EP0323468B1 - Headbox - Google Patents

Abstract

Headbox for paper machines, provided with an output channel in the form of a nozzle, with an output slot for producing a strip of pulp having the width of the machine. The output channel is delimited by two flow guide walls converging toward one another and as a rule by a rear wall. It is divided up into a number of channel sections (32), preferably autonomous, by subdivision of the flow guide walls into wall sections (30, 31) and preferably by subdivision of the rear wall into rear wall sections (37). Each channel section may have at least one input line (40) of its own, as well as (for adjusting the opening of the output slot) a section of part of the mobile wall (35) and an associated regulating system (38, 39).

Description

The invention relates to a headbox for a paper machine, in particular with the features specified in the preamble of claim 1. The essential components of such a headbox include two machine-wide, mutually converging flow guide walls which, together with a rear wall and two side walls, delimit a machine-wide, nozzle-like outlet channel. At the downstream end of the flow guide walls, which are also called the lower lip and upper lip, there is a machine-wide outlet gap through which the paper stock flows out in the form of a machine-wide stream of material onto a circumferential screen belt on which the paper web is formed.

• 1. CA-PS 849 817,
• 2. US-PS 3 373 080,
• 3. US-PS 4 198 270 (= DE 27 26 709),
• 4. US-PS 4 455 197,
• 5. US-PS 4 552 619,
• 6. US-PS 2 920 699,
• 7. US-PS 3 846 229 (= _DE 23 02 196).

Known headboxes of this type are described in the following publications:

• 1. CA-PS 849 817,
• 2. U.S. Patent 3,373,080,
• 3. US Pat. No. 4,198,270 (= DE 27 26 709),
• 4. U.S. Patent 4,455,197;
• 5. U.S. Patent 4,552,619,
• 6. U.S. Patent 2,920,699,
• 7. U.S. Patent 3,846,229 (= _DE 23 02 196).

A serious problem with such headboxes is to keep the clear width of the above-mentioned exit gap constant over the machine width with the greatest possible accuracy. Practice has shown that the presence of local deviations from the desired gap width impairs the quality of the resulting paper web, in particular results in an uneven basis weight cross section. For example, it has been found that a certain change in the gap width has an approximately tenfold increase in the basis weight of the paper web.

To make matters worse, certain changes in the gap width can only occur during operation and then usually only partially disappear again. Such changes in the gap width are caused, for example, by changes in temperature, in particular when the paper machine is started up again with heated material after a standstill. In this case, the flow guide walls of the headbox gradually take on the higher temperature of the paper stock.

Changes in the gap width that occur to different extents at different points in the machine width are also particularly unpleasant. For example, the flow of cold air into the machine room (when opening a gate) leads to a one-sided change in the gap width. Other non-uniform changes in the gap width are apparently caused by stiffening ribs, with which the current guiding walls have to be stiffened in some known constructions.

A headbox is described in document 1, the lower flow guide wall of which rests on a foundation by means of a hollow support. The problem on which this publication is based is that the hollow support mentioned assumes a higher temperature in its upper region adjacent to the current-carrying wall than in its lower region. It therefore expands more at the top than at the bottom, so that the lower current-carrying wall is bent. To eliminate this disadvantage, it is provided that the lower region of the hollow support mentioned is kept at the same temperature as the upper region by means of a heating device.

In the headbox according to document 4, a bundle of pipelines and flow guide walls connected thereto extend through the interior of a hollow headbox housing. A pivotable upper lip is formed by one of the walls of a hollow beam with a triangular cross section. In order to keep the components of the headbox at a certain temperature, water is passed through the interior of the headbox housing and through the upper lip hollow support, of course separately from the paper stock. According to FIG. 6, a rectangular hollow support is attached to the triangular hollow support, which serves to support adjusting spindles according to FIG. In order that a corresponding change in length of the triangular hollow beam can take place without hindrance when the temperature changes, the attached rectangular hollow beam 54 according to FIG. 7 is divided into numerous sections 55 distributed over the machine width.

In the headbox described in document 5, the flow guide walls are in turn reinforced by hollow beams. Similar to the document 1, measures are taken which result in the hollow beams and thus the current-carrying walls not bending due to temperature differences.

