EP0438685B1 - Former in a paper machine - Google Patents

Abstract

At the section of the papermaking machine where the pulp turns into paper, the widths and positions of the bars in contact with the fourdriniers are set in relation to each other together with the gaps between them so that every other gap between the bars at the upper fourdrinier is not covered wholly or partially by a bar at the lower fourdrinier.

Description

The invention relates to a former for a paper machine of the type described in the preamble of claim 1 and its advantageous use.

Formers of this type can be operated both in the immediate vicinity of a headbox, the suspension being injected in a flat jet into the space formed by the convergence of the sieves and then dewatered between the sieves, but they can also be provided with a pre-dewatering section in which Care is taken to ensure that the suspension coming from the headbox is guided over a certain distance on a sieve, with some of the water contained in the suspension being able to be led out, in particular downwards, even before it enters the former. The pre-dewatering section is then followed by the former of the type described here, the pre-dewatered suspension generally remaining on the sieve and the former being formed as a so-called twin-wire former while adding another sieve.

With previous forms of this type, e.g. from US-PS 4,769,111, DE-PS 31 38 133, DE-PS 31 53 305, DE-PS 35 46 629 and EP-OS 0 251 778 a variety of strips are known with which an improvement the formation is to be achieved, regularly distributed over the entire length of the formation section. It can happen that such distribution of the strips can lead to damage to the paper sheet formed, especially at the points of the formation section which are located behind the so-called water line.

In others, a ledge is inserted from the opposite side against the opposite drainage slot, most effectively at each slot and with considerable pressure so that the liquid is pressed out of the web can. Under certain circumstances, this pressure can be so high that the web to be formed is crushed between the sieves. If the pressure is absent or too low, the web can be re-moistened from above by the previously squeezed out liquid and thereby adversely changed. With the high pressures to be used, the sieves also suffer and are worn out earlier. Because of the narrow spacing between the strips, the spaces between the strips become dirty due to fibers and ashes being pushed out too much.

In the embodiment known from DE-PS 35 46 629, the lower strips are mounted on a flexible plate. There, however, the water drained downwards has to be discharged laterally between the strips, so that the amount is limited and there is a risk of contamination. No individual contact pressure of the strips can be set, and the reproducibility of the setting is insufficient.

If, as in accordance with EP-OS 0 251 778, a large number of strips are mounted on the lower sieve on a rigid frame, the risk of deformations, sieve damage etc. is particularly high. There, water drainage is also restricted, with a high risk of pollution.

In the previous arrangements, the strips on the lower sieve increase: sieve wear; the diarrhea, i.e. too many fines and ashes are washed out of the web; the formation impulses which are obtained on the lasts are inevitably too high in the previous devices, and often they lead to a formation deterioration in the case of substances sensitive to shear force.

The above-mentioned disadvantages of the previous devices are to be avoided with the invention. It should be possible to adapt the dewatering and the formation of the blade to the different technological conditions that arise on the way through the formation section, mainly the changing position of the water line of the web between the sieves by appropriate positioning of the strips relative to the web , ie to act on them only as gently as possible according to actual local needs, so that the sheet to be formed is not is damaged. The wear on the screens is also to be reduced and the water drainage is to be improved. The positioning to be carried out as required should also be possible during the operation of the paper machine.

According to the invention, this object is achieved on the former of the type described at the beginning by measures which are specified in claim 1.

The drainage and formation are carried out in a gentle manner according to the local technological conditions of the railway and according to the actual local need.

Advantageous exemplary embodiments of the former according to the invention are specified in the subclaims.

Fig. 1

schematisch einen Teil des Formers von der Seite gesehen;

Fig. 2

eine Kurve, entlang welcher eine Formstrecke gestaltet werden kann, und

Fig. 3

eine andere solche Kurve.

