EP0403577B1 - Twin-wire former and process for making a continuous web of fibrous material - Google Patents

Abstract

A twin-wire former for making a continuous web of paper (9) comprises two endless travelling screens (11, 12) which travel over breast rolls (13, 14) and form a wedge-shaped feed gap of a twin-wire region. In said region, the two travelling screens pass first over a curved stationary supporting device (15), then over a forming roll (16) which lies in the same screen loop (11) as the curved supporting device (15), and finally over a suction roll (18) which lies in the other screen loop (18). To provide a large storage volume for water, and additional lattice-like outer shell (27) surrounded by a coarse-mesh textile filter envelope (28) is arranged on the body (16a) of the forming roll. Approximately half the periphery of the suction roll (18) is surrounded by the two screens (11, 17). At the end of said region of the periphery, one screen (11) runs off followed by the other screen (12) together with the continuous web of paper (9) from the suction roll (18).

Description

The invention relates to a twin-wire former for producing a fibrous web, in particular paper web, in particular with the features specified in the preamble of claim 1. The invention also relates to a method for producing a fibrous web by means of such a twin wire former.

A twin-wire former is known under the name "Bel Baie III" (Preprints "B", 74th Annual Meeting, Technical Section, Canadian Pulp and Paper Association, p. B286-B289), see also FR-A-2131765. It essentially has the following structure: Two breast rollers guide two sieve belts from bottom to top in an essentially vertical twin-wire zone. The two screens converge towards one another in the area of a curved stationary support device which is arranged directly above one of the two breast rollers. A headbox arranged below the breast rollers conveys a stream of material into the inlet gap formed by the two screens. The material jet then forms a fibrous web in the twin-wire zone. (As is well known, this is done by removing most of the material water, mainly due to the tension of the screens and the curved shape of the twin-wire zone). The screen guide surface of the stationary support device has water discharge openings, at least some of which can be connected to a vacuum source. A forming roller designed as a suction roller is arranged behind the curved support device in the direction of wire travel. This forming roller and the curved support device are both within the same screen loop. At least one deflector is arranged in the other sieve loop for removing the water which has penetrated through the mesh of the sieve in the region of the curved support device.

The known arrangement has the following special features: the forming roller and the stationary support device lie in the loop of the second screen. In addition, the two screens separate in the upper area of the circumference of the forming roller. This is done in that the drainage point of the first screen lies in the web running direction before the drainage point of the second screen. The second screen then runs together with the fibrous web formed over a screen suction roll, where additional dewatering is to take place, and to the usual take-off point.

In the known arrangement one tries to achieve the following: By means of the stationary support device with a very large radius of curvature arranged at the beginning of the twin wire zone, the formation of the fibrous web should start as gently as possible despite the high working speed. Due to the simultaneous dewatering on both sides, the fibrous web formed, preferably paper web, should have the same possible properties on both sides (ie low two-sidedness). At the same time, the paper quality is to be increased by the fact that as little fiber and fillers as possible are lost during dewatering (ie the highest possible retention). Problems occur with this known twin-wire former, however, in that the "second" wire at the wrapping zone of the forming roller comes into direct contact with the suction roller jacket of the forming roller, which perforates in the usual way is. As a result, there is a risk that the perforation of the suction roll jacket in the paper web produces a so-called perforated shadow marking. This reduces the quality of the finished paper web. One could counter this danger by forcing the dewatering and web formation in the beginning area of the twin-wire zone, that is to say in front of the forming roller, in such a way that the web formation is at least largely completed when the forming roller is reached. Forcing the drainage on the stationary support device, however, would cause the risk of lower retention and also the creation of so-called pinholes in the paper web, in particular in the case of relatively thin types of paper. Such needle books presumably arise from the fact that the dewatering speed at individual points on the paper web is above average.

It should also be noted that the twin-wire zone is curved in only one direction. This means that there is no counter-curvature, e.g. is present in the S-shaped curved twin-wire zone of FIG. 2 of US Pat. No. 3,876,499. This creates the risk of a certain two-sidedness of the finished paper web; i.e. that the finished paper web does not have the same properties on both sides to the required extent.

