of tubes crossed, due to the build up of the temperature boundary layer.

The factors given below can be used for the various flow regimes; the factors for turbulent

flow are based on those given by Bell (1963).

Ncv is number of constrictions crossed = number of tube rows between the baffle tips; see

Figure.

1. Re > 2000, turbulent; take Fn from Figure 5.2.4

Figure 5.2.4 Tube row correction factor Fn

2. Re > 100 to 2000, transition region, take Fn = 1.0;

3. Re < 100, laminar region, Fn α (Nc’) -.18

Where Nc’ is the number of rows crossed in series from end to end of the shell, and depends

on the number of baffles. The correction factor in the laminar region is not well established,

and Bell’s paper, or the summary given by Mueller (1973), should be consulted if the design

falls in this region.

Fw, window correction factor This factor corrects for the effect of flow through the baffle window, and is a function of the

heat-transfer area in the window zones and the total heat-transfer area. The correction factor

is shown in Figure 5.2.5 plotted versus Rw, the ratio of the number of tubes in the window

zones to the total number in the bundle, determined from the tube layout diagram.

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For preliminary calculations Rw can be estimated from the bundle and window cross-

sectional areas.

Figure 5.2.5 Window correction factor

Fb, bypass correction factor This factor corrects for the main bypass stream, the flow between the tube bundle and the

shell wall, and is a function of the shell to bundle clearance, and whether sealing strips are

used:

Where α = 1.5 for laminar flow, Re < 100,

α = 1.35 for transitional and turbulent flow Re > 100,

Ab = clearance area between the bundle and the shell,

As = maximum area for cross-flow, equation 12.21,

Ns = number of sealing strips encountered by the bypass stream in the cross-flow zone,

Ncv = the number of constrictions, tube rows, encountered in the cross-flow section.

Equation applies for Ns < Ncv/2.

Where no sealing strips are used, Fb can be obtained from Figure 5.5.6.

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FL, Leakage correction factor This factor corrects for the leakage through the tube-to-baffle clearance and the baffle-to

shell clearance.

Figure 5.2.6 Bypass correction factor Figure 5.2.7. Coefficient for FL, heat transfer

Where βL = a factor obtained from Figure 5.2.7,

Atb = the tube to baffle clearance area, per baffle.

Asb = shell-to-baffle clearance area, per baffle.

AL = total leakage area = (Atb + Asb).

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2.3 Pressure drop The pressure drops in the cross-flow and window zones are determined separately, and

summed to give the total shell-side pressure drop.

Cross-flow zones The pressure drop in the cross-flow zones between the baffle tips is calculated from

correlations for ideal tube banks, and corrected for leakage and bypassing.

∆Pc = ∆Pi F’b F’

L

Where ∆Pc = the pressure drop in a cross-flow zone between the baffle tips, corrected

for by-passing and leakage,

∆Pi = the pressure drop calculated for an equivalent ideal tube bank,

F’b = by-pass correction factor,

F’L = leakage correction factor.

∆ Pi ideal tube bank pressure drop The number of tube rows has little effect on the friction factor and is ignored.

Any suitable correlation for the cross-flow friction factor can be used; for that given

in Figure 5.2.8, the pressure drop across the ideal tube bank is given by:

Where Ncv = number of tube rows crossed (in the cross-flow region),

us = shell side velocity, based on the clearance area at the bundle equator.

jf = friction factor obtained from Figure 5.2.8, at the appropriate Reynolds

number, Re = (ρusdo/μ).

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Figure 5.2.8 Friction factor for cross-flow tube banks

F’b, bypass correction factor for pressure drop

Bypassing will affect the pressure drop only in the cross-flow zones. The correction factor is

calculated from the equation used to calculate the bypass correction factor for heat transfer,

but with the following values for the constant ˛.

Laminar region, Re < 100, α = 5.0

Transition and turbulent region, Re > 100, α = 4.0

The correction factor for exchangers without sealing strips is shown in Figure 5.2.9.

F’L, leakage factor for pressure drop

Leakages will affect the pressure drop in both the cross-flow and window zones. The factor is

calculated using the equation for the heat-transfer leakage-correction factor, with the values

for the coefficient β’L taken from Figure 5.2.10.

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Window-zone pressure drop Any suitable method can be used to determine the pressure drop in the window area; see

Butterworth (1977). Bell used a method proposed by Colburn. Corrected for leakage, the

window drop for turbulent flow is given by:

Where uz = the geometric mean velocity,

uw = the velocity in the window zone, based on the window area less the area

occupied by the tubes Aw

Ws = shell-side fluid mass flow, kg/s,

Nwv = number of restrictions for cross-flow in window zone, approximately

Figure 5.2.9. Bypass factor for Figure 5.2.10. Coefficient for F’L,

pressure drop F’b pressure drop

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End zone pressure drop There will be no leakage paths in an end zone (the zone between tube sheet and baffle).

