EP0489094B2 - Twin-wire paper-web former - Google Patents

Abstract

Disclosed is a twin-wire former for the production of a paper web, in which two screens (11 and 12) together form a zone which is divided into three section (I, II and III). In the first section (I), the two screens (11, 12) run over a curved forming shoe (16) where they form a wedge-shaped inlet gate (15) with which a headbox (10) is directly associated. Abutting the lower screen (11) in the second section (II) are several flexibly mounted strips (27) and in the upper screen (12), between each of these strips (27), a rigidly mounted strip (28). In the third section (III), both screens (11, 12) run over a second curved forming shoe (23).

Description

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. From this publication, the features specified in the preamble of claim 1 are known. In other words, this means that the fibrous web is bathed from the stock suspension fed from the headbox only between two sieve 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 sieve belts 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 screen guide surface is made up of numerous strips with drainage slots in between. This forming shoe is followed (in a second section of the twin-wire zone) by a drainage bar arranged in the other wire loop and behind it a drainage 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) of the highest possible quality and with relatively high working speeds. Due to 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 a constant risk that fibers will clump together into flakes. Efforts are therefore made to form a stock suspension jet that is as free of flakes as possible (e.g. 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 a" deflocculation "(i.e. a dissolution of the flakes) 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 formation 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 the subsequent flat suction boxes. These have a high drainage capacity, so that the web formation can be ended in the area of the last flat suction device (ie ends in the area of the flat suction devices) the so-called main drainage zone, in which part of the fiber material is still in the form of a suspension). However, the flat suction cups are unable to prevent flocculation or to dissolve flakes that have arisen.

In order to cope with the last difficulties described, a web formation device known as "Duoformer D" has been 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 wire 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 that drains 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 only firmly supported strips are used) that a backflow forms in front of the strips in the fibrous suspension. 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 significantly recently, so further improvements are desirable.

The invention has for its object to design a twin wire former of the type described in such a way that the quality of the fibrous web produced, in particular with regard to formation (viewing), is further improved, and that the twin wire former easily adapts to different operating conditions (for example in terms of quantity and drainage behavior the fiber suspension) is adaptable.

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

• A. Doppelsiebformer ohne Einsieb-Vorentwässerungszone
• B. Beginn der Entwässerung in der Doppelsiebzone an einem gekrümmten Entwässerungselement, z.B. an einer rotierenden Formierwalze oder noch besser an einem gekrümmten stationären Formierschuh
• C. Weitere Entwässerung in der Doppelsiebzone zwischen Leisten, die entlang einer "Zick-zack-Linie" angeordnet sind und von denen die an dem einen Siebband anliegenden Leisten nachgiebig abgestützt sind

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

• A. Twin-wire former without single-wire pre-dewatering zone
• B. Start of dewatering in the twin wire zone on a curved dewatering element, for example on a rotating forming roll or even better on a curved stationary forming shoe
• C. Further dewatering in the twin-wire zone between bars which are arranged along a "zigzag line" and from which the bars resting on the one 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 good values for the retention of fillers and fines. In contrast, it had to be found again and again in known twin-wire formers 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 stock between adjacent strips compared to the "Duoformer D" (claim 2).

From DE-OS 31 38 133, Figure 3, a twin wire former is known whose twin wire zone has a curved stationary drainage element in a first section and strips arranged in a second section along a "zigzag line", which are also flexibly supported can and between which a relatively large stock is provided.However, the twin-wire zone is preceded by a single-wire pre-dewatering zone, in which the web formation initially only occurs in a lower layer of the fiber suspension supplied, while the upper layer remains liquid and tends to form flocs to a very great extent. It has been shown that these flakes cannot be dissolved to the required extent 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 waltz effect) further uneven drainage across the web width and thus undesirable fluctuations in the web quality (recognizable e.g. from annoying longitudinal stripes).

Further embodiments of the invention are explained below on the basis of exemplary embodiments which are illustrated in the drawing. Each of Figures 1 to 5 shows - in a simplified schematic representation - one of the different embodiments.