Another disadvantage of the known twin-wire former is that the separation of the two wires takes place before the wire suction roll, that is, at a point where the paper web formed still has a relatively low dry content. As a result, the first screen, when separated from the paper web, tends to forcefully remove many fibers from the paper web that is formed (and continues with the second screen). This in turn affects the quality of the finished paper web.

The invention is based on the task to improve the known twin-wire former so that the web formation - despite the highest possible working speed - at the beginning of Double sieve zone can take place even more gently than before in order to achieve the highest possible retention and to counter the risk of pinholes. At the same time, the aim is to eliminate the risk of perforated shadow marking on the forming roller. Finally, when the separation point of the two screens is reached, the fibrous web should have a higher dry content than previously, in order to counteract the risk of fibers being torn out of the fibrous web.

This object is achieved by the characterizing features of claim 1. Thereafter, it is characteristic of the twin-wire former according to the invention that the curved stationary support device and the forming roller are no longer arranged in the loop of the second but the first wire and that the twin-wire zone also extends over the wire suction roll, preferably from the periphery of the wire suction roll about half of both seven is wrapped. Another essential feature of the twin-wire former according to the invention is the water storage volume of the jacket of the forming roller which is increased many times over. As is known per se (DE-PS 32 10 320), this is achieved in that an additional grid-shaped outer jacket is arranged on the perforated roller body, preferably in the form of a honeycomb cover.

By combining these features with the still existing curved support device it is achieved that the zone of the main drainage (ie the zone of web formation, in which part of the fiber material is still in the form of a suspension) extends from the curved stationary support device to far in the looping zone on the forming roller can be extended. In other words: the drainage on the curved stationary support device can start much more slowly than before. This can be controlled according to claim 8 by varying the negative pressure in the stationary support device and or by varying the tension of the sieves. (Result: higher retention, no or much fewer pinholes as before). The formation of the web is then continued - due to the smaller radius of curvature of the forming roller - in its wrapping zone with a much higher intensity than before, so that the web formation is completed at the latest at the end of the wrapping zone. This can in turn be controlled according to claim 8 by the measures already mentioned above and additionally by varying the negative pressure in the forming roller. In this context, the greatly increased water storage capacity of the forming roller is important. This not only significantly increases their drainage performance, but also completely eliminates the risk of the shadow shadow marking occurring in the paper web.

Finally, in the end area of the twin-wire zone, namely on the wire suction roll, further dewatering of the finished paper web takes place. Thanks to the very long looping zone of the wire suction roll, a significantly higher dry content of the paper web can be achieved than with the known construction. This means that much less fibers are torn out of the paper web at the separation point of the two screens than before.

It is important to maintain the features that the forming roller is arranged in the same screen loop as the stationary support device and immediately behind it, and that at least one deflector is provided in the other screen loop in the area of the stationary support device for bundled water removal. The first-mentioned feature saves a lot of space, in particular in view of the fact that in many cases a deflector (with a large supporting body cross section) must be arranged in the other sieve loop exactly where the sieves have left the stationary support device. If one also wanted to arrange the forming roller in the other wire loop, then a very large distance from the stationary support device would have to be kept (in order not to collide with the deflector). The result would be: increased space requirements and a long unsupported wire run with the risk of disturbed web formation.

An additional advantage of the twin-wire former according to the invention results from the fact that the majority of the suction zone on the wire suction roll is covered not only by one wire, but by two wires: this significantly reduces the noise

Deviating from the known design, the twin-wire zone now has an S-shaped course. This counteracts the risk of a certain two-sidedness of the finished paper web; i.e. it is possible to ensure the same properties on both sides of the paper web with greater certainty than before.

From the magazine "Pulp & Paper" Sept. 1982 (pages 130-139, esp. Pp. 133 and 136) a twin wire former (under the name "Papriformer") is already known, in which two suction rolls in an S-shaped twin wire zone are connected in series. There, however, other essential features of the twin-wire former according to the invention are missing, in particular the curved stationary support device provided at the beginning of the twin-wire zone and the substantially increased water storage capacity of the first suction roller (same as forming roller) (achieved by the additional lattice-shaped outer jacket mentioned).

