BOARD AND PACKAGING HEADBOX TECHNOLOGYFOR THE 21STCENTURY
AuthorsPeter Haider and Robert Hutterer
Company and addressVoith Paper GmbH & Co KG, Paper Machine
Division Board and Packaging, P.O. Box 168, 3100 St. Poelten /
Austria
[email protected]@voith.com
Keywordsheadbox, lamella, microturbulence, dilution
ABSTRACTThe headbox as the link from the approach flow system to
the forming section is one of the most important tools for reaching
best paper sheet properties. To fulfill best sheet quality criteria
such as CD basis weight profile, fiber orientation, homogeneous
sheet structure, and so on, largely depends on the headbox
concept.The hydraulic concept, geometric dimensions and the
accuracy of the headbox are the key factors for optimum uniformity
of the sheet structure.The C-clamp design of the headbox ensures
that the slice opening remains parallel irrespective of the
pressure inside the nozzle and the jet velocity.The Voith Paper
dilution control system ModuleJetTMand the control system
ProfilmaticTMM on the headbox ensures the independent optimization
of the paper sheet properties CD basis weight profile and fiber
orientation.To deflocculate fiber flocs, the optimum intensity of
the microturbulence is the key parameter. Therefore the headbox is
equipped with replaceable inserts in the turbulence generator to
optimize the hydraulic conditions for any operating conditions for
board and packaging paper machines.The lamella technology,
available for all Voith Paper headbox types, further improves the
jet quality and influences the MD/CD ratio. By installing lamellas
in the headbox nozzle, the microturbulence in the jet will be
optimized and the macroturbulence in the jet will be eliminated in
the same time. In this way, "Tigerstripes" on board and packaging
papers can get rid of.The two-layer headbox technology, mainly used
on packaging paper machines, additional demands like layer purity
and the structured sheet formation are very important. To separate
the both layers inside the headbox nozzle, a rigid separating
element is used. Thus this rigid element, different jet velocities
in the layers can be realized. This additional feature, running the
headbox in "differential speed operation", allows further
improvements in paper sheet properties.INTRODUCTIONThe headbox, as
the link between the approach flow system and the forming section,
is one of the most important tools for reaching a homogeneous sheet
structure. Its main tasks are to distribute the stock uniformly in
cross direction and to create a homogeneous jet for achieving an
optimum sheet structure in terms of physical properties. Even stock
distribution with regard to uniform volume flow and stock
consistency in cross direction is one of the important criteria for
an optimized headbox. Another important criterion for good headbox
performance is the carefully optimized intensity of the jet
turbulence.To satisfy the high demands on product quality, optimum
flow guidance of the stock within the headbox is a fundamental
requirement. The main key factors and the basis for achieving a
uniform jet, and thus a uniform sheet structure, are the hydraulic
concept and the geometric dimensions and stability.Another
important task of a modern headbox is the possibility to adjust the
CD basis weight and fibre orientation profiles independently, thus
further improving the product quality.MECHANICAL DESIGN ASPECTSThe
quality and stability of the CD profiles, especially the fibre
orientation profile, depend largely on the precision and stability
of parallelism of the slice opening. To achieve a parallel and
stable slice opening under operating conditions, all Voith Paper
headboxes are designed according to the C-clamp principle (Figure
1).
Figure 1 C-clamp designThe nozzle forces due to the stock
pressure are introduced over a minimum distance via the turbulence
generator into the apron board. Due to this closed flow of the
forces inside the headbox, edge support of the top lip beam via the
lateral parts is avoided. The C-clamp design ensures that the lip
opening remains parallel irrespective of the pressure inside the
nozzle and of the jet velocity.The parallelism of the slice opening
under operating conditions is set within just a few hundredths of a
millimetre prior to the delivery of Voith Paper headboxes, thus
ensuring the best and most stable CD profiles right after start-up
(Figure 2).
