WO1993012291A1 - Twin-wire former - Google Patents

Abstract

The twin-wire former is used to produce a web of paper or cardboard. Two wires (endless) wire loops (11 and 12)) together form a twin-wire region in which one wire (12) runs over fixed rails (28) fitted at a distance apart on a dehydration box (18). In the twin-wire region the other wire (11) runs over a few rails (27) which lie opposite the fixed rails (28), are each supported by a pneumatic pressure cushion (24) and can be pressed against the other wire with an adjustable pressure. At least some of the rails (27, 28) may be exchanged for rails of another size whereby especially the width (b, B) of their wire support areas differs.

Description

 Twin wire former

The invention relates to a twin wire former for producing a fibrous web, in particular paper or cardboard web, from a fibrous suspension, in particular with the features specified in the preamble of claim 1. The following documents are referred to the state of the art: 1. GB 2174120 A (file P 4083) 2. EP 0371786 A2

3. WO 91/02842 (file P 4668)

4. DE-OS 40 05 420 = US-PS 5,045,153 (file: P 4713)

The two documents 1 and 2 disclose different ones. Twin-wire former, in which the top wire runs along the (essentially flat) underside of a drainage box. According to EP '786, this drainage box has rigid strips on its underside. Directly below these rigid strips, the bottom wire runs over flexible strips that can be pressed on. The latter are close together in some embodiments of the GB * 120, so that water cannot be drained down through the lower sieve in their area. The same applies to other versions of the GB '120, in which a flexible plate is provided instead of strips. In further embodiments of the GB '120, there are smaller or larger gaps between resiliently pressable strips, which can absorb smaller or larger amounts of water and discharge them laterally to the outside. This also applies to the twin wire former according to EP '786. In all these cases, the entire flat lower surface of the drainage box arranged in the upper sieve is covered by the resiliently pressable strips located in the lower sieve or by the said flexible plate, with the exception of at most the narrow zones of the spaces mentioned. As a result, the drainage of water down in the entire area of the flat bottom of said drainage box is more or less disabled.

Therefore, one of the disadvantages of all these known arrangements is that - in the area of the resiliently pressable strips (or the flexible plate) - the drainage takes place exclusively (or almost exclusively) upwards. Therefore, the quality of the fibrous webs produced, particularly with regard to "formation" or "transparency", leaves something to be desired.

It is also problematic that the gaps mentioned clog over time, so that the formation becomes uneven across the width of the web.

One had therefore switched to the construction method, in which only relatively few resilient strips are provided which can be pressed onto a sieve. There are large gaps between the strips that can hold large amounts of water. In addition, openings are provided so that these quantities of water can flow downwards in the shortest possible way. Twin wire formers of this type are described in documents 3 and 4. In general, the following applies to the twin wire formers according to documents 1-4: thanks to the resiliently supported strips, which are arranged in relation to the rigid strips, the following can be achieved: for example, if the amount of suspension flowing between the two sieve belts is increased, the resiliently supported strips can deviate somewhat . This eliminates the risk (which exists when using only firmly supported strips) that a back-up builds up in front of the strips in the fiber suspension. Such a backflow could destroy the fibrous layers so far formed on the two sieve belts. In other words: in the known double sieve formers according to Scripts 1-4, once the drainage pressure has been set, it remains constant thanks to the resiliently supported strips, even with one Change in the amount of suspension added or change in the dewatering behavior of the fiber suspension. There is therefore an automatic adjustment of the gap width between the fixed and flexible strips when one of the changes mentioned takes place. The known arrangements therefore allow the production of webs in a very large basis weight range, namely from relatively thin paper webs to relatively thick cardboard webs.

With the twin-wire formers known from documents 3 or 4, fibrous webs, in particular paper webs, with relatively good "formation" (i.e. with even fiber distribution - or in other words: with good "transparency") can be formed.

However, demands of this kind have recently been "screwed up" even higher. In addition, there is an additional requirement for many types of paper, namely that the properties (e.g. printability) should be the same on both sides of the web.

