EP0489094A1

EP0489094A1 - Twin-wire paper-web former.

Info

Publication number: EP0489094A1
Application number: EP90913173A
Authority: EP
Inventors: Rudolf Bueck; Dieter Egelhof; Klaus Henseler; Werner Kade; Albrecht Meinecke; Wilhelm Wanke; Hans-Juergen Wulz
Original assignee: JM Voith GmbH
Current assignee: JM Voith GmbH
Priority date: 1989-08-22
Filing date: 1990-08-09
Publication date: 1992-06-10
Anticipated expiration: 2010-08-09
Also published as: CA2053239A1; NO914134D0; NO178832C; DE59001020D1; DE3927597A1; NO178832B; NO914134L; EP0489094B1; EP0489094B2; ES2040604T3; BR9007185A; JP2898752B2; ES2040604T5; CA2053239C; JPH04507439A; WO1991002842A1

Abstract

Dans un former à deux toiles servant à la fabrication d'une bande de papier, deux bandes de toile (11 et 12) constituent ensemble une zone à deux toiles divisée en trois sections (I, II, II). Dans la première section (I), les deux toiles (11, 12) passent par-dessus un patin de formage courbe (16) où elles forment une fente d'entrée cunéiforme (15) à laquelle est directement associée une caisse de tête (10). Dans la deuxième section (II), plusieurs réglettes (27) élastiquement maintenues s'appliquent contre la toile inférieure (11), et entre ces réglettes, dans la toile supérieure (12), chaque réglette (28) est fermement supportée. Dans la troisième section (III), les deux bandes de toile (11, 12) passent par-dessus un autre patin de formage courbe (23).In a two-canvas forming used for making a paper web, two webs (11 and 12) together constitute a two-web area divided into three sections (I, II, II). In the first section (I), the two webs (11, 12) pass over a curved forming pad (16) where they form a wedge-shaped entry slot (15) to which is directly associated a headbox ( 10). In the second section (II), several rulers (27) resiliently held against the lower canvas (11), and between these rulers, in the upper canvas (12), each ruler (28) is firmly supported. In the third section (III), the two webs of fabric (11, 12) pass over another curved forming pad (23).

Description

Twin-wire former

The invention relates to a twin-wire former for producing a fibrous web, in particular paper web, from a fibrous suspension. The invention is based on the twin-wire former known from GB-PS 1,125,906. The features specified in the preamble of patent claim 1 are known from this publication. In other words, these say: The formation of the fibrous web from the stock suspension fed from the headbox takes place exclusively between two wire belts. A so-called single-screen pre-dewatering section is therefore missing. In a first section of the twin-wire zone, the two wire belts form a wedge-shaped inlet gap with one another; a material suspension jet coming from the headbox opens into this. This meets the two Siebbän¬ at a point where they run over a curved drainage element; in the case of the GB-PS mentioned, this is a stationary, curved forming shoe. Its curved sieve guide surface is formed from numerous strips with drainage slots in between. This forming shoe is followed (in a second section of the twin-wire zone) by a dewatering bar arranged in the other wire loop and behind this a dewatering bar arranged in the first wire loop (and formed by a first suction box). Finally, in a third section of the twin-wire zone, several stationary drainage elements designed as flat suction devices follow. For decades, twin wire formers of the known type have been trying to produce fibrous webs (in particular paper webs) with the highest possible quality and with relatively high working speeds. Because of the formation of the web between two screens, it is achieved in particular that the finished fibrous web has largely the same properties on both sides (low "two-sidedness"). However, it is difficult to achieve the most uniform possible fiber distribution in the finished fibrous web. In other words, it is difficult to get a good "formation". Because during the formation of the web there is always the danger that fibers will clump together to form flakes. Efforts are therefore made to form a material suspension jet which is as floc-free as possible in the headbox (for example with the aid of a turbulence generator). In addition, efforts are made to influence the dewatering of the pulp suspension during the formation of the web in such a way that "back-flocculation" is avoided as far as possible, or that after a possible formation of flocs, "deflocculation" (ie a dissolution of the flocs) takes place again.

