EP0195807B1 - Headbox system for a paper machine - Google Patents

Abstract

In a headbox system for a paper machine with an adjustable distribution of the pulp over the width of the web the flow in the delivery gap (8) is controlled in such a way that, over the width of the web, also the distribution of the fibres in respect of fibre orientation and quantity has a desired profile. For this, the flow of the pulp suspension is so controlled that in the event of a change on the pulp distribution over the width of the web the horizontal component of the flow direction in the delivery gap (8) remains, at all places, always in the direction of the machine, and thus transverse flows are avoided and the flow speed in the delivery gap (8) has a preset profile.

Description

Technical field

The invention relates to a headbox device for a paper machine with a distribution box for distributing a stock suspension fed over the web width of the paper machine, a guide device for the stock suspension having a plurality of holes or channels and a subsequent nozzle space with an adjustable outlet gap for distributing the stock suspension over the Web width of the paper machine, the mass distribution over the web width of the paper machine being adjustable to a predetermined basis weight over the width of the paper machine and for which purpose branches and a collecting container for the adjustable removal of the stock suspension and optionally branches for the adjustable feeding of the suspension in the distribution box are provided on the distribution box.

State of the art

Headbox devices of this type are known, for example, from US Pat. No. 4,087,321. They are used to feed a prepared pulp suspension to a paper machine and to distribute it over its entire web width in a certain way. This distribution of the stock suspension should take place in such a way that the paper web produced with the paper machine has certain properties across the web width, whereby usually an even basis weight (mass per unit area) after drying, an even moisture content, and an even fiber orientation over the entire web width are aimed for .

It is known from US Pat. No. 3,556,935 or US Pat. No. 4,089,739 to make the width of the outlet gap of the nozzle space adjustable by means of a plurality of adjusting devices distributed over the web width. This allows you to control the amount of material suspension over the web width, but not the fiber orientation.

Inadequacies that lie in the construction of the paper machine, as well as physical processes during the paper production process, are the reason why not all of the desired properties across the web width are usually at the same time as desired or even. For example, defects in the wire section or the shrinking of the paper web during drying, especially at the web edge, are compensated for by changes in the local mass flow, as are deficiencies in the geometry of the distributor, the guide device or the nozzle area. In order to change or set a specific local mass flow, the width of the outlet gap from the nozzle space is often changed locally. However, this also changes the pressure in the nozzle area locally and the pressure curve in the fiber suspension is uneven across the machine width. These local pressure differences in the nozzle space lead to cross-flows in the nozzle space, which have an effect right up to the outlet gap, in such a way that the direction of the flow in the nozzle space, seen across the width, is not exactly parallel and corresponds to the machine direction. Even small deviations from the machine direction lead to undesired nonuniformities in the fiber orientation of the paper web produced across the web width because of the transverse components of the speed vector which exist when the suspension jet emerges.

From AT-363 776 it is already known to compensate for pressure differences in the flowing stock suspension in the distributor over the web width by adding or subtracting stock suspension at suitable points, for example in the distribution box, so that the pressure curve in the distribution box over the entire Web width is constant. However, this measure alone cannot compensate for all pressure differences in the nozzle area.

From US-3,573,160 it is known to measure the speed profile of the stock suspension over the web width and the cross-section of the distributor, e.g. by means of an adjustable wall to adjust accordingly in order to achieve a uniform speed of the suspension in the outlet gap. From the European patent application EP-A 0 029 905, a similar solution for influencing the pressure profile in the distributor can be found, in which the shape of the distributor rear wall can be changed. These solutions too can only imperfectly prevent pressure differences and cross-flows.

As described in G B-1 216 114, attempts have already been made to achieve a uniform flow of material by means of several individually adjustable overflow pipes on the distribution box, whereby the disadvantages mentioned cannot be avoided either.

For example, additional pressure losses occur in the nozzle area at the edges due to friction losses on the side walls. which cause the pressure in the nozzle chamber and the flow of material through to the edge to drop. This also causes cross currents in the nozzle area, which means that the current threads are no longer aligned exactly parallel in the machine direction.

In DE-A-2151 906 it is proposed to achieve a uniform speed and quantity of material also in the edge area by providing a gap that can be individually adjusted over the width on the distributor. Differences in pressure and cross flows in the outlet gap are also present here.