Also, there will only be one baffle window in these zones; so the total number of restrictions

in the cross-flow zone will be Ncv + Nwv. The end zone pressure drop ∆Pe will therefore be

given by:

Total shell-side pressure drop Summing the pressure drops over all the zones in series from inlet to outlet gives:

∆Ps = 2 end zones + (Nb – 1) cross-flow zones + Nb window zones

∆Ps = 2∆Pe + ∆Pc (Nb – 1) + N b ∆Pw

Where Nb is the number of baffles = [(L/lB) -1].

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UNIT-6

SPREADSHEET – A COST-EFFECTIVE & TRANSPARENT

TOOL (5) Nowadays there are commercial applications such as HYSYS ® and ASPEN PLUS ® that

allows the user to simulate chemical plants in a very realistic way. Generally speaking, these

applications are very expensive and do not indicate exactly the simplifications upon which

the simulation models are based.

However, using a low cost tool as Spreadsheet i.e. MS Excel ®, it is possible to build and

solve simulation models that duplicate the results obtained using commercial simulators. In

order to develop practical simulations in Excel®, engineers must use detailed mathematical

models of unit operations, computer code for the calculation of thermodynamic properties,

and a computational tool designed to solve the highly nonlinear equation systems involved in

such models.

Presently, there are some free computer programs designed to construct and solve simulation

models. Among them, Ascend IV (6) is powerful mathematical modeling software with some

thermodynamics and distillation libraries; however, its interface is not very user friendly, an

aspect that causes some problems to the beginner.

Chemical process simulation involves the integration of three basic elements:

• Mathematical models of unit operations,

• Thermodynamic properties calculation methods,

• Numerical methods for the solution of non-linear equations systems.

Many papers present mathematical models for particular units in particular conditions, but it

is difficult to find works grouping general models that shear the same style and notation.

Additionally, even though there are many theoretical presentations (7), there are only a few

low cost tools for the calculation of thermodynamic properties of mixtures. Among them,

BibPhy®-ProSim and ProdeProperties®- Prode are two examples that allow thermodynamic

properties calculations using Excel®.

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Finally, some commercial tools such as MATLAB ® and MAPLE ® allow the solution of

general equations systems; however, these packages are relatively expensive and use

traditional numerical methods that can be inadequate for the solution of the highly non-linear

equations involved in the simulation of chemical processes (8).

Due to its low cost and highly acceptance in industry and academy, MS Excel is and ideal

platform for the construction and solution of mathematical models. The present work

presents how the three elements descried above can be integrated in MS Excel, offering a

practical method that allow the simulation of unit operations and complete processes at a

fraction of the cost of commercial process simulators.

6.1 The Uses of Microsoft Excel in Business and Engineering

Microsoft Excel is a spreadsheet tool capable of performing calculations, analyzing data and

integrating information from different programs. Microsoft Excel is comprised of

organizational units called workbooks. A standard workbook contains worksheets and chart

sheets. Worksheets perform calculations, store and organize data, present graphics and

controls like a web page; they are extremely versatile. A worksheet in turn is comprised of

millions of cells. The job of a cell is to store a formula that performs a calculation or

communicates with some other application (i.e. program) such as a database. They also store

and present data. A chart sheet's job is to present a chart or graph developed from data stored

on a worksheet.

• A typical business worksheet (A), its elements and the workbook that contains it are

presented in the illustration below. This simple example is a shipping status system

developed in Microsoft Excel in less than an hour utilizing the conditional formatting

feature, drawing shapes and worksheet functions like Vlookup.

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Figure 6.1.1 Business Example

• A typical engineering worksheet (B), its elements and the workbook that contains it

are presented in the illustration below. This simple example is a pressure vessel

design developed in Microsoft Excel in less than an hour.

Figure 6.1.2 Engineering Example

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Think of Microsoft Excel as a modular tool set that can be rapidly configured to accomplish a

desired task. A key force behind Microsoft Excel's capabilities is a powerful programming

language called Visual Basic for Applications (VBA) which comes standard with Microsoft

Excel. Using Microsoft Excel and VBA, a professional can accomplish important tasks like:

• Rapid analysis and charting

• Advanced modeling including numerical simulation

• Automated report generation

• Problem optimization using Solver and Crystal Ball

• Software design

• Team and model integration

• Database communication and control

• Project command and control

• Project constraint monitoring

• Information command and control

• Multi-language programming with FORTRAN and C code (DLL's)

• Real time integration with other applications

• Data sorting and analysis

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UNIT-7

SHELL & TUBE HEAT EXCHANGER DESIGN VIA

SPREADSHEET The present work is an initiation to bring both the previously discussed approaches to a

common platform and compare the same. MS Excel was selected as a tool to address the

above due to its inherent features like easy accessibility, simplicity and transparency.