It is known that a curved drainage element arranged in the first section of the twin-wire zone, in particular a stationary curved forming shoe designed according to the aforementioned GB-PS 1,125,906, counteracts the risk of flocculation. This also applies to the drainage strips arranged in the GB-PS in the second section of the twin-wire zone. Nevertheless, the risk of flocculation is not completely eliminated in the arrangement according to this GB-PS. Since the number of drainage strips there is very small, a large part of the web formation takes place in the area of subsequent flat suction boxes. Although these have a high drainage capacity, so that the web formation can be ended in the area of the last flat suction device (ie the so-called main drainage zone, in which part of the fiber material is still in the form of a suspension, ends in the area of the flat suction device). However, the flat suction cups are unable to prevent flocculation or to dissolve flakes that have arisen. In order to overcome the difficulties described last, a web forming device known under the name "Duoformer D" was developed (TAPPI Proceedings 1988 annual meeting, pages 75 to 80). This known web-forming device is part of a twin-wire former which has a single-screen pre-dewatering zone. In the double sieve zone, a plurality of fixed but adjustable supports are provided in one sieve loop, on the underside of a suction box draining upwards. In addition, a large number of resiliently supported strips are provided in the other wire loop. This resilience of the last-mentioned strips allows the following to be achieved: for example, if the amount of suspension flowing between the two sieve belts is increased, the flexibly supported strips can deviate somewhat. This eliminates the risk (which exists when using only firmly supported strips) that a backlog forms in the fiber suspension in front of the strips. Such a backflow could destroy the fibrous layers so far formed on the two sieve belts. In other words: with this known web-forming device, once the dewatering pressure has been set, thanks to the resiliently supported strips, it remains constant, even when the amount of suspension supplied changes or when the dewatering behavior of the fiber suspension changes. There is therefore an automatic adaptation of the path formation device to the changed conditions mentioned.

With this known web forming device, fibrous webs with relatively good formation can also be formed. In this regard, however, the requirements have increased considerably recently, so that further improvements are desirable. The invention is based on the object of designing a twin-wire former of the type described at the outset in such a way that the quality of the fibrous web produced, in particular the formation (viewing), is further improved, and that the twin-wire former is easily adapted to different operating conditions (for example with regard to the amount and dewatering behavior of the fiber suspension) is adaptable.

This object is achieved by the characterizing features of claim 1.

The inventors have recognized that a combination of known features, namely:

A. Twin-wire former without single-screen pre-dewatering zone

B. Start of drainage in the twin-wire zone on a curved drainage element, e.g. on a rotating forming roller or even better on a curved stationary forming shoe

C. Further drainage in the twin-wire zone between ledges arranged along a "zigzag line"

. are and of which the strips resting on the screen belt are resiliently supported

leads to an extraordinarily high increase in the quality of the finished fibrous web, so that it also meets the highest demands. At the same time, the twin-wire former according to the invention is insensitive to changes in the amount of suspension supplied and to changes in the dewatering behavior of the fiber suspension. Experiments have shown that the invention succeeds in achieving both a high increase in quality with regard to the formation and also good values for the retention of fillers and fines. In contrast, had to in the case of known twin-wire formers, it has repeatedly been found that an improvement in the formation was accompanied by a sharp decrease in the retention.

Experiments have also shown that the number of strips in the second section of the twin-wire zone can be significantly reduced compared to the "Duoformer D". However, this number is significantly larger than that of the twin-wire former known from GB 1,125,906. It is advantageous to increase the distance between adjacent strips compared to the "Duoformer D" (claim 2).

From DE-OS 31 38 133, Figure 3, a twin wire former is known, the twin wire zone of which has a curved stationary drainage element in a first section and strips arranged along a "zigzag line" in a second section, which also can be supported resiliently and between which a relatively large distance is provided. However, the twin-wire zone is preceded by a single-wire pre-dewatering zone, in which the web formation initially begins only in a lower layer of the fiber suspension supplied, while the upper layer remains liquid and tends to form flakes to a very great extent. It has been shown that these flakes cannot be dissolved to the required degree in the subsequent twin-wire zone. Another disadvantage is that the twin-wire zone behind the second section is deflected by a guide roller (14b). This causes (due to the so-called register roller effect) further uneven drainage across the web width and thus undesirable fluctuations in the web quality (recognizable e.g. from disruptive longitudinal strips).