Even in the case of several rows of bores or channels in the guide device which are offset with respect to one another, as disclosed, for example, in US Pat. No. 4,137,124, there are non-uniformities in the edge regions, such as lower material flows per unit of width, which also lead to pressure differences and cross-flows in the du and the gap. This could be compensated for by supplying additional material suspension or water on the sides through the side walls of the nozzle space, but this requires considerable mechanical and control-related expenditure.

A device according to US 4,285,767 shows the inflow of additional liquid into a high-consistency headbox. In another perforated roller headbox of the older type described by US 2 904 461, additional fibrous material is fed through a slot located at the bottom across the entire width.

From FR 1 519 450 it is also known to specifically add dilution water to the local stock density control in the headbox.

In the German design step DE-AS 11 65 397 it is described how the air which separates at the top of the perforated roller headbox can also be discharged in a controlled manner.

Even if these solutions can have an effect on the direction of flow at the outlet gap of the headbox, these effects cannot be controlled to a sufficient extent and cannot be separated from the other parameters that are also essential for the production of paper.

The invention sets itself the task of eliminating the aforementioned disadvantages of the prior art and, in particular, of creating a headbox device for a paper machine with which, despite the change in the local mass flow rate necessary to set the desired transverse profiles of the basis weight, a flow state in the nozzle chamber and reached at the outlet gap, which is free of cross flows. A paper web produced in this way should have a uniform fiber orientation across the web width.

Presentation of the invention

With the device according to the invention, this object is achieved by measures which are given in the characterizing part of claim 1.

These setting options ensure that the flow in the outlet gap is influenced in such a way that when the mass distribution is set to a desired cross profile, no cross flows nevertheless occur, and thus a paper with the desired mass distribution and with the desired, e.g. uniform fiber distribution with regard to quantity and orientation over the web width is obtained. However, it is conceivable that, if it were desired, other flow forms in the headbox device could also be set with the means according to the invention.

Brief description of the drawings

• Figur 1 zeigt eine Stoffauflauf-Vorrichtung in der Aufsicht,
• Figuren 2-4 zeigen verschiedene Ausführungen von Verteilkästen mit einstellbarem Druckverlauf in zwei Vertikalschnitten,
• Figur 5 zeigt eine Stoffauflauf-Vorrichtung im Vertikalschnitt,
• Figur 6 zeigt eine Stoffauflauf-Vorrichtung mit verschiedenen möglichen Zugabe- oder Entnahmestellen für Stoffsuspension oder Zugabestellen für Wasser,
• Figuren 7 und 8 zeigen Führungsvorrichtungen mit unterschiedlicher Geometrie über die Bahnbreite,
• Figuren 9-11 zeigen verschiedene Anordnungen von Zuleitungen und Abzweigungen an einem Stoffauflauf.

The invention and advantageous developments thereof are explained in more detail with reference to the exemplary embodiments shown in the figures.

• FIG. 1 shows a headbox device in a top view,
• FIGS. 2-4 show different designs of distribution boxes with adjustable pressure profiles in two vertical sections,
• FIG. 5 shows a headbox device in vertical section,
• FIG. 6 shows a headbox device with various possible addition or removal points for stock suspension or addition points for water,
• FIGS. 7 and 8 show guide devices with different geometries over the web width,
• Figures 9-11 show different arrangements of feed lines and branches on a headbox.

Description of exemplary embodiments of the invention

The headbox device shown in Figure 1 has a distribution box 1, the cross section of which decreases in the direction of flow F of the stock suspension. The substance suspension is e.g. to understand a mixture of fibers with additives for paper or cardboard manufacture, but also filler suspensions for paper coating. At the end of the distributor with the larger cross section, a connecting pipe 2 is connected for supplying the stock suspension, and at the other, opposite end with the smaller cross section, a return line 3. The distributor preferably has an approximately conical shape or can be designed as a truncated pyramid.

A guide device 5 is connected to a longitudinal side 4 of the distributor 1. In principle, this can be designed as any guiding device customary in paper machine technology, for example, as shown in detail in the figure, as a step diffuser with step-widening parallel channels 6, e.g. according to CH 518406 (US 3725197).

At the exit of these channels 6 there is a nozzle chamber 29 with an outlet gap 8, this being formed by two lips 7 or by a lip and a diaphragm mounted on the lip but freely movable relative to it. The stock suspension now passes through the channels 6, the nozzle chamber 29 and the outlet gap 8 to the wire section of the paper machine.