From US-PS 3,876,499 a twin-wire former is known, which is formed in that a top wire is placed on a conventional wire. The material is fed from top to bottom. In the (from top to bottom) starting area of the twin-wire zone, a stationary curved support device is arranged one after the other, further dewatering elements behind it and finally a forming roller. The twin-wire zone according to FIG. 1 is then curved in the other direction by a second forming roller arranged in the lower wire. Both forming rollers can be designed as suction rollers. The second forming roller is wrapped only about one eighth of its circumference by the two wire belts. Behind the second forming roller both sieve belts run over some suction boxes. At one of the suction boxes, the top wire separates from the paper web now formed and from the bottom wire. The latter guides the paper web over at least one further suction box and over a sieve suction roll to a take-off point.

The stationary support device of the US-PS has a relatively large and the first forming roller a relatively small radius of curvature. This in turn is intended to ensure that the formation of the fibrous web begins as gently as possible. After that, i.e. in the area of the smaller radius of curvature, the drainage is then forced. According to column 4, the dryness behind the second forming roller is "at least 1.5%". From this relatively low value it can be concluded that in the area between the two forming rolls, ie where the curvature is reversed, part of the fiber material is still in the form of a suspension; i.e. part of the fiber is still floating in the water. This has a very unfavorable effect on the further formation of the fibrous web. Another disadvantage of the known construction is the fact that the arrangement of the aforementioned suction boxes in the manner of a conventional fourdrinier wire takes up a lot of space.

In contrast, in the construction according to the invention (in particular by the already mentioned additional lattice-shaped outer jacket of the forming roller and by the very large suction zone of the forming roller) it is ensured that the formation of the fibrous web is completed at the latest at the end of the wrapping zone of the forming roller. In addition, the design according to the invention takes up much less space. This is achieved on the one hand by the fact that the forming roller is arranged directly (ie at a very short distance) behind the stationary support device and on the other hand that after the formation of the web, the further dewatering takes place exclusively on the sieve suction roller wrapped by the two sieves.

• Die Figur 1 zeigt einen Doppelsieb-Former mit im wesentlichen vertikaler Bahnbildungszone in einer schematischen Seitenansicht.
• Die Figur 2 zeigt die Doppelsiebzone eines Doppelsieb-Formers, der ähnlich dem Doppelsieb-Former der Figur 1 ist, jedoch in vergrößertem Maßstab.
• Die Figur 3 zeigt schematisch einen Doppelsieb-Former mit im wesentlichen horizontaler Bahnbildungszone.

Further advantageous embodiments of the invention are specified in the subclaims. These advantages are explained below in the context of the description of two exemplary embodiments.

• FIG. 1 shows a twin-wire former with an essentially vertical web formation zone in a schematic side view.
• FIG. 2 shows the twin-wire zone of a twin-wire former, which is similar to the twin-wire former of FIG. 1, but on an enlarged scale.
• FIG. 3 shows schematically a twin-wire former with an essentially horizontal web formation zone.

1 shows a headbox 10, two endless sieve belts, namely a first sieve 11 and a second sieve 12, each of which is guided into a twin-wire zone by means of a breast roller 13 and 14. In the twin-wire zone, the two screens 11 and 12 first run over a curved stationary support device 15 (with a very large radius of curvature) and immediately afterwards over a forming roller 16 (with a relatively small radius of curvature). These two elements 15 and 16 as well as a suction box 17, which can also be designed as a deflector, are all located within the loop of the first screen 11. Then the two screens run over a screen suction roller 18 located in the loop of the second screen 12. Nearby At the top vertex of this roller 18, the two screens 11 and 12 separate, the paper web being taken from the second screen 12 to a take-off suction roller 19. The remaining guide rolls for the first screen are designated 21, 21a and the guide rolls for the second screen 22, 22a. The guide rollers 21a and 22a are designed to vary the screen tension as screen tensioning rollers. Finally, a machine frame 23 for the first screen and a machine frame 24 for the second screen are indicated in FIG.

The twin-wire former is shown in a preferred arrangement, in which the outflow direction of the headbox 10 and the direction of the wires 11 and 12 run approximately vertically from bottom to top in the initial region of the twin-wire zone. However, other arrangements are also possible (see Figure 3).