Figure 2 Parallel slice opening under operating conditionsMODERN
HEADBOX HYDRAULICS:GENERAL PRINCIPLES OF HEADBOX HYDRAULICSThe main
components of a modern hydraulic headbox (either for fourdrinier or
gapformer) for an optimum flow guidance and intensity of
microturbulence are (in the direction of flow): Parabolic header
with constant pressure from tending side to drive side, for uniform
stock distribution across the headbox width. On fourdriniers and
hybrid formers, a pulsation dampening tank is installed in front of
the headbox. ModuleJet...dilution control system with dilution
header, valves with stepper motor, mixing chambers, throttles and
distributor blocks. Stilling chamber for optimum flow guidance and
flow uniformity. Turbulence generator, using the step diffuser
principle, to create an optimum intensity of microturbulence.
Nozzle with or without lamellas (depending on product quality
requirements) to accelerate the stock flow to jet velocity and to
avoid macroturbulence.The flow channel inside a state-of-the-art
headbox is shown schematically in Figure 3.
Figure 3 Flow channelUniform flow distribution in cross
directionA parabolic header of circular shape ensures uniform flow
distribution across the entire headbox width. Due to the parabolic
design, a constant pressure inside the header from tending side to
drive side is achieved.On a modern headbox, the possibility of
independent correction of basis weight profile and fibre
orientation profile is out of discussion. The fibre orientation
profile can be optimized by the slice lip, the basis weight profile
can be influenced through the ModuleJetTMdilution water system
(Figure 4) with ProfilmaticTMM control.
Figure 4 ModuleJetTMunitsThe ModuleJetTMunit adds dilution water
locally across the full headbox width. The dilution water white
water 1 is metered by an optimally designed valve. This design
avoids dead water areas inside the valve, and thus no solids can
deposit even in the closed position of the valve, because the valve
is always opened a minimum and flushed.The stock flow from the main
header and the dilution water flow from the dilution water header
are joined locally in the mixing chamber. A throttle at the mixing
chamber outlet ensures optimum mixing of the two flows, resulting
in a homogeneous average local stock consistency (Figure 5).
Figure 5 ModuleJetTMunitBefore the suspension enters the
turbulence generator, flow disturbances are reduced in cross
direction in the adjacent machine-wide stilling chamber.Since the
ModuleJetTMsystem was developed (nearly 10 years ago), the number
of installed ModuleJetTMheadboxes has increased continually (Figure
6). A ModuleJetTMheadbox is state-of-the-art on modern paper
machines. The ModuleJetTMvalve and the smart ProfilmaticTMM control
systemhave made Voith Paper the market leader in
dilutiontechnology.
Figure 6 Success of ModuleJetTMheadboxesOptimum free jet
qualityThe main goal for creating a uniform and optimum sheet
structure is an ideal free jet without any macroturbulence and with
a defined microturbulence. This process begins at the inlet of the
turbulence generator and ends in the free jet.The turbulence
generator, which is the heart of the headbox, distributes the stock
once more uniformly across the headbox width and generates
controllable and optimum turbulence, using the hydraulic principle
of the step diffuser to break up fibre flocs.The turbulence tubes
have a circular inlet and a square outlet. The inserts at the inlet
of the turbulence generator produce an optimum pressure drop. These
inserts can be replaced when upgrading the headbox later on.To
generate the desired turbulence intensity and a satisfactory free
jet, the position of the steps inside the turbulence tubes was
optimized in a Voith Paper development project.A schematic sketch
and a photo of a turbulence generator with lamellas are shown in
Figure 7 and Figure 8.
Figure 7 Turbulence generator with lamellas
Figure 8 Turbulence generator with lamellasThe lamella
technology for optimum free jet qualityThe main goal of the Voith
Paper R&D project avoiding macroturbulence and, at the same
time, optimizing microturbulence was reached by installing lamellas
in the nozzle. Without lamellas in the nozzle, the macrostructures
of the free jet result especially in the formation of more or less
irregular stripes. These stripes, called tiger stripes, are visible
as gloss differences at certain angles of light incidence.Figure 9
shows the effect of tiger stripes on the sheet quality.