Fig. 4

schematisch eine spezielle Anordnung der Leisten;

The subject of the invention and its advantages are described and explained in more detail below. The description refers to a drawing in which:

Fig. 1

schematically a part of the former seen from the side;

Fig. 2

a curve along which a molding path can be designed, and

Fig. 3

another such curve.

Fig. 4

schematically a special arrangement of the strips;

The part of a former for a paper machine relating to the invention is shown schematically. In a suitably adapted form, it can also be used for a board machine. A paper sheet, possibly a cardboard sheet, is formed in the former between an upper wire 1 and a lower wire 2. In operation, the suspension located between sieves 1 and 2 is dewatered, forming the sheet. This should maintain a homogeneous structure. Dewatering and formation are carried out using strips 3 and 4, one with reference number 3 above the upper sieve 1 and the other 4 below the lower sieve 2 are provided. The strips 3 and 4 are arranged along the formation section, run transversely to the machine direction and are adjustable relative to the screens 1 and 2, so that they touch the screens facing them, which run past them, supported during operation. Some of the conceivable adjustment movements are indicated by thick double arrows. Distances 5 and 6 are provided between the strips 3 and 4, so that slots are left open transversely to the running direction, through which the liquid to be removed from the suspension between the sieves 1 and 2 takes place. This happens on both sides, away from the top wire 1 and from the bottom wire 2. 1 and 4 show two examples of the strip arrangement relative to one another.

For the strips 3, which have to touch the top sieve 1, there are various known expedient forms that can be used. The sides of the strips 3 facing the sieve produce a conforming surface which forms the spatial shape of the formation section. It forms e.g. a rust-shaped bottom surface of a housing 12 facing the upper sieve 1 for collecting and removing the liquid flowing through the gaps 5 'during operation. The spatial shape of the formation section can be formed by a straight line, as is the case according to FIG. 1. However, the shape can also be formed by a concave or convex curve, which are shown in FIG. 2 or FIG. 3, or an approximately wavy or arbitrary curve.

The strips 3 are arranged along the surface at intervals 5, whereby the columns 5 'are formed.

The strips 4, which have to be coated on the bottom wire 2, are each designed in a suitable form. They can have smaller differences from one another, for example in the design of their head part facing the lower sieve 2. It is an advantage of the subject matter of the invention that, viewed transversely to the direction of travel, larger tolerances are permissible without disadvantage for the dimensional accuracy of the head part facing the lower wire because of the relatively large distance between these strips resulting from the characteristics. The Possibilities To Favorably Form The Sheet Formation Process influence, can also be improved by deliberately created inequalities across the width. It is also conceivable to correct the shape of the head part over the width, in particular during operation, for example by deforming it in zones.

The strips 4 are arranged on a table 7 which can be positioned relative to the lower wire 2 along the formation path at regular or irregular intervals 6. In a particularly advantageous embodiment, the individual distance 6 is in each case greater than the sum of the width of an individual bar 3 located in the top wire and the distance 5 to the adjacent bar located in the top wire, in the machine running direction.

The invention is based on a formation section with a length of 500 to 3000 mm. In this case strips 3 or 4 with a width of between 10 and 60 mm would be used. One advantage is seen in the fact that in the formation area with the strips 3, depending on the technological requirements, zones, e.g. 1 to 10 in number. It is also possible in each case to close off some of the successive drainage slots in the machine direction at intervals 5 with inserts 8 which match the same intervals 5, which means that a longitudinal section of the surface is without suction. This can be designed in such a way that these inserts 8, which delimit the individual zones, could also be inserted from the side into the relevant distances 5 as required during operation. The individual drainage gaps 5 'can also advantageously be acted upon with different vacuum. For this, e.g. the partitions 11 shown schematically here in the housing 12 are required and an extraction system which can be set to several negative pressures.