All of these known measures have not led to satisfactory results. On the one hand, the stiffening of the current-carrying walls with the help of hollow beams leads to very complex and space-consuming constructions, and on the other hand, the devices for producing uniform temperatures do not react quickly enough, at least when the paper machine starts up. Ie when starting up there is still the risk that the current supply walls expand faster than the opposite wall of the hollow beam. There is also often the risk that at least one of the two power supply walls is temporarily unevenly tempered on its own. It can happen that the inside of a current guide wall touched by the paper stock has a higher temperature than the outside. Thus, there is already the danger that the current guide wall in question bulges inwards in the central region of the width of the paper machine and reduces the clear width of the outlet gap. This danger is even greater if the current-carrying walls, as in the case of document 3, are simple, self-supporting plates are designed and executed without stiffening hollow beams. In the simplest case, the two converging flow guide walls are rigidly attached to a support element that extends across the machine width, for example on a rear wall of the outlet channel, in which a tube bundle for feeding the paper stock is arranged.

The invention has for its object to design a headbox of the type described in such a way that the clear width of the outlet gap can be kept constant with the greatest possible accuracy over the machine width without costly heating or cooling devices and / or stiffening structures for the flow guide walls are required .

This object is achieved by the characterizing features of the main claim. According to this, one of the essential ideas of the invention is to subdivide the outlet channel into a plurality of channel sections that are lined up over the machine width. It is essential that each of these channel sections is designed in the manner of a C-shaped, stiff, "clamp" so that it can withstand the internal pressure prevailing in the outlet channel. This means that the stiffeners that are required outside the outlet channel, e.g. in the form of the hollow beams mentioned. However, as usual, the outlet channel will generally be supported on a fixed component common to all channel sections, e.g. on a base plate.

The outlet channel can be divided into channel sections in different ways, as will be explained in more detail below. As a rule, however, each channel section is assigned a section of one current guide wall and a section of the other current guide wall. I.e. the two flow guide walls are preferably subdivided with the same division and thus into an equal number of wall sections (however, a division of the two flow guide walls into a different number of wall sections is also conceivable).

It is also common to all of the exemplary embodiments that the wall sections are releasably connected to one another (and to the two lateral boundary walls). This has the advantage that in the event of mechanical damage to a part of a current-carrying wall, the entire current-carrying wall no longer has to be replaced with a new one. Rather, only the wall section in question needs to be replaced. Thus, the material costs required for such an exchange are reduced to a small fraction of the previous value. Such an exchange can also take place in a much shorter time, so that an additional substantial saving can be achieved by shortening the downtime.

• Ist die Innenseite der Stromführungswände einer plötzlichen Temperatur-Änderung ausgesetzt, beispielsweise wenn beim Anfahren der Papiermaschine erwärmter Papierstoff beginnt, durch den Auslaufkanal zu strömen, und wenn somit für eine gewisse Zeit eine Stromführungswand an ihrer Innenseite eine höhere Temperatur aufweist als an ihrer Außenseite, so wölbt sie sich dank der Unterteilung in Wandsektionen nicht mehr als ganzes. Vielmehr wölbt sich unter der Wirkung dieser Temperaturdifferenz nur noch jede Wandsektion für sich allein, ohne die benachbarten Wandsektionen zu beeinflussen. Hierbei beträgt der Grad der Wölbung an jeder einzelnen Wandsektion nur einen kleinen Bruchteil der bisherigen Wölbung der gesamten Stromführungswand. Man braucht also die Anzahl der Wandsektionen, aus der gemäß der Erfindung eine Stromführungswand nunmehr zusammengesetzt ist, nur genügend groß zu wählen, um den Grad der Wölbung jeder einzelnen Wandsektion so klein zu halten, daß sie nicht mehr stört.

However, the main advantage lies in the principle effect of the invention described below:

• If the inside of the flow guide walls is exposed to a sudden change in temperature, for example if paper stock heated when starting up the paper machine starts to flow through the outlet channel, and thus if for a certain time a flow guide wall has a higher temperature on the inside than on the outside, then thanks to the division into wall sections, it no longer bulges as a whole. Instead, under the effect of this temperature difference, only each wall section bulges on its own without influencing the neighboring wall sections. The degree of curvature on each individual wall section is only a small fraction of the curvature of the entire current-carrying wall to date. The number of wall sections from which a current-carrying wall is now composed according to the invention need only be chosen large enough to keep the degree of curvature of each individual wall section so small that it no longer interferes.