Further embodiments of the invention are explained below on the basis of exemplary embodiments which are illustrated in the drawing. Each of FIGS. 1 to 5 shows - in a simplified schematic representation - one of the different exemplary embodiments. The twin wire former shown in FIG. 1 has an essentially horizontal twin wire zone; this comprises three sections I, II and III arranged one behind the other. The only partially shown endless sieve belts (bottom sieve 11 and top sieve 12) run in the immediate vicinity of a headbox 10, each over a breast roll 13 or 14, so that the two sieve belts form a wedge-shaped inlet gap 15 with one another at the beginning of the twin-wire zone. The material jet emitted by the headbox 10 comes into contact with the two sieve belts 11 and 12 only where the lower wire 11 runs in the first section I of the twin wire zone over a stationary, curved forming shoe 16. Its curved running surface is formed from a few strips 16 'with drainage slots located between them. The distance between the two breast rollers 13 and 14 can be varied. The forming shoe 16 can be operated with or without negative pressure.

In the second section II of the twin-wire zone, the two wire belts 11 and 12 (with the partially still liquid fiber suspension located between them) run through between a lower drainage box 17 and an upper drainage box 18. In the lower dewatering box 17 there is a row of at least two strips 27 (preferably with an approximately rectangular cross-section), which are pressed against the lower sieve 11 from below. For this purpose they are supported, for example, by springs 24 (or by pneumatic pressure cushions) on a preferably water-permeable plate 26. It goes without saying that the force of the springs (or the pressure prevailing in the pressure cushion) can be individually adjusted.

The upper drainage box 18 is suspended both at the front and at the rear end, as shown schematically by double arrows, on vertically displaceable support elements. On its underside there is a row of at least three strips 28, preferably with a parallelogram-shaped cross section, which abut the top of the top sieve 12 and are firmly connected to the box 18. Above the strips 28 are a front vacuum chamber 21 and in the drainage box 18 a rear vacuum chamber 22 is provided. In the area of the forming shoe 16, part of the water of the fiber suspension is drained downward; another part penetrates - due to the tension of the top wire 12 - up through the top wire and is deflected by the foremost of the strips 28 into the front vacuum chamber 21. The water penetrating upwards between the upper strips 28 reaches the rear vacuum chamber 22. The water penetrating between the lower strips 27 through the lower sieve 11 is discharged downwards. Between adjacent upper drainage strips 28 there is a minimum distance X of approximately 3 times the strip thickness Y. The same applies to the lower, resiliently supported strips 27. It is important that each of the strips 27 and 28 lies in the area of an intermediate space between two opposite strips, so that a "zigzag" arrangement is present. Preferably, the two screens 11 and 12 run on a straight path through section II. A gentle curvature of this path section is also possible; see FIGS. 2 and 5. In deviation from FIG. 1, the resiliently supported strips could also be arranged in the upper box 18 and the firmly supported strips in the lower box 17.

In the third section III of the twin-wire zone, both wire belts 12 and 13 run over a further curved forming shoe 23, which (as shown) is preferably arranged in the lower wire loop 11. An additional bar 29 with a vacuum chamber 30 can be provided behind this in the loop of the top wire 12. Flat suction devices 31 can also be provided in the loop of the bottom sieve. There (as shown with dash-dotted lines) the top wire 12 can be separated from the bottom wire 11 and from the fibrous web formed by means of a guide roller 19. Bottom wire and fibrous web then run over a wire suction roll 20. The guide roller 19 can, however, also be further back, so that the top wire 12 is only separated from the bottom wire 11 on the wire suction roll 20. It is important that two drainage boxes 17 and 18 with the alternately flexible and firmly supported strips 27 and 28 do not lie in the front or in the rear, but in the middle section II of the twin-wire zone. Because only here can they fully develop their effect, namely intensive dewatering of the fiber suspension supplied while maintaining the fine, flake-free fiber distribution. This is achieved in that the screen belt in question undergoes a slight (barely visible) deflection on each bar, so that turbulence is repeatedly generated in the still liquid part of the fibrous material. For success, however, it is also crucial that prior, in section I, a pre-dewatering, known per se, takes place on both sides and that this also takes place with the flock-free state of the fiber suspension being maintained as far as possible.