The invention is therefore based on the object of designing a twin-wire former in accordance with the preamble of claim 1 in such a way that the quality of the fibrous web produced is further improved, in particular with regard to the formation (see-through) and with regard to the same properties on both web sides (ie reduction of the so-called two-sidedness) ).

This object is solved by the features of claim 1. In other words, they say the following:

An already existing twin screen former designed according to the invention can be modified by simply exchanging one or more strips for strips of different widths in such a way that a paper web with one another previously modified quality characteristics arise.

The selection of different widths of strips has an influence not only on the ratio of the amounts of water that flow through the two sieve belts (= "control of the main drainage direction"), but also on a certain migration of the fine and fillers (during the Dewatering process) within the thickness of the resulting fibrous web. In other words, it can be determined on which side of the finished paper web a higher or lower proportion of the fine and fillers should be present. This makes it possible, for example, to influence the printability of the paper web, for example to ensure that the printability of both paper sides is as uniform as possible. In some cases it can happen that a section of the paper machine following the twin wire former, e.g. the press section, in the paper web creates an undesirable two-sidedness, and that the cause of it cannot be eliminated there. In such a case - thanks to the invention - in

Twin wire formers can be counteracted with the measures described above to prevent two-sidedness.

The invention is applicable to many different twin wire former designs, e.g. in so-called hybrid formers (which are known to have a single-screen pre-dewatering section in front of the twin-wire zone), to which the fiber suspension is fed in the usual manner by means of a headbox; in addition - for the production of a two-layer fibrous web - a secondary stock headbox can be arranged directly in front of the twin wire zone. Another example are the so-called gap formers, in which the formation of the web takes place essentially from the beginning between the two wire belts. In this case, the path of the two sieve belts through the

Twin sieve zone can be arranged in any direction, for example approximately horizontally or vertically (from bottom to top or vice versa) or, for example, rising obliquely. The path can be straight or can be composed in any way of different curved areas or of curved and straight areas.

Various embodiments of the invention are described below with reference to the drawings.

Each of Figures 1 to 8 shows one of the various embodiments in a schematic representation in longitudinal section.

In FIG. 1, two sieve belts 11 and 12 (with the fibrous suspension 1, which is still partially liquid therebetween) run in the direction of arrow R between a lower dewatering box 17 and an upper dewatering box 18. The lower drainage box has a rigid strip 8 at its front end (with respect to the direction of wire travel), which can, however, also be omitted. This is followed (at a certain variable distance) by a plurality of strips 27, which are resiliently pressed onto the lower sieve 11 from below. For this purpose, they are supported on the rigid, water-permeable plate 26, for example, via support strips 127 and pneumatic pressure cushions 24. It goes without saying that the pressure prevailing in the pressure pads can be individually adjusted for each individual bar 27. A preferred construction for the strips 27, 127 and for their vertical guidance is described in DE 40 19 884 = US 5 078 835 (file P 4734). The following alternative is not shown: The strips 27 rest on a flexible plate which is supported by several pneumatic pressure cushions.

The upper drainage box 18, on which a guide roller 14 for the top wire 12 is mounted, can be used both at the front and at the rear end, as schematically with Double arrows P and P * indicated, to be hung on not shown, approximately vertically displaceable support elements. Thus, the position of the guide roller 14 and the box 18 is adjustable, if necessary also during operation. On the underside of the box 18 there is a row of strips 28 which abut the top of the top sieve 12. Each bar 28 is attached to a support bar 128 which is fixedly connected to the box 18. Above the strips 28, 128, a front vacuum chamber 21 and a rear vacuum chamber 22 are provided in the drainage box 18. In front of the upper dewatering box 18, the top wire 12 runs over the mentioned guide roller 14. In FIG. 1 it is therefore assumed that the bottom wire 11 has a substantially horizontal single-wire screen between a headbox (not shown) and the meeting with the top wire.