At least one of the lips 7 or an orifice attached to it is provided with a plurality of adjustment devices 30 distributed over the width, with which the gap width can be adjusted individually over the width manually or by means of a regulating device in order to flow with the material suspension flow rate flowing out of the gap 8 Set the desired oven-dry basis weight profile of the paper web thus produced over the width. However, if the gap width is changed locally via adjusting devices 30 at the outlet gap 8, the local flow conditions, such as pressure and Ge, also change in addition to the local mass flow and the basis weight profile speed in the nozzle area. The result of the pressure differences are cross flows in the nozzle space, which are present up to the outlet gap 8 and act on the fiber storage on the sieve of the paper machine until the entire fiber structure is fixed in the sheet. Because of the different cross currents across the web width of the machine, there is then a different fiber storage in the sheet over the web width. In order to prevent such cross-flows of the stock suspension, certain precautions are necessary. One of these options is to readjust or readjust the pressure of the stock suspension over the web width of the paper machine or the width of the nozzle area while maintaining the set gap width profile. On the other hand, however, the gap width can also be kept constant and instead the material density can be changed locally so that that with constant pressure across the width, the desired cross profile, for example the oven dry basis weight, is created in the paper web. To do this, it is necessary to add or remove material suspension or water at suitable points across the width.

Similar to gap width adjustments, another phenomenon has an effect, which arises from the fact that at the edges of the paper machine, as a result of frictional losses on the side walls 9, there is also a pressure loss in the stock suspension. Instead of compensating for this pressure loss by supplying suspension or water through the side walls 9 through additional lines 10, which means considerable mechanical and control expenditure, the required compensation can be carried out in a simpler and more precise manner by the same pressure control or consistency control the same change in geometry takes place as is required to maintain a certain basis weight profile with uniform fiber orientation. In the same way, the shrinkage that occurs at the edges when drying paper webs can also be compensated for, so that a paper web is produced that is uniform over the entire web width.

Pressure compensation can be carried out individually or in combination with various measures. One possibility for pressure compensation over the web width of the machine is that, as shown in FIG. 1, on a longitudinal side of the distribution box 1, distributed over its length, a plurality of branches formed as branch lines 11, 12, 13 and 14 are connected to the inside of distributor 1 are connected. In these branch lines, controllable valves 15, 16, 17, 18, 19 are provided, by means of which the flow of the branch lines and the return line can be changed. The branch lines 11, 12, 13 and 14 and the return line 3 are e.g. connected to a common manifold 20. The material suspension branched off at the individual connection points of the branch lines and via the return line can be discharged via this. Depending on the amount of suspension removed in the individual branch lines, the hydrostatic pressure profile in the distributor 1 changes from the location of the connection of the relevant branch lines. The extent of the change in pressure can be adjusted by means of the valves 15, 16, 17, 18 and 19, so that by actuating these valves it is possible to set the required pressure curve over the length of the distributor or the width of the paper machine. The shape of the distributor 1 can initially roughly predefine a specific pressure profile in the distributor, so that only the remaining deviations need to be corrected by actuating the valves.

The pressure setting at the various setting points can either be done manually, by means of a visual pressure indicator or after observing the paper web generated or according to the measurement results for certain paper properties, which were determined with suitable sensors across the width of the paper web, e.g. the basis weight, or the fiber orientation, or, as indicated in FIG. 1, with the aid of a control device 26 connected to the various measuring devices. For this purpose, pressure measuring devices 21, 22, 23, 24 and 25 are provided along the flow direction F on the distributor 1, with which the hydrostatic pressure of the suspension in the distributor 1 or its course in the longitudinal direction is determined. In principle, such pressure measuring devices can be designed and arranged as desired, wherein the pressure measurement can also take place in the nozzle area. In the simplest case, the measuring devices can be assigned to the individual valves and control them directly, or the measuring devices can supply their measured values to the control device 26 independently of the valves, which controls and adjusts the individual valves in a suitable manner, for example by means of a process computer, until as long as the desired pressure prevails at all measuring points.

