DE3927597A1 - DOUBLE SCREEN SHAPER - 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 production a fibrous web, in particular paper web, made of a fiber stock suspension. The invention is based on the GB-PS 11 25 906 known twin wire former. From this published Lichung are given in the preamble of claim 1 NEN characteristics known. In other words, they say: That Form the fibrous web from that fed from the headbox The stock is only suspended between two sieve belts other. So-called one-sieve pre-dewatering is missing route. Form in a first section of the twin wire zone the two sieve belts together form a wedge-shaped inlet gap; in this one comes out of the headbox Fabric suspension jet. There it meets the two sieves tapes, where these over a curved drainage element fen; in the case of the aforementioned GB-PS, this is a stationary, ge curved formation shoe. Whose curved screen guide surface is made up of numerous strips with in between Drainage slots. This formation shoe follows (in one second section of the twin wire zone) one in the other wire loop arranged drainage bar and behind this one arranged in the former sieve loop (and by a first suction box formed) drainage bar. In the end several follow in a third section of the twin-wire zone stationary drainage elements designed as flat suction cups.  

One has been trying with twin wire formers of the known type for a long time Decades of fibrous webs (especially paper webs) possible of the highest quality and with a relatively high working speed manufacturing skills. Because of the path formation between two Seven is achieved in particular that the finished fibrous material properties largely the same on both sides points (slight "two-sidedness"). It creates difficulties however, the most uniform possible fiber distribution in the fer to achieve the fibrous web. In other words: it does Difficulty getting a good "formation". Because wait Due to the formation of the web there is a constant risk that Fa ball into flakes. One tries therefore in Headbox a fluff-free material suspense ion beam (e.g. with the help of a turbulence generator gers). In addition, efforts are made to form the To influence dewatering of the fiber suspension in such a way that "back flocculation" is avoided as far as possible or that after a possible formation of flakes again a "deflocculation" (i.e. disintegration of the flakes) takes place.

It is known that one in the first section of the twin wire zone arranged curved drainage element, in particular a according to the aforementioned GB-PS 11 25 906 trained stationary curved formation shoe counteracts the risk of flocculation works. This also applies to those in the GB-PS in the second Ab cut the twin-wire zone arranged drainage strips. Nevertheless, there is a risk with the arrangement according to this GB-PS the flocculation is not completely eliminated. Because there the number of drainage strips is very small large part of the path formation in the area of subsequent fla chen suction boxes instead. These have a high drainage performance, so that in the area of the last flat suction Path formation can be ended (i.e. in the area of the flax sow the so-called main drainage zone ends in the Part of the fiber material still in the form of a suspension lies). However, the flat suction cups are not able to Avoid flocculation or flakes that arise again to solve.  

In order to master the difficulties described last, is a web known as "Duoformer D" educational institution (TAPPI Proceedings 1988 annual meeting, pages 75 to 80). This well-known path formation device is part of a twin wire former, which is an on sieve pre-dewatering zone. In the twin wire zone is in one sieve loop a variety of firm but one adjustable supports provided, namely at the Un bottom of a suction box draining upwards. Furthermore are a lot of compliant in the other wire loop supported strips provided. Due to this compliance the The last mentioned last can be achieved: At for example with an increase in between the two sieve belts The inflowing amount of suspension can be flexibly adjusted dodging supported strips. This will make the danger eliminated (the only when using firmly supported Last there is) that in the fiber suspension before Last forms a backlog. Such a backlog could up to this point the fiber material formed on the two sieve belts destroy layers again. In other words: with this be Known path formation device remains a once set Drainage pressure thanks to the resiliently supported strips constant, even if the suspension is changed amount or if the drainage behavior changes Fibrous suspension. So it finds an automatic adjustment solution of the path formation device to the mentioned changed Relationships instead.

With this known web forming device, fibers can also webs of material with relatively good formation are formed. This however, the requirements are recent increased dramatically, so that further improvements are desirable are worth.

The invention has for its object a double sieve mer of the type described in such a way that the Quality of the fibrous web produced, in particular the Formation (review) is further improved, and that the twin wire former can easily operate at different locations conditions (e.g. in terms of quantity and drainage behavior of the fiber suspension) is adaptable.

This task is characterized by the characteristics of the contractor Proverb 1 solved.

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 roller or even better on a curved stationary forming shoe
C. Further drainage in the twin-wire zone between "zig-zag" arranged strips, of which the strips attached to the sieve belt are supported resiliently to an extraordinarily high increase in the quality of the finished fiber web, so that these also meet 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 good values for the retention of fillers and fines in spite of the high quality increase with regard to the formation. In contrast, it had to be found again and again with known twin wire formers that an improvement in the formation was accompanied by a sharp decrease in the retention.