Pre-drainage section forms. The pre-dewatered, but still partially liquid fiber suspension 1 is shown in FIG. 1 in an exaggerated thickness. However, it can be seen that the box 18 and guide roller 14 are adjusted such that the top wire 12 between the guide roller 14 and the first upper bar 28 comes into contact with the top of the fiber suspension, namely e.g. at point K. The zone in which the upper strips 28 lie opposite the lower strips 27 is the so-called "sandwich zone".

1, the following is provided as an example: in the area of the upper drainage box 18, the number of rigid strips 28 is larger (preferably approximately twice as large) than the number of lower, resiliently supported strips 27. In other words: on the upper drainage box 18 is the strip division t approximately half of the strip division T of the lower box 17. Within the length of the upper box 18, each of the lower strips 27 lies opposite a gap between two upper strips 28. Each two upper strips 28 lie opposite a gap between two lower strips 27. 1, the upper and lower strips can also be arranged in approximately the same division, at least in a part of the sandwich zone, preferably in its rear part, see FIGS. 2-5. Each of the strips 27, 28 has one coming into contact with the sieve belt 11 or 12

Sieve support surface with a certain width B or b. According to FIG. 1, this width is the same on all ^" upper strips 28 and the same on all lower strips 27; it is also the same size on the lower strips or only slightly larger than on the upper strips.

The drainage boxes 17 and 18 are followed, for example, by a curved suction box (not shown) arranged in the upper sieve 12 in the form of an extension of the box 18.

During operation of the twin wire former, intensive double-sided drainage (downwards and upwards) takes place in the area where the lower and upper strips 27, 28 lie opposite one another, each of the strips 27, 28 being deflected by a slight (barely visible) deflection of the sieve belt 11 or 12 in question introduces a pressure pulse in the still liquid part of the fibrous material, which causes a more uniform fiber distribution (for example, splits existing flakes). It can therefore be expected that the paper web produced using the twin-wire former has a relatively good "formation" (see-through). If, however, it is found that the surface properties of the two paper web sides are not sufficiently uniform, this can be remedied by replacing at least some of the strips 27, 28 on the twin-wire former according to FIG. 1 with other strips whose screen support surface is different Width.

An example of this type is shown in Figure 2. This differs from Figure 1 essentially only in that instead of the lower strips 27 with relatively smaller Width 227 strips are now used, the screen support surface has a much larger width B *. As a result, the drainage through the lower sieve 11 is somewhat slowed down compared to FIG. 1.

FIG. 3 shows another modification of the exemplary embodiment according to FIG. 1. Here, the upper strips 28 have been replaced by strips 283 whose screen support surface has a greater width b ', again compared with FIG. 1. The width B of the screen support surface on the lower strips 273 is only slightly changed compared to FIG. 1. Only the shape of the strips has changed; it is now approximately symmetrical. Compared to FIG. 1, it can be expected according to FIG. 3 that ^" the drainage of the fiber suspension in the upward direction is somewhat weaker.

The embodiment according to FIG. 4 again has the same upper drainage box 18, which has already been shown in FIGS. 1-3. The only difference from FIG. 1 is that approximately the first half of the upper strips 28 is replaced by strips 284, the sieve support surface of which is somewhat larger Width b ". The bar division t in the upper box 18 has remained unchanged. In contrast, the number of lower bars 27 has been increased in the front part of the lower box 17, on the one hand by providing additional bars before the top box begins , and in that the division in the front part of the box is approximately halved, thus approximately corresponds to the upper strip division T. Another variant can also be seen, according to which the first two strips are replaced by a plate 9 which rests on, for example, two pneumatic pressure cushions 24 by means of two movable support strips 127. Normal individual strips 274 follow, their sieve support area B ^{1 is} relatively large, then some strips 27, the shape of which is unchanged compared to FIG. Deviating from Fig. 4 could in total lower box 17, the bar division t be the same size as in upper box 18.