A major advantage of the invention is therefore that the heating or cooling devices provided in the prior art can at least largely be dispensed with and that the clear width of the outlet gap can be kept constant over the machine width with much higher accuracy than before.

The various design options for dividing the outlet channel into channel sections are specified in claims 2 to 7. A first group of exemplary embodiments is characterized in that each channel section has the shape of an essentially C-shaped bracket, that is to say mechanically an autonomous element, so that when the headbox is assembled, an independent channel section is placed next to the other. In this case, the rear wall of the outlet channel is also divided into “rear wall sections”, the division of the rear wall sections preferably being equal to the division of the wall sections. However, the division of the rear wall sections could also be carried out, for example, twice the division of the wall sections. I.e. in this case an autonomous channel section would be composed of a rear wall section and a total of four wall sections.

In another group of exemplary embodiments, a rear wall which is arranged at the inlet end of the outlet channel and is uniform for the entire headbox is provided. I.e. the rear wall extends as a one-piece component across the machine width, each of the wall sections being fastened to this rear wall.

• 1. Die beiden Wandsektionen sind zu einem einstückigen, vorzugsweise durch Gießen hergestelltes Element vereinigt (Anspruch 3).
• 2. Die beiden Wandsektionen sind an ihrem zulaufseitigen Ende derart abgeformt, daß sie unmittelbar aneinandergeschraubt werden können.
• 3. Die beiden Wandsektionen sind allein mittels einer separaten Rückwand oder Rückwandsektion aneinander gekoppelt (Anspruch 4).
• 4. Die beiden Wandsektionen sind mittels eines Einbauelements (Strömungsgitter, Turbulenzrohrbündel oder dergleichen) oder mittels mehrerer derartiger Einbauelemente aneinandergekoppelt (Anspruch 5).
• 5. Die beiden Wandsektionen sind sowohl mittels einer Rückwand als auch mittels wenigstens eines Einbauelementes aneinandergekoppelt.

However, the various exemplary embodiments of the invention can also be subdivided from other points of view, namely according to the way in which the two wall sections of a channel section are combined with one another in order to increase the structure that withstands the internal pressure form. Some options are listed below:

• 1. The two wall sections are combined into a one-piece, preferably made by casting element (claim 3).
• 2. The two wall sections are molded at their inlet end in such a way that they can be screwed together directly.
• 3. The two wall sections are coupled to one another solely by means of a separate rear wall or rear wall section (claim 4).
• 4. The two wall sections are coupled to one another by means of a built-in element (flow grid, turbulence tube bundle or the like) or by means of several such built-in elements (claim 5).
• 5. The two wall sections are coupled to one another both by means of a rear wall and by means of at least one built-in element.

As with known headboxes, a device for adjusting the inside width of the outlet gap will usually also be provided in the construction according to the invention, e.g. in the form of an adjustable panel or in the form of a movable, downstream part of the current guiding wall. According to a further idea of the invention, the adjusting device is divided into sections in the same way as the associated current guiding wall. This achieves an important additional advantage, which is explained below using an embodiment with a movable current-carrying wall:

In known headboxes, the one-piece, movable flow guide wall (usually the upper lip), which extends across the machine width and is usually supported at both ends by means of a lifting spindle, represents a beam of considerable length that is loaded by the internal pressure. As is known, the deflection of such a bar is proportional to the third power of the length of the bar. Such a one-piece upper lip must therefore be reinforced, for example by means of a strong support and / or by means of stiffening ribs. According to the invention, however, such a movable flow guide wall is now divided into numerous movable wall sections arranged in a row, the length of which, measured transversely to the machine longitudinal direction, is only a fraction of the length of a one-piece movable flow guide wall. The deflection of each of these movable wall sections under the internal pressure is therefore negligible. This fact even allows the movable wall sections to be made relatively thin-walled. A considerable saving of material is thus achieved. For example, stiffening ribs, which - as already mentioned above - have been the cause of uneven changes in the clear width of the outlet gap, have also been eliminated. In addition, the lifting devices previously required for adjusting the one-piece movable upper lip are no longer required.