The twin wire former shown in Fig. 1 has a substantially horizontal twin wire zone; this comprises three sections I, II and III arranged one behind the other. The only partially shown endless sieve belts (lower sieve 11 and upper sieve 12) run in the immediate vicinity of a headbox 10 via a breast roller 13 or 14, so that the two sieve belts form a wedge-shaped inlet gap 15 with each other at the beginning of the twin-wire zone. The material jet emitted by the headbox 10 only comes into contact with the two sieve belts 11 and 12 where the lower sieve 11 runs over a stationary, curved forming shoe 16 in the first section I of the twin-wire zone. Its curved tread is formed from a few strips 16 'with drainage slots in between. The existing between the two breast rollers 13 and 14 is variable. 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 in between) run between a lower dewatering box 17 and an upper dewatering box 18. In the lower drainage box 17 there is a row of at least two strips 27 (preferably with an approximately rectangular cross-section), which are resiliently pressed onto 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 pads) can be individually adjusted.

The upper drain box 18 is at both the front and rear ends, as schematically shown with double arrows, suspended on vertically movable support elements. On its underside there is a row of at least three strips 28, preferably with a parallelogram cross-section, which bear against the top of the top sieve 12 and are firmly connected to the box 18. Above the strips 28, a front vacuum chamber 21 and a rear vacuum chamber 22 are provided in the drainage box 18. In the area of the forming shoe 16, part of the water of the fiber suspension is discharged downwards; another part penetrates - due to the tension of the upper sieve 12 - up through the upper sieve 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, a minimum distance X of approximately 3 times the strip thickness Y is provided. 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 a 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 Figures 2 and 5. Deviating 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 lower wire. 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 also be further back, so that the top wire 12 is only separated from the 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 are not 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 to this, 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 double-sided pre-dewatering, 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) if 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 a sieve belt runs polygon-like from bar to bar, with each bar not only draining water, but also creating 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. This option will be used when the highest productivity is 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 renewed 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 sieves 11 and 12 is drawn in an exaggerated manner in the twin-wire zone. This should make it clear that the two screens 11 and 12 converge towards one another over a relatively long distance within the twin-wire 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, (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 shown only by way of example in FIGS. 1 to 3. The end of the sieve convergence is symbolically represented by the point E; there the dry matter content of the paper web has reached approximately 8%. However, this point can also be on one of the flat suction cups 31, for example. Beyond this point, you try to find the dry content, preferably before separating the two Sieves to increase still further. 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, however, be very different and depends, among other things, on the type of paper 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 the removal of the water extracted from the fiber suspension; because the water can be drained off evenly to both sides. In the middle section II of the twin wire zone in particular, 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, guide plates 44 assigned to the fixed strips 28 and a water discharge strip 45. The remaining elements are provided with the same reference numerals as the corresponding elements in FIG. 1. The same applies to the FIG. 3. A conceivable modification of FIG. 3 can consist in that a forming roll is arranged instead of the screen suction roll 20 and the screen suction roll is arranged instead of the guide roll 19. A similar arrangement is known from DE-GM 88 06 036 (Voith file: P 4539). with this exception and apart from the 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 one does without the relatively expensive forming roller (in terms of acquisition and operation) can. Thus, the screen suction roll 20 is usually 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. 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 there is a second curved stationary forming shoe 16a at a distance. 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 is a component of a vacuum chamber 51 in a known manner. This vacuum chamber is, similar to the upper drainage box 18 of FIG. 1, on its front and hung at its rear end in vertically movable brackets. 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 depth of penetration is low, the strip 50 is used only for water removal, and if the depth of penetration 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 drainage on both sides; it is only continued after the bar 50 has removed the water which has penetrated upwards on the first forming shoe 16 from the top wire 12. This enables higher working speeds.

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 strip support bodies 55 are pivotally mounted, the strip 57 being resiliently pressed against the underside of the lower sieve 11 under the force of springs 54. 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 these vacuum chambers 56 corresponds to that of the vacuum chamber 51. It is important that each of the strips 57 and 58 rests on its screen belt 11 and 12 with a certain contact pressure (corresponding to the suspension pressure). The setting of the strips 57 and 58 is carried out such that a wine deflection of the sieve belts preferably takes place on each strip. Thanks to the resilient support of the lower strips 57, the setting once made is insensitive to changes in the quantity or quality of the fabric, so that no back pressure forms in front of the strips and that turbulence forces are nevertheless effectively introduced into the fiber suspension. In contrast to FIGS. 1 to 3, there is the possibility of individually adjusting each of the strips 57/58 with regard to altitude and inclination relative to the wire frame. This makes it even easier to influence the quality of the paper produced, both in terms of formation and in terms of 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 be possibly omit.