Both the approaches can be very well formulated as CAD (Computer Aided Design)

using inbuilt functions like IF condition, VLOOKUP, etc.

7.1 SPREADSHEET PROCEDURE

7.1.1 First of all one has to select the process fluid of shell side & tube side from the

dropdown menu.

Ø Shell side fluid

Ø Tube side fluid

Figure 7.1.1 Selecting process fluid from dropdown menu

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7.1.2 The process variables which are to be manually supplied are temperature of the process

fluid at the inlet & outlet of the shell & tube heat exchanger design, flow rate of the process

fluids.

Figure 7.1.2 Preliminary manual inputs

7.1.3 After providing preliminary variables input, the spreadsheet generates the physical

properties generated for the systems from the databanks, which is included in this work (9)

Figure 7.1.3 Generating physical properties of the process fluids (10)

7.1.4 Now, shell & tube heat exchanger design is the very iterative process, so we have to

give some assumed or initial values for some of the tentative set of parameters.

These parameters are no. of passes at the tube side, overall heat transfer coefficient (U0), type

of pitch, type of head, etc.

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Figure 7.1.4 Assuming the values for different parameters.

7.1.5 Then the select the type of method for designing the shell & tube heat exchanger.

Figure 7.1.5 Selecting the method of design.

7.1.6 The spreadsheet then computes & validate the necessary and sufficient conditions for

the determining the result of design parameters like the area of heat exchanger, no. of tubes

required, shell side heat transfer coefficient & pressure drop, tube side heat transfer &

pressure drop, etc.

Figure 7.1.6 Results

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7.2 Color code used in spreadsheet: The following color codes are used in the excel sheet for making it more user friendly.

Color Inference

Manual inputs

Values obtained from the databank

Assumed values

Resulting parameters

Values of resulting parameters obtained by spreadsheet procedure

Different result values due to different approaches

Table 7.2.1 Color code in spreadsheet

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UNIT-8

CASE STUDIES 8.1 CASE STUDY -1 Design an exchanger to sub-cool condensate from a methanol condenser from 95ºC to 40°C.

Flow-rate of methanol 100,000 kg/h. brackish water will be used as the coolant, with a

temperature rise from 25°C to 40°C.

Figure 8.1.1 Problem inputs

Now, the spreadsheet will estimate the values of necessary physical & chemical properties

of methanol and water from its databank. The follwing figures are displaying the same thing.

Figure 8.1.2 Physical properties of process fluids (9) & (10)

+

Now take the initial values for the tentative set of parameters, like

No. of passes in the tube side = 2,

Overall heat transfer coefficient = 600 W/m2 °c,

Type of pitch = triangular pitch,

Type of head = split ring floating head,

Type of shell side fluid = organic liquid,

Type of tube side fluid = sea water, etc.

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The follwing figures are displaying the same thing.

Figure 8.1.3 Selecting no. of passes in tube side

Figure 8.1.4 Selecting the type of pitch

Figure 8.1.5 Selecting the type of head

Figure 8.1.6 Selecting the type of shell side & tube side fluid

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Next, the spreadsheet will compute and validate the necessary and sufficient conditions for

determining the design parameter values based on both the approaches refer to the following

figures.

Figure 8.1.7 Results obtained by kern’s method

Now, choose other method of design the shell & tube heat exchanger, we get the different

result values for the shell side heat transfer coefficient & pressure drop. The follwing figures

are displaying the same thing.

Figure 8.1.8 Selecting other method of design

Figure 8.1.9 Results obtained by Bell-Delaware’s method

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8.2 CASE STUDY-2

Design an exchanger to sub-cool condensate from a Methanol condenser from 95ºC to 40°C.

Flow-rate of methanol 100,000 kg/h.

Now, let’s take the other tube side fluid i.e. Hexane as the coolant, with a temperature rise

from 25°C to 40°C.

Figure 8.2.1 Selecting the other coolant for same problem

Now, the spreadsheet will estimate the values of necessary physical & chemical

properties of methanol and water from its databank. The follwing figures are displaying the

same thing.

Figure 8.2.2 Physical properties of process fluids (9) & (10)

Now take the different initial values for the tentative set of parameters, like

No. of passes in the tube side = 4,

Overall heat transfer coefficient = 800 W/m2 °c,

Type of pitch = square pitch,

Type of head = fixed & U-tube head,

Type of shell side fluid = organic liquid,

Type of tube side fluid = organic liquid, etc.

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The follwing figures are displaying the same thing.