In Figure 2 it can be seen that the curved stationary support device 15 can be divided into two separate drainage boxes 15a and 15b. Each of these boxes has a curved screen guide surface formed from different strips 25. There are slots 26 between the strips, through which part of the water is removed. The lower drainage box 15a can be divided into a lower and an upper area by means of an intermediate wall 45, only the upper area having a suction connection 46 which leads to a vacuum source V. The upper drainage box 15b is formed uniformly as a suction box. Lateral water outlets are labeled 47. The distance A between the upper end of the upper drainage box 15b and the point where the screens 11, 12 run onto the forming roller is relatively small. Furthermore, it can be seen from FIG. 2 that almost the entire upper right quadrant of the forming roller 16 is wrapped in the two screens 11 and 12. It is shown schematically that the forming roller 16 has a perforated roller body 16a and a suction box 16b therein. Its suction zone also extends almost over the entire upper right quadrant of the forming roller. A honeycomb cover 27 is arranged on the roller body 16a of the forming roller 16 and is composed of upright strips or bleaching strips. This honeycomb cover forms an additional lattice-shaped outer jacket that has large-volume cells that are open in the radial direction; that is, these cells are open to both the screen 11 and the bores of the roller body 16a. A fabric jacket is arranged on the surface of the honeycomb train 27, in the form of a coarse-mesh screen fabric 28.

In FIG. 2, the thickness d of the material jet leaving the headbox 10 and the distance a between the two screens 11 and 12 (shown, for example, at the outlet from the stationary support device 15b) are exaggerated. This should make it clear that the two screens 11 and 12 converge not only in the area of the curved stationary support device 15, but also in the wrapping zone of the forming roller 16. This makes it clear that the process of forming the web on the support device 15 starts relatively slowly and only ends on the forming roller 16. The end of the zone in which the two sieves converge (and thus the end of the web-forming process) can, for example, be approximately in the middle of the looping zone of the forming roller 16, as is shown only by way of example in FIG. 2. 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%.

Deflectors 29, 30, 31 serve to strip off water which has penetrated through the mesh of the second sieve 12. A deflector 29 is arranged where the two sieves 11 and 12 are unsupported from the lower (15a) to the upper (15b) drainage box to run. Another deflector 31 is arranged where the two screens 11 and 12 run unsupported from the upper dewatering box 15b to the dewatering roller 16. Another deflector 30 can be arranged in the lower region of the lower drainage box 15a. The deflectors are important in order to remove the water penetrating through the mesh of the second sieve 12 as early as possible in the initial region of the double sieve zone, so that the subsequent dewatering through the second sieve 12 can take place unhindered. The deflectors bundle the water, ie remove it in the form of water jets that are as compact as possible. This counteracts the tendency that (at the desired very high working speed, order of magnitude approx. 1500 m / min).

the water emerging from the sieve mesh is sprayed into fog. This would not only be uncomfortable for the operating personnel, but there would also be the risk of rewetting the paper web on its way from the screen suction roller 18 to the take-off roller 19. The deflectors 29, 30, 31 are pivotally mounted and can therefore be more or less attached to the second screen 12 can be employed. The water thrown off in the looping zone of the forming roller 16 is collected and removed by means of a deflection plate 50.

The screen suction roller 18 has at least two suction zones 18a and 18b. A first large suction zone 18a is located in the area which is surrounded by two screens 11 and 12. A smaller suction zone 18b, in which a generally higher vacuum is set, is located behind the point where the first screen 11, guided by one of the screen guide rollers 21, stands out from the paper web 9 formed. The entire suction device can also be divided into three suction zones. Scrapers that remove water and any material particles from the rollers 13, 14 and 21 are designated 33, 34 and 41 in FIG.

The arrangement shown in FIGS. 1 and 2, in which the screens 11, 12 run essentially from bottom to top in the web formation zone, is preferred for various reasons: the stock feed to the headbox 10 is much simpler than e.g. in the arrangement according to US Pat. No. 3,876,499. In the area of the stationary support device 15, the water (emerging from the sieves 11 and 12) is initially discharged in a predominantly horizontal direction relatively evenly on both sides. This reinforces the tendency towards a high uniformity of the initial web formation on the two screens and thus towards a uniformity of the paper properties on the top and bottom.