The strips 4 to be touched on the lower wire 2 are mounted on the table 7, which can advantageously be raised, inclined and longitudinally displaceable relative to the lower wire during operation, so as to be positionable in the machine running direction. Ie they are not permanently connected to the table 7 at one point. For example, on the surface of the table there are several grooves running transversely to the machine direction, into which grooves with dovetail connection strips 4 can be pushed in and out. In this way, the strips 4 can, if required, be moved to another location during operation by pulling them out of a previous position from the side of the machine and pushing them into a new desired position, that is to say into a different groove.

The strips 4 to be touched on the bottom wire 2 are individually adjustable, supported on the table, in the direction of the bottom wire 2. The height of each can be adjusted individually. It when the strips 4 are pneumatically or hydraulically adjustable is particularly advantageous. They can be supported elastically and resiliently against the table 7 during operation on an air cushion 9, for example by have two telescopically nested parts, between which an inflatable air bellows is arranged.

An advantageous possibility of adapting the device to the requirements of paper technology results from the possibility of adjusting the table 7 in or against the direction of wire travel. As a result, the position of the strips 4 can be changed relative to the strips 3, which leads to positive effects in sheet formation. Depending on the application, it is conceivable that the first and / or the last bar of the formation section is a bar 4 acting on the lower wire 2 in the direction of wire travel.

It is a further advantage if the table 7 with the displaceable and flexibly supported strips 4 for positioning vertically to the lower sieve 2 is supported against its foundation on air bellows and is thus supported in a pneumatically elastic manner.

The measures described, which can also be carried out during operation: the zone formation along the fitting surface with the strips 3, the positioning of the strips 4 on the table 7, the individual adjustment of the strips 4 and the positioning of the table 7 enable the papermaker, the drainage and the formation of the sheet between the screens according to the technological requirements and the situation in a large operating area. All of the problems mentioned at the beginning the process can be overcome. It is possible to make the process gentle, as far as the paper sheet to be formed and the surrounding sieves are concerned. The pneumatically elastic support of the strips 4 and the table 7 also contributes to this, which is able to compensate elastically for a suddenly occurring increase in the thickness of the suspension web between the sieves without damaging the sheet or the sieves.

Claims (24)