FIG. 5 shows a further possibility of modification of the original construction shown in FIG. 1. Thereafter, every second of the strips 28 has been removed without replacement in the rear part of the upper drainage box 18. In the lower drainage box 17, relatively narrow strips 275 are provided in the front area, whereas relatively wide strips 275 'are provided in the rear area (both with an approximately rectangular cross section, for example). In contrast to this, in the exemplary embodiment according to FIG. 6, a row of unusually wide strips 286 is arranged in the rear part of the upper drainage box 18, each of which replaces two original strips 28.

With all these different modifications of the original construction according to FIG. 1, the drainage is either forced upwards or downwards. Accordingly, the fine and fillers are shifted either upwards or downwards within the partially still liquid fiber suspension. This ultimately results in changed properties of the two sides of the finished paper web.

While FIGS. 1-6 explain the principle of the invention using the example of a hybrid former, a gap former is shown schematically in FIG. There, the two screens 11 and 12 are brought together via a breast roller 11a, 12a. A headbox 10 is arranged between them. The twin-wire zone begins at a curved drainage box 13 (for example arranged in the lower screen 11). This is followed by a sandwich zone with an upper drainage box 18, which essentially corresponds to that of FIG. 1, and with a lower drainage box 17 ', which in principle is the same Box 17 corresponds to Figure 1. With this gap former, they can Strips 275, 28 of the two drainage boxes 17 'and 18 can be exchanged in the same way for any other strips, as was explained above with reference to FIGS. 2-6; the last division can also be varied as required.

In all the exemplary embodiments shown (FIGS. 1-7), the two sieve belts 11, 12 run essentially rectilinearly through the so-called sandwich zone (e.g. between the strips 27 and 28 in FIG. 1). In other words: the screen support surfaces, in particular the rigidly supported strips 28, lie at least approximately in a single plane. However, you can also deviate from this. In particular, it can be advantageous to arrange the sieve support surfaces of the strips 28 along a slightly convexly curved (that is, bulged downward) sieve raceway, similar to that known from FIG. 5 of WO 91/02842 (file P 4668).

Claims

Claims 1. Twin wire former for the production of a fibrous web, in particular paper or cardboard web, from one Fibrous material 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 the twin-wire zone, the one belt (12) runs over rigid strips (28) which are arranged at a distance from one another on a drainage box (18); c) in the twin-wire zone also runs the other wire belt (11) over a few bars (27), which are opposite the rigid bars (28), by means of resilient elements (springs, pneumatic pressure pads 24 or the like) and supported with a selectable force the other sieve belt can be pressed on; d) each of the strips (27, 28) has one with the Sieve belt (11, 12) coming into contact with a sieve support surface with a width (b or B) (e) measured in the direction of sieve travel, characterized in that in the so-called sandwich zone, rigid strips (28) and resiliently supported strips (27 ) face each other, at least in one of the two sieve loops (11 and / or 12) at least one of the strips (27, 28) can be exchanged for a strip whose sieve support surface has a different width (eg B ¹ or b ¹ ). 2. twin-wire former according to claim 1, characterized in that the screen support surfaces of all strips (27 or 28) of a belt (11 or 12) have a greater width (B ¹ , b ¹ ) than the screen support surfaces of the strips of the other belt or the other way around. 3. twin wire former according to claim 1 or 2, characterized in that the strip division t in the two wire loops (11, 12) is the same. 4. twin wire former according to claim 1 or 2, characterized in that the strip division (t) in one sieve loop (12) from the strip division (T) in the other sieve loop (11) deviates. 5. Twin wire former according to claim 1 or 2, characterized in that the strip division within at least one wire loop (11, 12) is smaller in the beginning area of the sandwich zone than in its end area. 6. twin wire former according to claim 1 or 2, characterized in that the strip division within at least one wire loop (11, 12) in The initial area of the sandwich zone is larger than in its end area. 7. Twin wire former according to one of claims 1-6, characterized in that the sieve support surfaces of the strips (27, 28) within one and / or the other sieve loop (11, 12) have different widths (eg B and B ¹ ) (Fig. 4). 8. Twin wire former according to one of claims 1-7, characterized in that it is a hybrid former. 9. Twin wire former according to one of claims 1-7, characterized in that it is a gap former.