However, according to FIG. 3, the invention can also be implemented with predominantly horizontal guidance of the screens 11 ', 12' in the web formation zone. The first sieve 11 'can here as "Upper sieve" and the second sieve 12 'are referred to as "lower sieve". Preferably, as shown, the arrangement of the stationary support device 15 'and the forming roller 16' in the top wire 11 'and the screen suction roller 18 in the bottom wire 12'. Otherwise, the paper web 9 would hang behind the screen suction roller 18 on the underside of the second screen 12 '. The stationary support device 15 'is provided with a (known per se) water lifting device 48. At the forming roller 16 'the wrapping zone (and thus also the suction zone 16b) is somewhat larger than in Figure 2. Otherwise, however, the elements of Figure 3 are essentially the same as those of Figures 1 and 2 and therefore provided with the same reference numerals.

Claims (8)

1. A twin-wire former for making a continuous web of fibrous material, in particular a web of paper, having the following features:

   a) Two endless wire webs (a "first wire" 11 and a "second wire" 12) are guided in the outlet region of a head box (10) by means of a breast roll (13, 14) each in the initial region of a twin-wire zone, in which they converge, and in fact at at least one curved stationary forming device (15), the wire guide surface of which comprises water discharge apertures (26) which can be connected to a negative pressure source (V);

   b) inside the same wire loop as the stationary forming device (15) - and only a small distance (A) behind it - is disposed a forming roll (16) constructed as a suction roll, the radius of curvature of which is smaller than the radius of curvature of the stationary forming device (15) and around which both wires (11, 12) are wound at least over one sixth (and preferably over roughly one quarter) of its circumference, the suction zone (16b) of the forming roll extending substantially over the entire winding zone;

   c) in the other wire loop, and in fact in the region of the curved forming device (15), is disposed at least one deflector (e.g. 31) which can be adjusted at the wire (12);

   d) in the wire direction behind the forming roll (16) is disposed a further suction roll (18) (hereinafter referred to as "wire suction roll"), from which the continuous web of fibrous material (9) runs together with the second wire (12) to a web transfer position (19);

   characterised by the following features:

   e) the curved stationary forming device (15) and the forming roll (16) are disposed in the loop of the first wire (11), but the wire suction roll (18) is disposed in the loop of the second wire (12);

   f) on the perforated roll body (16a) of the forming roll (16) is disposed a latticed outer shell (27), which forms large-volume cells open in the radial direction, for the purpose of the temporary storage of water;

   g) at least one third of the circumference of the wire suction roll (18) has both wires (11 and 12), which one after the other run away from the wire suction roll (18), wound around it.
2. A twin-wire former according to Claim 1,
   characterised in that the region of the twin-wire zone, in which the two wires (11 and 12) converge, extends from the bottom to the top (Fig. 1 and 2).
3. A twin-wire former according to Claim 1,
   characterised in that the region of the twin-wire zone, in which the two wires (11′ and 12′) converge, extends roughly horizontally, the downwardly curved stationary forming device (15′) and the forming roll (16′) lying in the upper wire (11′) (Fig. 3).
4. A twin-wire former according to one of Claims 1 to 3,
   characterised in that roughly half the circumference of the wire suction roll (18) has the two wires (11 and 12) wound around it.
5. A twin-wire former according to one of the preceding Claims,
   characterised in that the latticed outer shell (27) of the forming roll (16) is formed by a honeycomb covering composed of edgewise tapes, on which a fabric shell (28) is disposed.
6. A twin-wire former according to one of the preceding claims,
   characterised in that in the loop of the second wire (12) in that region of the twin-wire zone where the two wires (11 and 12) run from the curved forming device (15) to the forming roll (16) is disposed a deflector (31), which can be adjusted at the second wire.
7. A twin-wire former according to one of the preceding claims,
   characterised in that in the loop of the first wire (11) between the forming roll (16) and the take-up point of the two wires on the wire suction roll (18) is disposed a suction box (17), the front strip of which acts as a deflector.
8. A process for making a continuous web of fibrous material by means of a twin-wire former, which is constructed according to one of the preceding claims,
   characterised by the following process steps:

   a) the negative pressure effective in the water discharge apertures (26) of the curved stationary forming device (15) is adjusted to such values and the tension of the wires (11, 12) is adjusted to the only slight curvature of the stationary forming device in such a way that only a part of the web formation occurs in its vicinity;

   b) the negative pressure in the forming roll (16) is adjusted to such values that the remaining formation of the web,occurs in its winding zone, so that the formation of the web has been completed at the end (E) of the winding zone of the forming roll at the latest.

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