The pressure profile specified by the control device 26 can be selected so that the pressure shows a desired course, for example is kept constant over the entire width, or certain edge corrections can be included, for example a somewhat higher pressure at both edges, to prevent pressure losses to compensate for the side walls. In addition, the control device 26 can also be controlled by further sensors, not shown, which, for example, determine or measure the thickness, the basis weight, the curl (curl) or the fiber orientation of the paper produced along the width of the paper web and deliver corresponding additional control signals 28 by means of an additional pressure correction can be carried out or a pressure profile can be specified. If suitable operating personnel are available, such a control can also be carried out in a simpler manner manually or semi-automatically by observing the measured values of the individual measuring devices and adjusting accordingly. Such a simpler control is often sufficient, since a one-off adjustment after starting up a paper machine is usually sufficient and readjustment is only necessary if operating parameters, e.g. the pressure or the composition of the stock suspension, or if the production is changed , for example if the machine is switched to a different type of stock suspension, which results in a change in the pressure distribution. With known headbox devices, this was only possible with difficulties and complications.

The cross section of the branch lines 11, 12, 13 and 14 can be different, depending on the necessary correction. Likewise, the mutual distance between these branch lines can be selected differently. However, it is also readily possible to provide lines of the same diameter at the same distance from one another, in a number that ensures the necessary controllability.

Instead of discharging part of the stock suspension via the branch lines, the branch lines can also be used to supply additional stock suspension. In this case, a feed line is to be provided instead of a return line 20. This can be connected to the supply line 2, which is present anyway, or else to a supply line separate therefrom, the devices for flow control 15, 16, 17, 18, 19 being designed as suitable, controllable pumps. If necessary, however, only a part of the branches can be used to remove stock suspension, while the other branches can supply stock suspension. The control members can be designed so that the flow direction and the flow rate can be controlled.

FIG. 2 shows a variant in two vertical sections, in which the distribution box 1 is connected to a collecting container 20 via connecting lines 11-14. The height of the end of the connecting lines 11, 12, 13, 14, which goes into the collecting container 20, is adjustable, for example by sleeves 66 which can be displaced from the outside or by a telescopic design of the connecting lines which extends over at least part of the connecting lines. This creates a weir that is adjustable in height and thus makes the local pressure in the distributor individually adjustable.

Figure 3 shows a modification in which the connecting lines 11, 12, 13 are designed as slots with underlying, adjustable surface elements 60, 61 and 62, which serve as overflow weirs. By adjusting these surface elements, the effective weir height can be changed locally, so that a local pressure correction in the distribution box 1 also takes place here.

Another possibility for influencing the pressure conditions is shown in FIG. 4, in which material is not drawn off via individual branch lines arranged across the machine width, but via an outlet slot 64 between distribution box 1 and collecting container 20, the outlet slot 64 covering the entire machine width goes. A vertical overflow weir 63 extending over the entire width is also provided in the collecting container 20, the height of which can be adjusted locally over the width from the outside. A change in the hydrostatic pressure in the distributor or in the nozzle can thereby be achieved simply by changing the weir height of the outlet slot 64. Instead of a vertical weir, a displaceable screen 65 can also be provided, by means of which the outlet slot 64 can be individually adjusted over the width.

Instead of attaching the branch lines or connecting slots to the side of the distribution box 1, these can also be provided at other points of the headbox device, for example on the diffuser block 5, or in the nozzle chamber 29 immediately after the diffuser block or just before the outlet gap 8 or in between.

FIG. 5 shows an analog headbox device in vertical section, in which a distributor 1, a diffuser 5 with a row of parallel channels 6, and a nozzle chamber 29 with an outlet gap 8 are again provided. The stock suspension flows from the nozzle chamber 29 onto the screen 32 of a paper machine which is guided over a breast roller 31. The front part 33 of the upper delimitation lip 7 of the nozzle chamber 29 can be individually adjusted over the width by means of a joint 34 and by adjusting device 35 distributed over the width of the outlet gap 8. In many cases, the exit gap is delimited at the top by a diaphragm which is mounted on the upper lip, but is freely movable to a certain extent in relation to the upper lip. The gap width can then be changed using the adjustment devices attached to the panel. As a result, the amount of suspension emerging from the gap can be controlled locally independently over the width. The supply or discharge of fibrous suspension again takes place via branch lines 11 on the distributor 1 and also or optionally via branch lines 51 on the nozzle chamber 29, valves or pumps 36 or the designs shown in FIGS. 2-4 being provided for quantity regulation.