Experiments also showed that in the second Ab cut the number of strips in the twin-wire zone, compared with the "Duoformer D" can be significantly reduced. However, this number is much larger than that from GB 11 25 906 known twin wire former. It is advantageous that Distance between adjacent strips compared to the "Duoformer D", to enlarge (claim 2).

From DE-OS 31 38 133, Fig. 3, a twin wire former is known whose twin wire zone has a curved stationary drainage element in a first section and "zigzag" strips arranged in a second section, which can also be supported in a flexible manner and between which a relatively large distance is provided. However, there the double sieve zone is preceded by a single-sieve 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 flake to a very large extent. It has been shown that these flakes cannot be dissolved again 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 ( 14 b). 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, for example, from disruptive longitudinal stripes).

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 simplified simplified representation - one of the various 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 (lower sieve 11 and upper sieve 12 ) run in the immediate vicinity of a cloth run 10 via a breast roller 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 abge given from the headbox 10 jet comes with the two sieve belts 11 and 12 only in contact where the lower wire 11 in the first section I of the twin wire zone over a stationary, curved forming shoe 16 runs. 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 12 and 13 (with the partially still liquid fiber suspension in between) run between a lower drainage box 17 and an upper drainage 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 pads) on a preferably water-permeable plate 26 . It is understood that the force of the springs (or the pressure prevailing in the pressure pads) can be individually adjusted.

The upper dewatering box 18 is both vordereren on and suspended from the rear end, as shown schematically by double arrows illustrates Darge to vertically slidable members. On its underside there is a row of at least three strips 28 , preferably with a parallelogram cross-section, which abut 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 formation shoe 16 , part of the water of the fibrous suspension is discharged 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 thickness of the strips 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. The two screens 11 and 12 preferably run on a straight path through section II. A gentle curvature of this path section is also possible, however; see Fig. 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 . Behind this, an additional bar 29 with Unterdruckkam mer 30 can be provided in the loop of the top wire 12 . In addition, flat suction cups 31 can be provided in the loop of the lower sieve. There (as shown with dash-dotted lines) the top wire 12 can be separated by means of a guide roller 19 from the bottom wire 11 and from the formed serstoffbahn Fa. 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 bottom wire 11 on the wire suction roller 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 from the twin-wire zone. Because only here can they develop their full potential, 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, it is also crucial that previously, in section I, a known pre-dewatering takes place on both sides and that this is also done while maintaining the flake-free state of the fiber suspension as far as possible.

For this bilateral pre-dewatering, you will see in the first section I of the twin-wire zone (see FIGS . 1 and 3 to 5) a stationary curved forming shoe whenever it comes down to compliance with the highest quality requirements with regard to the formation. This effect of the forming shoe is based on the fact that at least one sieve belt runs polygon-like from bar to bar, with each bar not only draining water, but also generating turbulence in the still liquid substance. With such a forming shoe, however, it sometimes presents certain difficulties in achieving a stable operating state when starting up the paper machine. It can therefore be advantageous to provide a known forming roller 40 in section I instead of the stationary forming shoe and the breast roller lying in front of it, as shown in FIG. 2. 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 be used either solely for water discharge 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 fiber is still in a liquid state.

In Figs. 1 to 3 in the twin the distance between the two wires 11 and 12 is exaggerated large net Drawn. This is to illustrate that the two screens 11 and 12 converge to each other over a relatively long distance within the double pelsiebzone. This illustrates 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), for example, lie approximately in the middle of the looping zone of the second forming shoe 23 , as shown only by way of example in FIGS. 1 to 3 is. 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 located on one of the flat suction devices 31 , for example. Beyond this point, an attempt is made to increase the dryness even further, if possible before the two sieves are separated. A goal is namely that the separation of the sieves 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. However, the type and number of the dewatering elements required for this within the twin-wire zone can 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 carried off evenly to both sides. In particular in the middle section II of the double screen zone, no vacuum chambers are required. However, the forming roller 40 of FIG. 2 is generally designed as a suction roller. The formation shoes, 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 , the guide strips 44 assigned to the fixed strips 28 and a water discharge strip 45 . The other elements are provided with the same reference characters as the corresponding elements in FIG. 1. The same applies to FIG. 3. A conceivable modification of FIG. 3 may consist in that instead of the suction suction roller 20, a forming roller and instead of the guide roller 19 the sieve suction roller is arranged. A similar arrangement is known from DE-GM 88 06 036 (Voith file: P 4539). Except for this exception and apart from the embodiment shown in 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 it is relatively expensive (in terms of acquisition and operation) Forming roller can do without. Thus, the screen suction roll is usually available as the sole 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 embodiment of 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 16 a is arranged 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 16 a, which is part of a vacuum chamber 51 in a known manner. This vacuum chamber, similar to the upper drainage box 18 of FIG. 1, is suspended at 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. At a low penetration depth, the strip 50 is used only for water removal, at a greater depth of penetration it is also used to generate turbulence in the suspension and thus to improve the formation. By the presence of two separate forming shoes 16 and 16 a, the bilateral drainage is temporarily interrupted; it is only continued after the bar 50 has removed the water that has penetrated upwards on the first forming shoe 16 from the upper sieve 12 . As a result, higher working speeds are possible.