Figure 9 Effect of macroturbulence on sheet qualityThese tiger
stripes are caused by local fibre orientation non-uniformity in the
surface of the sheet because of crossflows at the impingement point
induced by the macrostructures in the free jet. The interactions
between flow velocity profiles out of the single turbulence tubes
are unsteady in three dimensions, thus forming eddies inside the
nozzle.These eddies formed by the unsteady flow process can be seen
as macroturbulence in the free jet, causing tiger stripes in the
sheet.The flow configuration inside the headbox nozzlewithout
lamellas is shown in Figure 10, and Figure 11 shows the effect of
eddies on the free jet.
Figure 10 Flow configuration inside the headbox nozzle without
lamellas
Figure 11 Structure of macroturbulence in the jetThe
prerequisite for an optimum sheet is an ideal free jet without
macroturbulence and with a defined and optimized free jet.To reach
this target of an optimum free jet, the lamella technology is the
solution. On the one hand, the lamellas eliminate the
macroturbulence; on the other, lamellas increase the intensity of
microturbulence. Depending on the microturbulence intensity, the
MD/CD tensile ratio can be adjusted.The lamellas of different
lengths (60% up to nearly 99% of nozzle length) are mounted between
each row of the turbulence generator. Because of their specific
mounting, the lamellas are self-centered inside the nozzle due to
the stock flow (Figure 12, Figure 7 and Figure 8).
Figure 12 Lamellas in the nozzleInstalling lamellas between each
row of the turbulence generator will reduce the degree of freedom
of the flow inside the nozzle. Interactions between the single
turbulence tubes will occur only in cross direction. The
three-dimensional interaction in cross- and z-directions of the
single flows at the turbulence tube outlet will be avoided (Figure
13), thus eliminating the eddies inside the nozzle and resulting in
a very stable flow with optimized microturbulence.
Figure 13 configuration inside the headbox nozzle with
lamellasThe hydraulic effect of various lamella lengths on the
quality of the free jet and the correlation between lamella length
and macro- and microturbulence are clearly shown in Figure 14.
Figure 14 Turbulence structure depending on lamella lengthThe
macroturbulence in the free jet decreases as the length of the
lamella is increased. At the same time, the microturbulence
increases as the length of the lamella is increased. This effect
allows to reduce the MD/CD tensile ratio. For most packaging
grades, a low MD/CD tensile ratio is one of the targeted sheet
properties (Figure 15).
Figure 15 Influence of lamellas on MD/CD tensile ratioRegarding
the free jet quality when long lamellas are used, the tip of the
lamella is the most important design criterion.Lamella vibrations
and flow disturbances caused by flow eddies right behind the
lamella tip can be avoided by a special design of the lamella tip
patented by Voith Paper in Europe.Using a long lamella (lamella
length more than 80% of nozzle length) with a flat tip will cause
the end of the lamella to vibrate, thus resulting in disturbances
in the free jet and also in the sheet structure. There are two
kinds of vibrations on long lamellas without serrated tip:1)
Vibrations along the width with steady nodes on the lamella tip
result in steady streaks in the free jet (Figure 16).2) Vibrations,
as described under 1, are superimposed by vibrations of the lamella
across the whole lamella width. These combined vibrations cause a
rhomb structure in the free jet (Figure 17).
Figure 16 Effects on the free jet structure by using
non-optimized lamella tip on long lamellas
Figure 17 Effects on the free jet structure by using
non-optimized lamella tip on long lamellasTo prevent such effects
as shown in Figure 16 and Figure 17, Voith Paper has developed a
specially designed lamella tip. The tip of the lamella has a very
fine saw tooth structure on the last few millimetres at the end of
the lamella. This special tip allows to install long lamellas
ending just a few centimetres before the slice lip.Combining long
lamellas with a length of nearly 99% of the nozzle length and short
lamellas with approximately 60% of the nozzle length arranged in a
certain pattern gives the optimum jet quality and, as a result,
optimum sheet quality without macroturbulence and with an optimized
intensity of microturbulence. The free jet quality improvements are
shown in Figure 18.