As shown in FIG. 6, further possibilities are that the branch lines are provided either selectively or in combination on the distributor 1, on the diffuser block 5 or on the nozzle chamber 29. Changes in the pressure curve can be compensated for as close as possible to the point of origin. In the example shown, the possibility is provided to provide branch lines 37 on the upper side of the distributor 1 or at another location. Instead, the branch lines can also be led directly out of the diffuser block 5. In the example shown, lines 39 are in the first diffuser stage, lines 40 into the second diffuser stage immediately after the paragraph to the first stage, or lines 41 into the third diffuser stage immediately after the paragraph to the second stage. However, supply lines can also be provided in the nozzle space 29 itself, for example lines 42 in the beginning of the nozzle space, directly behind the outlet openings 43 from the diffuser channels or lines 44, which are in the vicinity of the outlet gap 8 of the nozzle space 29 in the movable part 33 of the upper part Lippe flows out. As already mentioned, these lines can be used to add or remove water or white water with a lower consistency or suspension, depending on the requirements for the purpose of pressure control or to change the consistency.

Another possibility of regulating the pressure in the distributor 1 can be created by changing the distributor cross-section, e.g. through a movably designed rear wall 45 of the distributor. The mobility can be achieved in that the rear wall 45 can be adjusted either manually or by means of a control device over the width of the distribution box, as a result of which the pressure profile in the distributor can be set, or in that the rear wall 45 is designed as a flexible and deformable membrane which is automatic , for example with the help of a pressure pad on the back so that the same pressure prevails over the entire width of the distribution box.

In combination with other adjustment and control options or alone, it may be necessary to change the nozzle geometry to achieve the same flow velocities and pressures across the width of the nozzle area. For this purpose, adjustment options 52 are necessary, which are to be provided immediately after the guide device, in front of the outlet gap or also in between, wherein the adjustment devices can also be attached at two and more positions in the machine direction.

Further control options consist in providing adjustable screens 46 in front of the inlet openings 38 of the diffuser channels 6 or corresponding adjustable screens 47 in front of the outlet openings 43 of the diffuser channels. These can consist of sliding elements, iris diaphragms, inflatable bodies or other elements. Suspension or water can also be supplied via lines 50 directly at the inlet 38 of the channels, or the channels can be narrowed by controllable pressure bodies 27.

The increased pressure loss at the edges, e.g. due to friction on the side walls 9 can be compensated for by changing the pressure loss of the guide device in the edge zones, e.g. is reduced, which is achieved by changing the passage cross section per unit width.

FIG. 7 shows an example of a diffuser block 5 with a plurality of rows of stacked diffusers arranged one above the other. To reduce the pressure loss of the diffusers, the cross section of the diffusers in the edge zone 48 on the side wall 9 is larger than in the adjoining central zone 49, so that the pressure loss in the stock suspension in the edge zone is somewhat smaller than in the middle and thus the increased Pressure losses in the peripheral zone can be compensated.

In the case of the diffuser block 5 shown in FIG. 8, the individual rows of step diffusers are offset from one another, which means that a lower nozzle pressure is created in the edge zone 48 due to the diffusers missing relative to the center 49. A larger material flow per unit of width is achieved in the edge zone by making additional bores or by reducing the pressure loss of the individual element of the guide device in the edge area, so that a pressure correction is also possible here. In this way, fundamental defects in the distribution board design or at another location on the paper machine can also be compensated for by changing and adapting the hole diameter or pressure loss across the web width.

'' Modifications and appropriate further training are possible and are within the scope of the inventive concept. Thus, as shown in FIG. 9, the branch lines 11 connected to the collecting line 20 can advantageously also be connected at the highest point of the distributor 1, which ensures that any air excretions can be safely removed. Immediately next to it, the connection for the diffuser 5 can be provided, so that the pressure control takes place in the immediate vicinity of the inlet of the diffuser.

The collecting line can also be guided directly along the distributor 1, so that the branches 11 are formed by openings which connect the distributor 1 and the collecting line 20.

FIG. 10 shows an example in which the diffuser is designed as a guide device which is known per se and which connects a distributor 1 with the lips 7 and an outlet gap 8 and which has two pipe systems 53 and which are connected in series and are separated by a space 55 bypassing the suspension 54 has. This intermediate space 55 is connected through branch openings 56 with an independently controllable or adjustable cross section to an overflow line 57, which at the same time shows a damping effect.

FIG. 11 shows an analogous example in which controllability is achieved by means of slide 58. that change the cross section of the branch openings. The openings 59 can optionally also be brought forward to the beginning of the nozzle chamber 29.