Another difference from Fig. 1 is that in the second section from section II of the twin wire zone, the lower, flexibly supported strips 57 and the upper, firmly supported strips 58 are formed as single strips. This means that each bar has its own support body 55/56 . The lower strips support body 55 are pivotally mounted, the strip 57 is pressed resiliently under the force of springs 54 to the underside of the lower wire 11 . The support body 56 of each of the upper strips 58 is formed like that of the strip 50 as a vacuum chamber. The suspension of this vacuum chamber 56 corresponds to that of the vacuum chamber 51st It is important that each of the strips 57 and 58 with a certain pressure force (corresponding to the suspension pressure) is applied to its sieve belt 11 and 12 respectively. The setting of the strips 57 and 58 is carried out in such a way that a small 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 amount or quality of the fabric, so that no back pressure forms in front of the strips and that an effective introduction of turbulence forces into the fiber suspension is nevertheless successful. In contrast to FIGS. 1 to 3, there is the possibility of individually adjusting each of the strips 57/58 with respect to altitude and inclination relative to the screen track. This makes it even easier to influence the quality of the paper produced, both with regard to the formation and with regard to the surface finish (printability). Deviating from FIG. 4, the upper strips 58 can resiliently and the lower strips are firmly supported 57th 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 an average of approximately 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, deviating from Fig. 1, not a curved but a flat forming shoe 23 '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 of this, however, the top wire 12 can also be guided up to the wire suction roller 20 . It can, as shown, loop around a small part (or alternatively a larger part) of the circumference of the suction roller and then be returned to the deflection roller 19 .

In the embodiment according to FIG. 5, the twin- wire zone extends essentially in the horizontal direction. The individual elements are essentially the same as in the exemplary embodiment according to FIG. 4. A 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 curved path 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.

It can be seen from all the exemplary embodiments that in the second section II of the twin-wire zone preferably n flexibly supported strips 27/57 and n + 1 fixed strips are present. However, it is also possible to make the number of flexibly supported strips equal to or greater by 1 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 2 (see FIG. 4). However, the number n = 3 or 4 is preferred.

Claims (12)

1. Double-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 ) bil with each other a twin-wire zone;
• b) in a first section (I) of the twin-wire zone, in which the two screens ( 11 , 12 ) run over a curved Ent watering element (for example via a stationary forming shoe ( 16 ), a rotating forming roller 40 or the like), form the two sieve belts with one another 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 (for example upper bar 28 and lower bar 27 ) is arranged in each of the wire 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 (eg 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, on each wire belt ( 11 , 12 ) at least two strips ( 27/28 ; 57/58 ) are provided and that on one of the two Sieve belts adjacent strips ( 27 ) are resiliently supported.

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 hen hen. 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 strips ( 27 , 57 ) firmly supported strips ( 28 , 58 ) are provided, the position of which band ( 12 ) relative to their sieve is adjustable (for example by moving or pivoting Ver), characterized in that the support body ( 18 , 56 ) of the firmly supported bar ( 28 , 58 ) rests in two pivot bearings, each of which can be moved ver 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 the one wire loop ( 11 ) behind a first curved stationary forming shoe ( 16 ) at a distance a second curved stationary forming shoe ( 16 a) is angeord net, and that in the other wire loop ( 12 ) in the area between the two stationary forming shoes ( 16 , 16 a) a bar ( 50 ) is arranged. 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 ( 16 ) provided in the first section (I) and that behind this curved drainage element in the opposite sieve loop, a bar ( 29 ) is arranged. 6. twin-wire former according to one of the preceding claims, characterized in that at least that area 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 wire run direction essentially vertically from bottom to top increases. 9. twin wire former according to one of claims 1 to 7, characterized characterized in that the twin wire zone in the wire running direction rises from the bottom to the top, with an inclination towards the horizontal from about 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 is preferably the curvature of the first section (I) arranged drainage element is opposed. 11. twin wire former according to claim 10, with a substantially horizontal extending twin wire zone, characterized in that the first section (I) of the twin wire zone curved upwards, the second section (II) curved down and the third section (III ) is curved upwards again ( Fig. 5). 12. twin wire former according to the preamble of claim 1 characterized in that in the second section of the double Sieve zone provided with at least two strips on each sieve belt hen are, and that the support body of each bar in two Swivel bearings are resting, each of which is transverse to the screen direction tion is movable.