Figure 18 Improvements in jet quality due to lamellasFigure 19
shows the convincing result of the lamella technology in the sheet.
This comparison makes clear that only optimal flow guidance inside
the headbox leads to complete elimination of tiger stripes.
Figure 19 Improvements in sheet quality due to
lamellasMULTI-LAYER TECHNOLOGY FOR GAPFORMERThe multi-layer
technology used for packaging paper grades has the advantage to
produce a stratified sheet on one forming unit only. Due to this
technology, different furnishes can be placed on the top and bottom
sides of the two layers. Compared to multi-ply concepts, more
mixing between the two layers leads to better plybonding.Figure
20shows the design of a MasterJet M2 two-layer headbox in operating
and cleaning positions. By opening the whole nozzle area including
the turbulence generator, machine -wide access is possible to the
inserts of the turbulence generator and to the stilling
chamber.
Figure 20 MasterJet M2 two-layer headboxOn the two-layer
headbox, the two layers are separated through a rigid separating
element with an exchangeable tip. This system patented by Voith
Paper allows to operate the headbox in the differential speed mode.
The rigid separating element provides mechanical stability even at
high differential pressures in both layers, thus allowing different
jet velocities in the layers.With other types of layer separating
by a flexible lamella, differential speed operation is not possible
because of bending of the lamella due to pressure differences.A
comparison of Voith Paper's rigid separating element and a flexible
lamella is shown in Figure 21.
Figure 21 Two-layer technology Comparison of rigid separating
element and flexible lamellaOn gapformers, some portion of the jet
energy of the outer layer is used to open the gap between the two
wires (Figure 22).
Figure 22 Differential speed operation and operation with
identical jet velocitiesIf the headbox is operated with identical
jet velocities in both layers, a difference in the jet-to-wire
ratio between the outer and inner layers will occur, resulting in a
higher z -orientation in the outer layer and thus a higher
plybond.The possibility of differential speed operation neutralizes
the effect of different jet-to-wire ratios, thus achieving almost
the same z-orientation in the inner and outer layers and a lower
plybond compared to operation with identical jet velocities.With
differential speed operation, however, several improvements in
strength properties can be achieved and are confirmed in trials on
the pilot paper machine.Operating the two-layer headbox with a
higher speed on the outer layer will improve the strength
properties as follows: Higher breaking lengthgeom. mean Higher
RCTCD Higher SCTCDThe two-layer technology using a rigid separating
element and the possibility of differential speed operation, in
combination with the lamella technology, is a big step forward in
papermaking and gives the maximum flexibility and potential for
optimizing the papermaking process.SUMMARYThe demands on sheet
uniformity and sheet properties are mainly based on the quality of
the headbox. The geometric dimensions and stability as well as the
hydraulic concept determine the quality of the headbox.To reach all
state-of-the-art sheet strength properties as well as surface
properties, the free jet quality is one of the most important
criteria.The volume flow rate and the stock consistency
distribution in cross direction, combined with a uniform and
optimum microturbulence generated in the stock, result in an
optimum free jet quality.Due to the well-tuned system of turbulence
generator, nozzle geometry and lamella technology, a distinct
improvement in free jet quality and also in sheet quality is
achieved.The multi-layer technology, in combination with the
optimized hydraulic concept with lamella technology, allows to
produce a specific stratified sheet on one forming unit only.Due to
this optimized hydraulic concept with lamella technology in
combination with the optimum mechanical design, slogans like
optimum free jet quality or optimum sheet quality are no longer
slogans, but will become reality now and in the future.