Document 7 describes a headbox with a large number of adjacent feed pipes. According to FIGS. 5 and 6 referred to as a schematic representation, sections of the upper and lower wall of a machine-wide outlet channel are connected to each inlet pipe. In contrast, FIG. 7, which shows "a practical realization", shows one-piece walls 161 and 171. For this purpose, the description states: "A top wall 161 and a bottom wall 171 serve in common as the top and bottom walls, respectively, of each of the flow path tubes 15" (column 8 from line 27). According to the task of document 7, this known headbox in the material flow should generate microturbulence so that the fiber material in the paper is distributed as evenly as possible. For this purpose, a "spin inducing vane" is provided in each inlet pipe, followed by a "rectifier vane" (rectifying vane) in the outlet duct. So it is a completely different task. The problems explained above and the essential inventive features serving to solve these problems are not disclosed in document 7.

Further advantageous embodiments of the invention are described in claims 12 and 13.

The solution principle according to the invention, namely subdivision of the outlet channel into sections, can also be combined with known measures. So you can a headbox according to the invention e.g. Place on a base plate that extends across the machine width or on a cross member that is held at the temperature of the paper stock by means of a heating device.

• Die Figur 1 zeigt einen Längsschnitt durch einen Stoffauflauf.
• Die Figur 2 zeigt einen Schnitt entlang der Linie 11-11 der Figur 1.
• Die Figur 3 zeigt eine einzelne Kanalsektion eines von den Figuren 1 und 2 abweichenden Ausführungsbeispiels in einer Schrägansicht.
• Die Figur 4 zeigt ein weiteres Ausführungsbeispiel im Längsschnitt.
• Die Figur 5 zeigt eine Ansicht von oben auf den Stoffauflauf der Fig. 4.
• Die Figur 6 zeigt einen Teil-Querschnitt nach Linie VI-VI der Fig. 4.
• Die Figur 7 zeigt eine als einteiliges Element ausgebildete Kanalsektion.
• Die Figur 8 zeigt ein Ausführungsbeispiel für eine Doppelsieb-Papiermaschine im Längsschnitt.

Embodiments of the invention are described below with reference to the drawing.

• Figure 1 shows a longitudinal section through a headbox.
• FIG. 2 shows a section along the line 11-11 of FIG. 1.
• FIG. 3 shows an individual channel section of an embodiment differing from FIGS. 1 and 2 in an oblique view.
• Figure 4 shows a further embodiment in longitudinal section.
• FIG. 5 shows a view from above of the headbox of FIG. 4.
• FIG. 6 shows a partial cross section along line VI-VI of FIG. 4.
• FIG. 7 shows a channel section designed as a one-piece element.
• FIG. 8 shows an exemplary embodiment of a twin-wire paper machine in longitudinal section.

The headbox of a paper machine, shown schematically in simplified form in FIGS. 1 and 2, has a lower flow guide wall 10 and an upper flow guide wall 11, which delimit an outlet channel 12 which is machine-wide in the usual manner. In the drawing, this becomes essentially horizontal from left to right flows through. The flow guide walls, which initially run parallel and then converge in their downstream part to one another, form a machine-wide exit gap 14. The machine-wide stream of material emerging from this passes in the usual manner in the area of a breast roll 15 to an endless, rotating screen belt 16, the part of the Sieve section of the paper machine is. Instead of the illustrated arrangement of the headbox with an approximately horizontal flow direction, all other possible flow directions can also be realized within the scope of the invention, for example vertical or, for example, an upward flow direction. The latter is used, for example, in connection with a twin-wire section.

The flow guide walls 10 and 11 are rigidly connected at their upstream end by means of a rear wall 17, likewise in the middle area by two flow grille blocks 18 and 19 arranged one behind the other. The headbox rests on a base plate 20, the temperature of which can be adjusted to a certain value, e.g. by supplying heat via heating channels 21.

As can be seen from FIG. 2, the lower current guiding wall 10, the rear wall 17 and the flow grid blocks 18 and 19 are divided into sections of the same width. The sections of the lower current-carrying wall are designated by 10a, 10b, 10c and 10d and the sections of the rear wall by 17a, 17b, 17c and 17d. Likewise, the sections of the flow grid blocks are designated 18a, 18b, 18c and 18d and 19a, 19b, 19c and 19d. The upper current-carrying wall 11 is divided in the same way.