For this preliminary drainage on both sides, a stationary curved forming shoe will always be provided in the first section I of the twin-wire zone (according to FIGS. 1 and 3 to 5) when the highest quality requirements with regard to the formation are important. This effect of the forming shoe is based on the fact that at least one sieve belt runs polygon-like from bar to bar, each bar not only removing water but also generating turbulence in the still liquid material. With such a forming shoe, however, it is sometimes difficult to achieve a stable operating state when starting up the paper machine. It can therefore be advantageous, according to FIG. 2, to provide a known forming roller 40 instead of the stationary forming shoe and the breast roller lying in front of it in section I. Use will be made of this possibility if the highest productivity is primarily required of the paper making machine. In the third section III, the bar 29 already mentioned can either be used solely for the drainage of water upwards or in addition for the further generation of turbulence (for the purpose of further quality improvement). The latter is possible if at this point part of the fibrous material is still in a liquid state.

In FIGS. 1 to 3, the distance between the two screens 11 and 12 is exaggeratedly large in the twin-wire zone. This is intended to illustrate that the two screens 11 and 12 converge towards one another over a relatively long distance within the double screen zone. This makes it clear that the process of forming the web on the first forming shoe 16 (in section I) starts relatively slowly and only ends in section III. The end of the main dewatering zone in which the two sieves converge to one another (and thus the end of the web formation process) can be, for example, approximately in the middle of the looping zone of the second forming shoe 23, as is only exemplary in FIGS 3 is shown. The end of the sieve convergence is symbolically represented there by the point E; there the dry matter content of the paper web has reached approximately 8%. This point can also e.g. lie on one of the flat suction cups 31. Beyond this point, attempts are made to increase the dryness even further, if possible before the two sieves are separated. One goal is namely that the sieving takes place with the highest possible dry web content, so that as few fibers as possible are torn out of the web during the separation. The type and number of the dewatering elements required for this within the twin-wire zone can be very different, among other things. depending on the paper type and its raw material components as well as on the working speed.

The exemplary embodiments shown in FIGS. 2 and 3 differ from the others primarily in that the twin-wire zone rises essentially vertically from bottom to top in the direction of wire travel. This simplifies it Removing the water extracted from the fiber suspension; because the water can be drained off evenly to both sides. In particular in the middle section II of the twin-wire zone, no vacuum chambers are required. However, the forming roller 40 of FIG. 2 is generally designed as a suction roller. The forming shoes 16, 23, in particular those arranged in the third section III, can be provided with a suction device if necessary.

Further elements of the twin wire former shown in FIG. 2 are water collecting containers 41, 42 and 43, baffles 44 assigned to the fixed strips 28 and a water discharge strip 45. The remaining elements are provided with the same reference symbols as the corresponding elements in FIG the same applies to FIG. 3. A conceivable modification of FIG. 3 can consist in that a forming roller is arranged instead of the suction suction roller 20 and the sieve suction roller is arranged instead of the guide roller 19. A similar arrangement is known from DE-GM 88 06 036 (Voith file: P 4539). Apart from this exception and apart from the exemplary embodiment according to FIG. 2 (with forming roller 40), however, the invention will - whenever possible - be used to design the twin-wire former in such a way that the (in purchase and operation) can do without relatively expensive forming roller. Thus, the screen suction roll 20 is generally present as the only suction roll. Furthermore, it can be ensured in all embodiments of the invention that there is no guide roller deflecting the twin-wire zone (with the above-mentioned harmful register roller effect).