Claims (17)

1. A headbox system for a paper-making machine with a distribution box (1) for distributing a supplied pulp suspension across the width of the web of the paper-making machine, a guide device (5) for the pulp suspension having a plurality of holes or channels (6) and a subsequent nozzle chamber (29) with an adjustable delivery gap (8) for distributing the pulp suspension across the width of the web of the paper-making machine, the distribution of the pulp across the width of the web of the paper-making machine being adjustable to a predetermined weight per unit surface area across the width of the paper-making machine, and for which purpose branch pipes (11-14) and a collecting container (20) for adjustable removal of the pulp suspension and optionally branch pipes (37) for the adjustable supply of the suspension into the distribution box (1) are provided, characterised in that in order to set the flow of the pulp suspension in the headbox system with the aim that, even with a change in the distribution of pulp, by altering operating parameters, viewed across the width of the web of the paper-making machine, the horizontal component of the direction of flow of the pulp suspension at the delivery gap (8) is parallel in all places to the direction of the machine, and the rate of flow in the delivery gap (8), viewed across the width of the machine, is the same, additional branch pipes (39, 40, 41, 42, 44, 50) selectively distributed across the width of the headbox for the adjustable localised removal or supply of pulp suspension or water and/or devices (27, 45, 46, 47, 51, 52, 56, 58, 59) for the localised alteration or formation of the flow profile and the pressure profile in the related chambers of the headbox system are provided not only in the distribution box (1) but in the headbox system also between the distribution box (1) and the delivery gap (8), the flow of the pulp suspension being able to be set by changing the flow profile in the distributor, or in the guide device (5) or in the nozzle chamber (29) or by feeding in or removing pulp suspension, or feeding in water relative to the distributor (1) the guide device (5) and/or relative to the nozzle chamber (29) by the use, combined as required, of the means provided locally for this purpose.

2. A system according to Claim 1, characterised in that the branch pipes (11-14) arranged on the distribution box (1) project into the collecting container (20) with their ends which face away from the distribution box, the length of their parts which project into the collecting container (20) being changeable.

3. A system according to Claim 1, characterised in that the branch pipes (11-14, 64) on the distribution box (1) are formed with at least one opening and that an overflow weir (60, 61, 62) which is locally adjustable in height is provided at the openings in the collecting container (20).

4. A system according to Claim 1, characterised in that the branch pipes on the distribution box (1) consist of a slot (64) running in the longitudinal direction between the distribution box (1) and the collecting container (20), the width of which slot is locally adjustable.

5. A system according to Claim 1, characterised in that the branch pipes (39-41 ) and the devices (56, 58) are connected to the guide device (5, 55).

6. A system according to Claim 1, characterised in that the branch pipes (39) are connected to the first step of a channel of the guide device (5) which is constructed as a stepped diffusor.

7. A system according to Claim 1, characterised in that the branch pipes (40, 41 ) are connected to a second or to a subsequent step of a channel of the guide device (5) which is constructed as a stepped diffusor directly after a step.

8. A system according to Claim 1, characterised in that the branch pipes (59) are connected to the nozzle chamber (29).

9. A system according to Claim 1, characterised in that an automatic control device (26) is equipped to control the pressure across the width of the headbox to give a predetermined pressure profile in the distribution box (1).

10. A system according to Claim 1, characterised in that the devices (27) are provided for altering the flow profile of the channels (6) of the guide device (5).

11. A system according to Claim 1, characterised in that the branch pipes (50) are provided for supplying water to the entrances (38) to the channels (6).

12. A system according to Claim 1, characterised in that the devices (46, 47) in front of the entrance openings (38) of the guide device (5) or after its exit openings (43) are provided for adjustable flow control through the guide device (5).

13. A system according to Claim 1, characterised in that the cross-section of the channels of the guide device (5) differ in shape across the width of the web.

14. A system according to Claim 13, characterised in that the channels of the guide device (5) in the edge zones (48) of the headbox have a different pressure drop than those in the central zone (49).

15. A system according to Claim 1, characterised in that the guide device (5) has the same number of channels (6) per unit width in the edge zone, the cross-sections of the channels in the edge zone (48) being different from those in the central zone (49).

16. A system according to Claim 14, characterised in that additional holes are provided in the edge zone (48) as channels in the guide device (5).

17. A system according to Claim 1, characterised in that the geometry of the distribution box (1, 55) and/or the nozzle chamber (29) is locally and individually changeable across the width of the headbox by means of the device (45 or 52) to change or form the flow profile and the pressure profile in the chambers (1, 29).

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