Each of the lower wall sections, e.g. 10b, forms a so-called channel section 8b with the associated upper wall section 11b and with the associated rear wall section 17b and with the flow grille sections 18b and 19b. The wall sections 10b and 11b are screwed to the sections 17b, 18b and 19b of the rear wall and the flow grid blocks in such a way that the channel section forms an element of the outlet channel which withstands the internal pressure. The screws are indicated schematically in the drawing, e.g. at 29

At least one inlet line 23 opens into each channel section 8a-8d. This can be detached, e.g. attached to the rear wall 17 by means of a flange 9. The feed lines 23 are connected to a mass distribution system in a known manner. The latter can have the shape of a distribution tube extending across the machine width or (according to document 6) a vertical tube or a so-called damping container with air cushion. A control valve (omitted in the drawing) can be installed in each feed pipe 23 for local regulation of the amount of substance to be fed. The adjustment of the control valves can be triggered by an automatic control device that keeps the basis weight of the paper web as constant as possible over the machine width (the so-called cross-sectional area weight).

To adjust the clear width of the exit gap 14, each upper wall section 11a-11d d has a weak point 11 '. The associated adjustment device is omitted in FIG. 1.

The channel sections 8a-8d abut one another in a liquid-tight manner; Sealing elements are not visible in the drawing. The channel sections can be braced with one another and with the side walls by means of tie rods 24 which extend across the machine width. One of the two side walls is indicated at 13 in FIG. 1. If necessary, the inside of the flow guide walls can be coated with a film 25 in the nozzle-like part of the outlet channel 12 in order to bridge any unevenness at the joints 22. If necessary, a material with the lowest possible linear expansion coefficient is selected for this. The film can be firmly connected to the relevant wall sections (e.g. 10a to 10d), for example by gluing, so that the film only ever expands together with the wall sections under the action of heat.

The headbox according to FIGS. 1 and 2 can be modified if necessary in such a way that the channel sections 8a-8d are subdivided to a certain extent by partitions 26 in their inlet-side area. Such partition walls 26, indicated by dash-dotted lines in FIG. 2, can be combined with the rear wall sections 17a-17d.

The individual channel section 32 shown in FIG. 3 comprises a lower wall section 30, an upper wall section 31 and a rear wall section 37. In contrast to FIGS. 1 and 2, the wall sections 30 and 31 are now rigidly connected to one another solely via the rear wall section 37 (by means of schematically shown Screws 34). Also deviating from FIG. 1, the two wall sections 30 and 31, insofar as they form the structure that withstands the internal pressure, extend parallel to one another over their entire length. In this case, a nozzle-like outlet channel is formed in that an intermediate wall section 35 converging towards the lower wall section 30 is arranged on the inside of the upper wall section 31. This intermediate wall section is flexible, for example with the aid of a weak point 36. The downstream end of the intermediate wall section 35 is coupled to an adjusting device 38 arranged on the part 31 'of the upper wall section 31, for example by means of at least one lifting spindle 39.

An inlet line penetrating the rear wall section 37 is designated by 40. Not shown in FIG. 3 is a flow grid which serves to make the flow of material even. 3, an additional flexible intermediate wall section could be arranged on the lower wall section 30, which is symmetrical to the upper intermediate wall section 35.

The outlet channel 12 of the illustrated exemplary embodiments tapers essentially linearly as far as the outlet gap 14. Deviating from this, other shapes can also be selected, for example based on document 2. There, the flow guide walls have S-shaped inner surfaces in a symmetrical arrangement in order to reach a small gap at a short distance, which is then maintained over a relatively long distance up to the outlet opening. It is also possible - in an asymmetrical arrangement - to provide only one of the two current-carrying walls with an S-shaped inner surface and the other with an essentially flat inner surface.

The headbox shown in FIGS. 4 to 6 has a lower flow guide wall 50 and an upper flow guide wall 51. Between these there is a bundle of turbulence tubes known from conventional headboxes and designated 52 as a whole. In the example shown, there are two rows 53 and 54 of turbulence tubes one above the other, which converge at approximately the same angle to one another as the two flow guide walls. The turbulence tubes are embedded in two transverse walls 55 and 56.