The exemplary embodiment in FIG. 4 differs from FIG. 1, inter alia, in that in the first section I of the twin-wire zone in the loop of the lower wire 11, behind a first curved stationary forming shoe 16, a second curved stationary forming shoe 16a is arranged at a distance is. In addition, a single bar 50 is arranged in the loop of the upper wire 12 in the area between the two stationary forming shoes 16 and 16a, which in a known manner is part of a Vacuum chamber 51 is. Similar to the upper drainage box 18 of FIG. 1, this vacuum chamber is suspended in its front and rear ends in vertically displaceable holders. As a result, both the depth of penetration of the bar 50 into the raceway of the top wire 12 and the angle of attack of the bar 50 can be varied. If the penetration depth is low, the strip 50 only serves to remove water, and if the penetration depth is greater, it is also used to generate turbulence in the suspension and thus to improve the formation. The presence of two separate forming shoes 16 and 16a temporarily interrupts the preliminary drainage on both sides; it is only continued after the bar 50 has removed the water from the upper sieve 12 which has penetrated upwards on the first forming shoe 16. As a result, higher working speeds are possible.

Another difference from FIG. 1 is that in the second section II of the twin-wire zone, the lower, flexibly supported strips 57 and the upper, firmly supported strips 58 are designed as individual strips. This means that each bar has its own support body 55/56. The lower ledge support bodies 55 are pivotally mounted, the ledge 57 being resiliently pressed under the force of springs 54 onto the underside of the lower sieve 11. The support body 56 of each of the upper strips 58, like that of the strip 50, is designed as a vacuum chamber. The suspension of this Unterdruckkam¬ chambers 56 corresponds to that abuts the negative pressure chamber 51. Important isti that each of the strips 57 and 58 with a certain pressing force An¬ (corresponding to the suspension pressure) at its screen belt ^"11 and 12 respectively. The adjustment of the strips 57 and 58 is carried out in such a way that a slight deflection of the sieve belts preferably takes place on each bar Thanks to the resilient support of the lower bars 57, the setting once made is insensitive to changes in the quantity or quality of fabric, so that there is no backlog in front of the bars forms and that nevertheless an effective introduction of turbulence forces into the fiber suspension succeeds. In contrast to FIGS. 1 to 3, there is the possibility of individually adjusting each of the strips 57/58 with respect to the altitude and inclination relative to the wire frame. As a result, the quality of the paper produced can be influenced even better, both with regard to the formation and with regard to the surface quality (printability). 4, the upper strips 58 could be flexible and the lower strips 57 could be firmly supported. Another alternative could be that not only the upper ledges 58 but also the lower ledges 57 are secured in vertically displaceable brackets (as shown on the vacuum chamber 51). In this case, the springs 54 can possibly be omitted.

Another difference between FIGS. 1 and 4 is that in FIG. 4 the twin-wire zone rises from bottom to top in the direction of wire travel with an inclination of on average about 20 ° with respect to the horizontal. This makes it possible to keep the overall height of the twin wire former relatively low. In the third section III of the twin-wire zone, deviating from FIG. 1, it is not a curved but a flat forming shoe 23 'that is provided. The separation of the top wire 12 from the bottom wire and from the fibrous web formed can take place on one of the flat suction devices 31, as in FIG. 1. Instead, however, the top wire 12 can also be led up to the wire suction roll 20. It can, as shown, loop around a small part (or alternatively a larger part) of the circumference of the screen suction roll and then be returned via the deflection roll 19.

In the embodiment according to FIG. 5, the twin-wire zone as a whole extends essentially in the horizontal direction. The individual elements are essentially the same as in the exemplary embodiment according to FIG. 4. One difference, however, is that the drainage strips 57 and 58 located in the second section II of the twin-wire zone are arranged along a downwardly curved section of the twin-wire zone. Accordingly, an upwardly curved forming shoe 16, 23 is provided in the first section I and in the third section III of the twin-wire zone. This embodiment is particularly recommended for the modernization of existing Fourdrinier paper machines.

The exemplary embodiments shown have in common that in the second section II of the twin-wire zone there are preferably n f l e x i b e l supported bars 27/57 and n + 1 f e s t supported bars. It is also possible, however, to make the number of the fl e x i b e l supported bars equal or greater than the number of the f e t t supported bars. Instead of a firmly supported strip, an inlet or outlet edge of a drainage box could also be provided. The minimum number n of flexibly supported strips is two (see Fig. 4). However, three or four flexibly supported strips are preferred.