1. Forming unit for a paper machine for forming the paper sheet between a top wire (1) and a bottom wire (2) when the suspension is dewatered between the wires using bars (3 and 4), which are arranged on both sides of the wires, at the top and bottom, along the line of the formation section, extend transversely to the machine direction, can be adjusted relative to the wires and contact the wires during operation, spacings (5 and 6) being provided between the bars so that gaps (5') are left for the drainage of the liquid to be removed upon dewatering, and the widths and positions of the bars (3 and 4) contacting the wires during operation as well as the spacings (5 and 6) between the bars being adapted to one another such that at least each second gap (5') between the bars (3) disposed in the top wire (3) is neither completely nor partly covered by a bar (4) disposed in the bottom wire, characterised in that the spacing (6) between the bars (4) disposed in the bottom wire is in each case greater than the sum formed by the width of one bar (3) disposed opposite in the top wire (1) and its spacing (5) from the adjacent bar disposed in the top wire (1), and that the bars (4), which are to contact the bottom wire (2), can be mounted with a table (7) along the formation section.
2. Forming unit according to claim 1, characterised in that the bars (3), which are to contact the top wire (1), are all of the same shape and joined together to form an osculating surface along which they are arranged at regular spacings (5).
3. Forming unit according to claim 1, characterised in that the bars (4), which are to contact the bottom wire (2), can be mounted so as to be displaceable in relation to the table (7) in the machine direction, as required.
4. Forming unit according to claim 1 or 3, characterised in that the bars (4) contacting the bottom wire (2) can be individually displaced on the table (7) in the direction of the bottom wire (2).
5. Forming unit according to claim 1, 3 or 4, characterised in that the bars (4) can be displaced by pneumatic or hydraulic means in a gas-cushioned manner, so that they are flexibly supported against the table (7) by means of a gas cushion in the direction of displacement.
6. Forming unit according to claim 1, 3, 4 or 5, characterised in that the table (7) with the bars (4) can be positioned relative to the running direction of the wires (1, 2) in the horizontal direction.
7. Forming unit according to claim 1, 3, 4, 5 or 6, characterised in that the table (7) with the bars (4) can be positioned relative to the bottom wire (2) in the vertical direction.
8. Forming unit according to one of the preceding claims, characterised in that the bars (4), which are to contact the bottom wire (2), are mounted so as to enable water which is removed from the area of the bottom wire to be discharged directly downwards and directly reach a collecting device (13).
9. Forming unit according to one of the preceding claims, characterised in that the bars (3, 4) can be mounted and dismounted while the forming unit is operating.
10. Forming unit according to one of the preceding claims, characterised in that the formation section is created along a curve or a series of curves.
11. Forming unit according to one of the preceding claims, characterised in that the bars (3), which are to contact the top wire (1), and the bars (4), which are to contact the bottom wire (2), in each case create an osculating surface on the side which faces the wire, and that these osculating surfaces do not overlap.
12. Forming unit according to one of claims 1 to 10, characterised in that the bars (3), which are to contact the top wire (1), and the bars (4), which are to contact the bottom wire (3), in each case create an osculating surface on the side which faces the wire, and that these osculating surfaces overlap.
13. Forming unit according to one of the preceding claims, characterised in that a variable vacuum can be applied to gaps (5') created between the bars (3), which are to contact the top wire (1).
14. Forming unit according to one of the preceding claims, characterised in that the length of the formation section above the top wire (1) is 500 to 3000 mm, the width of the bars (3) is 10 to 60 mm and they are up to 40 in number, that one to ten zones is or are created along the formation section and that the number of bars (4) below the bottom wire (2) is 1 to 20.
15. Forming unit according to one of the preceding claims, characterised in that the formation section above the top wire (1) can be altered by closing several or, in extreme cases, all the gaps (5') following one another in the machine direction by inserts (8) which fit into the spacings.
16. Forming unit according to one of the preceding claims, characterised in that adjusting elements are provided laterally of the twin wire to adjust the spacing between the mounting part which bears the bars (4) contacting the bottom wire (2) and the mounting part which bears the bars (3) contacting the top wire (1).
17. Forming unit according to one of the preceding claims, characterised in that the first and/or last of the bars contacting the wires during operation is a bar (4) which contacts the bottom wire (2).
18. Forming unit according to one of the preceding claims, characterised in that a suction device (10) follows the formation section to ensure that the moist web lies on the bottom wire (2).
19. Forming unit according to claim 1 or 2, characterised in that, viewed widthwise, the bars (3, 4) contacting the wires (1, 2) during operation are not plane on their surface which faces the wire, at least in part.
20. Forming unit according to claim 1 or 2, characterised in that the bars (3, 4) contacting the bottom wire (1, 2) during operation can be individually deformed widthwise at their surface which faces the wire, at least in part.
21. Use of the forming unit according to one of the preceding claims in a combination of a Fourdrinier and twin-wire paper machine, characterised in that the suspension to be dewatered is subjected to preliminary dewatering on an endless wire before reaching the forming unit, that the top wire (1) is advanced in the area of the forming unit such that the receiving space for the suspension which is to be further dewatered is created between these wires, and that the endless wire achieves the effects of the bottom wire (2) within the device according to the invention.
22. Use of the forming unit according to one of claims 1 to 20 in a twin-wire paper machine, characterised in that the suspension to be dewatered is introduced as a flat jet via a head box into the stock receiving space produced by bringing together the wires (1, 2).
23. Use of the forming unit according to claim 21 or 22, characterised in that a forming cylinder which is encircled by the top wire (1) is provided before the forming unit.
24. Use of the forming unit according to claim 21 or 22, characterised in that a forming cylinder which is encircled by the bottom wire (2) is provided before the forming unit.

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