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 enables the overall height of the twin wire former to be kept relatively low. In the third section III of the twin-wire zone, not a curved, but a flat forming shoe 23 'is provided, in contrast to FIG. 1. 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. Instead, however, the top wire 12 can also be guided up to the wire suction roll 20. There, as shown, it can 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. However, one difference 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 there are preferably n flexibly supported strips 27/57 and n + 1 firmly supported strips in the second section II of the twin-wire zone. However, it is also possible to make the number of flexibly supported strips the same or one greater than the number of firmly supported strips. 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 (11)

1. Twin-wire paper-web former for the manufacture of a length of fibre material, in particular a paper length of fibre-material suspension, having the following features:

   a) two sieve belts (continuous sieve loops 11 and 12) forming a double-sieve zone;

   b) in a first section (I) of the double-sieve zone, in which both sieves (11, 12) run over a curved water-drainage element (for example a stationary forming element (16), a rotating forming roller (40) or the like), the two sieve belts form a wedge-shaped entry gap (15) which receives the fibre-material suspension directly from a material run-up (10);

   c) in a second section (II) of the double-sieve zone, at least one water-drainage ledge (for example top ledge 28 and bottom ledge 27) is arranged in each of the sieve loops (11, 12), and that one ledge is transposed relative to the other in the movement direction of the sieve;

   d) in a third section (III) of the double-sieve zone, both sieve belts (11, 12) move over at least one stationary water-drainage element (for example forming element 23 and/or flattening vacuum 31);

   e) characterised in that in the second section (II) of the double-sieve zone, at least two ledges (27/28; 57158) are provided in a conventional manner for each sieve belt (11, 12), and that the ledges (27), which abut one of the two sieve belts, are resiliently supported.

   f) that the ledges within the one sieve belt are set on gap relative to the ledges of the other sieve belt.
2. Twin-wire paper-web former according to Claim 1, characterised in that in the second section (II) of the double-sieve zone between adjacent water-drainage ledges (28) is provided a minimum gap (X) of approximately three times the ledge thickness (Y).
3. Twin-wire paper-web former according to Claim 1 or 2, wherein in the second section (II) of the double-sieve zone are provided ledges (28, 58), which are fixedly supported opposite the resiliently supported ledges (27, 57) and the position of which is adjustable relative to their sieve belt (12) (for example by displacement or pivoting), characterised in that the carrier unit (18,56) of the fixedly supported ledge (28, 58) is mounted in two pivot mounts, each of which being displaceable transversely to the running direction of the sieve.
4. Twin-wire paper-web former according to one of Claims 1 to 3, characterised in that in the first section (I) of the double-sieve zone are provided in the one sieve loop (11) behind a first curved stationary forming element (16) at a distance a second curved stationary forming element (16a), and that in the other sieve loop (12) in the area between the two stationary forming elements (16, 16a) is arranged a ledge (50).
5. Twin-wire paper-web former according to one of the above claims, characterised in that a stationary water-drainage element (23) in the third section (III) of the double-sieve zone is curved, preferably in the same way as the water-drainage element (16) provided in the first section (I), and that behind this curved water-drainage element a ledge (29) is arranged in the opposite sieve loop.
6. Twin-wire paper-web former according to one of the above claims, characterised in that at least the area of the double-sieve zone (between 15 and E) in which the length is formed is free of all rollers (Fig. 1 and 3 to 5) which curve the double-sieve zone.
7. Twin-wire paper-web former according to one of Claims 1 to 6, with a sieve-suction roller, characterised in that a portion of the periphery of the sieve-vacuum roller (20) is surrounded by both sieves (11 and 12).
8. Twin-wire paper-web former according to one of the above claims, characterised in that the double-sieve zone rises substantially vertically from the bottom to the top in the movement direction of the sieve.
9. Twin-wire paper-web former according to one of Claims 1 to 7, characterised in that the double-sieve zone rises in the movement direction of the sieve from the bottom to the top with an inclination of between approximately 10 and 30° relative to the horizontal.
10. Twin-wire paper-web former according to one of the above claims, characterised in that the water-drainage ledges (27/28; 57/58) in the second section (II) of the double-sieve zone are arranged along a curved line, the curvature of which is preferably in opposition to the curvature of the water-drainage element (16) in the first section (I).
11. Twin-wire paper-web former according to Claim 10, having a substantially horizontally extending double-sieve zone, characterised in that the first section (I) of the double-sieve zone is curved upwards, the second section (II) is curved downwards, and the third section (III) is again curved upwards (Fig. 5).

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