Claims

Twin-wire former for the production of a fibrous web, in particular paper web from a fibrous suspension, with the following features:

a) two sieve belts (endless sieve loops 11 and 12) form a twin sieve zone with one another;

b) in a first section (I) of the twin-wire zone, in which the two screens (11, 12) run over a curved dewatering element (for example over a stationary forming shoe (16), a rotating forming roller 40 or the like) the two sieve belts together form a wedge-shaped inlet gap (15) which receives the fiber suspension directly from a headbox (10);

c) in a second section (II) of the twin-wire zone, at least one drainage bar (e.g. upper bar 28 and lower bar 27) is arranged in each of the sieve loops (11, 12), one bar being offset from the other in the direction of wire travel;

d) in a third section (III) of the twin-wire zone, both wire belts (11, 12) run over at least one stationary dewatering element (e.g. forming shoe 23 and / or flat suction device 31);

e) characterized in that in the second section (II) of the twin-wire zone, as is known per se, at least two strips (27/28; 57/58) are provided on each wire belt (11, 12) and that on one of the are resiliently supported on the two sieve belts (27).

2. Twin wire former according to claim 1, characterized in that in the second section (II) of the twin wire zone between adjacent drainage strips (28) a minimum distance (X) of approximately 3 times the strip thickness (Y) is provided.

3. twin-wire former according to claim 1 or 2, in which in the second section (II) of the twin-wire zone with respect to the resiliently supported bars (27, 57) firmly supported bars (28, 58) are provided, the position of which relative to their sieve belt (12 ) is adjustable (eg by moving or swiveling), characterized in that the supporting body (18, 56) of the firmly supported bar (28, 58) rests in two swivel bearings, each of which can be displaced transversely to the direction of wire travel .

4. Twin wire former according to one of claims 1 to 3, characterized in that in the first section (I) of the twin wire zone in a wire loop (11) behind a first curved stationary forming shoe (16) a second curved stationary forming shoe at a distance (16a) is arranged, and that a bar (50) is arranged in the other sieve loop (12) in the area between the two stationary forming shoes (16, 16a).

5. Twin wire former according to one of the preceding claims, characterized in that the stationary drainage element (23) arranged in the third section (III) of the twin wire zone is curved, preferably in the same sense as the drainage element provided in the first section (I) ( 16) and that a bar (29) is arranged behind this curved drainage element in the opposite sieve loop.

6. twin-wire former according to one of the preceding claims, characterized in that at least that region of the twin-wire zone (between 15 and E) in which the web formation takes place is free of any rollers which bend the twin-wire zone (FIGS. 1 and 3 to 5).

7. twin-wire former according to one of claims 1 to 6, with a wire suction roll, characterized in that part of the circumference of the wire suction roll (20) is wrapped by two sieves (11 and 12).

8. twin-wire former according to one of the preceding claims, characterized in that the twin-wire zone in the Sieblauf¬ direction increases substantially vertically from bottom to top.

9. Twin wire former according to one of claims 1 to 7, characterized in that the twin wire zone rises in the wire running direction from the bottom up, with an inclination to the horizontal of approximately 10 to 30 °.

10. Twin wire former according to one of the preceding claims, characterized in that the drainage strips (27/28; 57/58) lying in the second section (II) of the twin wire zone are arranged along a curved line, the curvature of which preferably corresponds to the curvature of the first Section (I) arranged drainage element (16) is opposite.

11.- twin wire former according to claim 10, with a substantially horizontally extending twin wire zone, characterized in that the first section (I) of the twin wire zone curved upwards, the second section (II) curved downwards and the third section (III) is curved upwards again (FIG. 5).

12. twin wire former according to the preamble of claim 1, da¬ characterized in that in the second section of the double screen zone at least two strips are provided on each screen belt, and that the support body of each strip rests in two pivot bearings, each of which is transverse to Sieblaufrich¬ direction is displaceable.