The 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 ModuleJetTM and the control system ProfilmaticTM M 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.
The 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 ASPECTS
The 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 design
The 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 conditions
MODERN HEADBOX HYDRAULICS: GENERAL PRINCIPLES OF HEADBOX HYDRAULICS
The 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
- 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 channel
Uniform flow distribution in cross direction
A 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 ModuleJetTM dilution
water system (Figure 4) with ProfilmaticTM M control.
Figure 4 ModuleJetTM units
The ModuleJetTM unit 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 ModuleJetTM unit
Before the suspension enters the turbulence generator, flow disturbances are reduced in cross direction in the adjacent machine-wide stilling chamber.
Since the ModuleJetTM system was developed (nearly 10 years ago), the number of installed
ModuleJetTM headboxes has increased continually (Figure 6). A ModuleJetTM headbox is state-of-the-art on modern paper machines. The ModuleJetTM valve and the smart ProfilmaticTM M control system have made Voith Paper the market leader in dilution technology.
Figure 6 Success of ModuleJetTM headboxes
Optimum free jet quality
The 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 lamellas
The lamella technology for optimum free jet quality
The 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 quality
These 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 nozzle without 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 jet
The 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 nozzle
Installing 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 lamellas
The 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 Turbulence structure depending on lamella length
The 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 ratio
Regarding 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 lamellas
To 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 lamellas
Figure 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 lamellas
MULTI-LAYER TECHNOLOGY FOR GAPFORMER
The 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.
shows 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 headbox
On 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 Two-layer technology Comparison of rigid separating element and flexible lamella
On 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 velocities
If 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 length geom. mean
- Higher RCTCD
- Higher SCTCD
The 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.
The 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.
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