As can best be seen from FIG. 6, the current guiding walls 50 and 51 and the transverse wall 55 (likewise the transverse wall 56) are divided into sections with the same division. Only two of these sections are fully visible; these are designated 50a, 50b and 51a, 51b and 55a, 55b. The wall sections are connected to the transverse wall sections by means of screws 60 and 61 and are thus joined to form a rigid channel section 48a, 48b. Comparing FIGS. 4-6 with FIGS. 1 and 2, in FIG. 4-6 the inlet-side transverse wall (= "inner rear wall") 55 has assumed the stiffening function of the rear wall 17 of FIGS. 1 and 2. In this case, an “outer rear wall” 57 extends in one piece over the entire machine width; it is fastened to the upper wall sections 51 by means of joints 57 'and can be folded up for cleaning purposes. The outer rear wall 57 thus does not contribute (or does not significantly) to stiffening the channel sections. At least one inlet line 59, which in this case is connected to the lower wall section 50, opens into each channel section.

The channel sections and the (not visible) side walls are clamped liquid-tight with the aid of tie rods 58. The tie rods 58 extend in the transverse direction through the transverse walls 55 and 56. 4 to 6, the wall sections (50a, 50b etc. and 51a, 51b etc.) could also be fastened to a turbulence tube bundle which is not divided into sections and whose transverse walls thus extend in one piece across the machine width. In this case the tie rods 58 would be omitted.

A diaphragm 62 (62a, 62b), which is divided into sections, is provided to adjust the clear width of the outlet gap. Each diaphragm section is fastened to a diaphragm holder 63 (63a, 63b), which is also divided into sections, on which at least two adjustment spindles 64 engage. Each diaphragm section 62a, 62b, together with its diaphragm holder section 63a, 63b, is held against the liquid pressure on the relevant wall section 51a, 52a by means of an abutment 65 which is also divided into sections (which is omitted in FIG. 5). The adjusting spindles 64 can be operated together or independently of one another. Thus, each aperture section 62a, 62b can be moved in parallel or can be inclined within certain limits. As is known, conventional screens extend in one piece over the entire machine width, the local fine adjustment of the outlet gap width being carried out by deforming the screen. Such a deformation is not provided in the embodiment according to FIG. 5. However, if required, each orifice section 62a, 62b can also be assigned more than two adjusting spindles, so that the orifice sections can also be deformed.

The schematic representation of FIG. 7 shows that the structure of each channel section that withstands the internal pressure can also be designed as a one-piece element 70. Such an element can be made by casting. Since, as in the exemplary embodiments described above, it has only a small width in the cross-machine direction, it is entirely possible to smooth the inner surfaces touched by the material flow by mechanical processing. If necessary, a flow grille 71 can be inserted into the outlet channel, similar to that in FIGS. 1 and 2. In contrast to FIG. 1, however, this need not contribute to stiffening the channel section. As a result of the omission of the screws on the flow grille, an enlarged flow cross section is available.

The previously described exemplary embodiments have in common that the headbox with the lower flow guide wall rests on a foundation, a base plate or the like. FIG. 8 shows an embodiment which differs from this.

Here the headbox rests on a foundation 88 via the rear wall 87. The direction of flow here is not horizontal, but directed obliquely upwards, as is required for known twin-wire paper machines. The current guiding walls 80 and 81, which are in turn divided into sections, are screwed to the rear wall 87. As in FIG. 2 or 3, the rear wall can also be divided into sections. However, it can also extend in one piece over the entire machine width. In both cases, two associated sections of the flow guide walls 80 and 81 form a channel section 89 together with the rear wall 87. A conventional cross-distribution pipe 86 is provided as a device for generating a machine-wide material flow. Connected to this are a plurality of inlet pipes 83 which are distributed uniformly over the machine width and which extend through the rear wall 87 and open into the outlet channel 82. A control valve 85 is provided in each inlet pipe. Just as in FIG. 7, a flow can take place in the outlet channel grid 84 are provided. Like the power supply walls, this can be divided into sections. However, their division does not have to match the division of the channel sections 89. Rather, it may well be expedient to arrange, for example, two half-width grid sections in each channel section.

For simplification, the tie rods extending across the headbox (similar to tie rods 24 in FIG. 1) and the adjusting devices for the clear width of the outlet gap are omitted in FIGS. 7 and 8.