Figure 8.2.3 Selecting no. of passes in tube side

Figure 8.2.4 Selecting the type of pitch

Figure 8.2.5 Selecting the type of head

Figure 8.2.6 Selecting the type of shell side & tube side fluid

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Next, the spreadsheet will compute and validate the necessary and sufficient conditions for

determining the design parameter values based on both the approaches refer to the following

figures.

Figure 8.2.7 Results obtained by kern’s method

Now, choose other method of design the shell & tube heat exchanger, we get the different

result values for the shell side heat transfer coefficient & pressure drop. The follwing figures

are displaying the same thing.

Figure 8.2.8 Selecting other method of design

Figure 8.2.9 Results obtained by Bell-Delaware’s method

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UNIT-9

SUMMARY

Ø A Shell & Tube Heat Exchanger Design Spreadsheet is easy to learn, & offers a quicker

way for preliminary calculation of the Shell & Tube Heat Exchanger design.

Ø By using spreadsheet method, we obtain a quick view of results in tabulated format,

which is easy to visualize the result.

Ø A Spreadsheet also eliminates human error in doing the iteration calculation, which is

commonly used in design calculation.(11)

Ø In addition to this, it can prove to be a vital tool for practicing Chemical Engineers and

Designers for the design of Shell & Tube Heat exchanger with the minimum inputs.

Ø Here the by using the lookup function of excel, we can easily visualize the physical

properties of different compounds, by selecting corresponding compound by the use

component dropdown function.

Ø This method also offers an easy way for the designer to scale up and optimize the

process.

Ø The Spreadsheet also provides a cheaper alternative to the designer compared to costly

commercial software. & also gets the transparent results of the given process.

Ø For lab level or pilot plants, this spreadsheet method provides a less costly path of

examining the changes in parameters for changes in process conditions.

• Here in the case study-1 of methanol in shell side & water in tube side, gets the same

area for the two different approaches i.e. kern & Bell-Delaware. But there is

difference in pressure drop & heat transfer coefficient on the shell side.

• Now, in the case study-2, by changing the other tube side fluid i.e. Hexane as a

coolant, gets the different values for the design parameters.

Ø Although this is a primitive stage, the calculations can be extended to include more

complicated systems and design.

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Ø The present work is an initiation to compare the fundamental and practical approaches

on a common platform using spreadsheet. Owing to the salient features of spreadsheet,

the computation tool developed can be extensively used for teaching the concepts of

certainly augment the traditional classroom teaching.

Ø This type of spreadsheet application can serve the dual purpose of being helpful to the

students in understandings the effects of different operating variables on the system and

the industrial personnel for the verification of the design aspects.

Ø Further, the scope can be expanded if spreadsheet application is merged with mechanical

designing aspects of Heat Exchanger, tubes, tube sheets, head, nozzles, etc. this will

make it a complete spreadsheet design application for the Shell & Tube Heat Exchanger

designing.

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REFERENCES

1. Coulson & Richardson’s Chemical Engineering, vol.6., “Chemical Engineering

Design”, 4th Edition, R.K.Sinnott, (2005).

2. Kern, D.Q., “Process Heat Transfer”, McGraw-Hill, New York, (1950).

3. Taborek, J., Evolution of heat exchanger design techniques, Heat Transfer Eng. 1,

No. 1, 15-29 (1979).

4. Sadik Kakac & Hongtan Liu, “Heat Exchangers, Selection Rating & Design”, CRC

Press, 2nd Edition, (2002).

5. Bernard V. Lineage, “A Guide to Microsoft Excel for Scientists and Engineers”,

Second Edition, (2000).

6. P. C. Piela, T. G. Epperly, K. M. Westerberg and A. W. Westfzrberg, “Ascend: an

object-oriented computer environment for modeling and analysis: The modeling

language”, Computers & Chemical Engineering, Vol. 15, No. 1. pp. 53-72, (1991).

7. Prausnitz JM, Lichtenthaler RN, De Azevedo EG, Molecular Thermodynamics of

fluid-phase equilibria, 3rd edition, Pretence hall PTR, New Jersey, (1999).

8. M. F. Cardoso, R. L. Salcedo, S. Feyo de Azevedo, D. Barbosa, “Optimization of

reactive distillation processes with simulated annealing”, Chemical Engineering

Science,55,pp. 5059-5078, (2000).

9. Don W. Green & Robert H. Perry, “Perry’s Chemical Engineer’s Handbook”, 8th

Edition, McGraw-Hill, New York, (2008).

10. PERRY, R. H., GREEN, D.W. and MALONEY, J. O. “Perry’s Chemical Engineers

Handbook”, 7th edition, (McGraw-Hill), (1997).

11. Jolius Gimbun, A.B.Dayang Radish, T.G. Chuah, Bioreactor design via spreadsheet-

a study on the monosodium glutamate (MSG) process, journal of food engineering,

64, p.277-283, (2004).