Claims (13)

1. Headbox for papermaking machines, having the following features:

a) a nozzle-like outlet-channel (12, 82) as wide as the machine is connected to a device (23; 86, 83) for producing a flow of stock as wide as machine, the outlet-channel being defined by two converging flow-guidance walls (10, 11) as wide as the machine, by a rear wall (17) and by two lateral walls (13), and an outlet-gap (14) as wide as the machine being located at the downstream end of the outlet-channel;

b) the flow-guidance walls (10, 11) comprise a plurality of wall-sections (10a to 10d, 11a to 11d) being arranged in a row over the width of the machine and being connected to each other and to the two lateral walls (13) in liquid-tight fashion;

c) characterized in that, as seen in a longitudinal section, each pair of wall-sections (e.g. 10b and 11 b; 30 and 31) are forming - together and with the rear wall (e.g. 17; 37) - a channel-section (e.g. 8b; 32) which is formed in the manner of a C-shaped and rigid clamp capable of withstanding the internal pressure; and in that the channel-sections (8a to 8d) are detachably connected to each other and to the two lateral walls (13).

2. Headbox of claim 1, characterized in that the rear wall (17) is also divided into sections (17a to 17d), the spacing (between the joints) of the rear- wall-sections preferably being the same as the spacing (between the joints) of the wall-sections (10a to 10d, 11a to 11d).

3. Headbox of claim 1, with or without built-in elements (e.g. flow-grating 71, bundle of turbulence-tubes or the like) arranged in the outlet-channel, characterized in that the structure of each of the C-shaped channel-sections, adapted to withstand the internal pressure, is in the form of a one-piece, inherently rigid element (70).

4. Headbox of claim 1, with or without built-in elements (e.g. flow-grating 84) arranged in the outlet-channel, characterized in that the structure of each of the C-shaped channel-sections (32; 89), adapted to withstand the internal pressure, consists only of two wall-sections (30, 31; 80, 81) belonging to each other and of the rear wall (37; 87).

5. Headbox of claim 1 or 2, with at least one built-in element (e.g. flow-grating 18, 19, bundle of turbulence-tubes 52 or the like) arranged in the outlet-channel (12), characterized in that the built-in element (18,19; 52) is in the form of a stiffening tie-element for two wall-sections (e.g. 10b and 11b; 50b and 51b) belonging to each other.

6. Headbox of any one of claims 1 to 5, with at least one built-in element (e.g. flow-grating 18, 19, bundle of turbulence-tubes 52 or the like) arranged in the outlet-channel (12), characterized in that the built-in element (18, 19) is also divided into sections (18a to 18d, 19a to 19d) the spacing (between the joints) of the built-in-element-sections preferably being the same as the spacing (between the joints) of the wall-sections (10a to 10d, 11 a to 11 d).

7. Headbox of claim 6, characterized in that at least some of the built-in elements overlap the joints (22) between adjacent wall-sections (e.g. 10a to 10d).

8. Headbox of any one of claims 1 to 7, with a device for adjusting the internal width of the outlet-gap (14), e.g. an adjustable slice lip (62), a mobile downstream part (35) of the flow-guidance wall (31), or the like, characterized in that the adjusting device (35; 62) is also divided into sections, each of which is associated with one of the said channel-sections (32; 48a, 48b).

9. Headbox of claim 8, characterized in that, in each channel-section (32), the downstream part (35) of at least one (31) of the two wall-sections is movable for the purpose of adjusting the internal width of the outlet-gap.

10. Headbox of claim 9, characterized in that, in each channel-section, a wall-section (31) comprises an outer part (31') pertaining to the rigid structure of the channel-section and a mobile part (35) arranged on the inside thereof (Figure 3).

11. Headbox of claim 8, with a slice lip (62) used to adjust the internal width of the outlet-gap, characterized in that the slice lip (62) is divided into slice lip-sections (62a, 62b), a slice lip-section being associated with each channel-section (48a, 48b).

12. Headbox of any one of the preceding claims, characterized in that the inside of at least one of the flow-guidance walls (10 and/or 11), divided into sections, is covered by a strip of flexible material (foil 25, thin sheet metal or the like) for the purpose of evening-out any irregularities at the joints (22) between the wall-sections (10a-10d, 11a-11d).

13. Headbox of claim 12, characterized in that the strip of material (25) is attached, over the whole of the covered internal surface of the flow-guidance wall (10 and/or 11), rigidly to the wall-sections, e.g. by gluing.

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