CA2158039A1

CA2158039A1 - Process and twin-wire former to form a fibre material web

Info

Publication number: CA2158039A1
Application number: CA 2158039
Authority: CA
Inventors: Jurgen Banning
Original assignee: Individual
Current assignee: Voith Sulzer Papiermaschinen GmbH
Priority date: 1994-01-13
Filing date: 1995-01-12
Publication date: 1995-07-20
Also published as: FI953352A; WO1995019467A1; BR9505817A; JPH08508069A; DE4400782A1; EP0688375A1; FI953352A0

Abstract

In a twin-wire former to form a thick paper or cardboard web, a head-box (1) is arranged at the beginning of a substantially horizontal path of an endless lower wire (S1) to feed a stream of suspension thereto. Guides (9, 6, 6a, 9a) for an endless upper wire (S2) are arranged in such a way that the latter comes into contact (contact point K) with the suspension on the travelling lower wire at a short distance from the head-box (1). The two wires (S1, S2) together form an at least approximately straight and horizontal twin-wire region. Starting immediately behind the contact point (K) there are several successive strips, fixed strips (6) on one (S2) and flexible ones (3) on the other (S1) on the inside of both wires, which extend transversely to the direction of travel of the web.

Description

PA 10022 Wo SPECIFICATION

METHOD AND TWIN-WIRE FORMER FOR THE FORMING
OF A FIBER WEB

The present invention relates to a method of forming a fiber web, particularly a thick paper or cardboard web, from a suspension of fibers. The invention also relates to a twin-wire former for the carrying out of said method.
The starting point for the invention is WO 93/12291. In it, several different twin-wire formers are described which have the following features in common: Within a horizontal twin-wire zone, the one rotating wire (the upper wire) comes into contact with rigid drainage strips, while the other wire (the lower wire) comes into contact with resiliently supported strips each of which can be pressed with an adjustable force against the wire. Since the wires travel on essentially straight paths over the drainage strips, a suspension pressure the amount of which can be freely - selected and is independent of the amount of the longitudinal tension of the wire (in contradistinction to completely or partially curved twin-wire zones) can be produced in the fiber suspension by the resilient drainage strips. By means of the drainage strips pressure pulses are constantly exerted on the fiber suspension so that the fiber material is SPEC\123965 constantly reoriented in the portion of the suspension which is still liquid. In this way, the production of flocculations (agglomerations of fiber) is avoided, or flocculations which have occurred are broken up again.
Furthermore, the resilient drainage strips can move away somewhat if the dilution of the fiber suspension is increased or if the drainage behavior of the pulp changes or if, in an exceptional case, a clot passes through the twin-wire zone.
Many of the known twin-wire formers are developed as so-called hybrid formers. Such formers have a single-wire pre-drainage zone. The formation of a fiber web commences already on this one wire, it then passing, together with the liquid suspension remaining on it, into the following twin-wire zone. Directly in front of the start of the twin-wire zone, a so-called secondary headbox can be present which feeds an additional layer of suspension. One disadvantage of hybrid formers is that generally there is a very great tendency towards flocculation in the one-wire pre-drainage zone, this flocculation leading to an undesired cloudy or flocculent "formation" of the final fiber web. In principle, to be sure, the twin-wire zone described above tends to redissolve flocculations. However, the twin-wire zone loses this property more and more the higher the desired specific basis weight of the final fiber web is.

SPEC~123965 Other known twin-wire formers are developed as so-called gap formers. In this case a jet of suspension is introduced (or "shot") by means of the headbox into the wedge which is present at the start of the twin-wire zone between the wires which converge towards each other. In many of these gap formers, the two wires travel directly at said wedge over a curved wire guide element, for instance a curved stationary drainage box or a rotating forming roll. This type of former has proven excellent for the production of relatively thin fiber webs (for instance, printing papers) at relatively high operating speeds. The presence of said curved first section of the drainage zone has the result, however, that, from the very start, a relatively high drainage pressure is present which is dependent of the radius of curvature and of the longitudinal wire tension which is necessarily always present.
In general, the "shooting" of a jet of suspension requires a given minimum operation speed, which in many cases is unsuitable for the production of extremely thick fiber webs. This is true also of the gap former known from EP
0318107 in which a curved first section of the twin-wire zone is absent and in which, therefore, the "shooting" of the jet of suspension takes place between two straight wire travel paths, and this from the bottom to the top.

SPEC\123965 _ 2158039 The reverse arrangement in accordance with US 5,259,929 with the feeding of a jet of suspension from the top to the bottom is, in principle, to be sure, suitable also for relatively low operating speeds. Here, however, there is the disadvantage that the amount of the drainage pressure in the fiber suspension cannot be freely set since a geodetic pressure portion is always present. Furthermore, in that case, the two wires must be conducted over a roller within the twin-wire zone, so that an additional drainage pressure is present there as a result of the tension of the wire.
The object of the present invention is further to develop the known method and the known twin-wire former in such a manner that thick paper or cardboard webs of improved "formation" can be produced, i.e. have a higher freedom from flocculation than previously, so that the final product is imparted an improved smoothness of surface and a more uniform thickness.
This object is achieved by the method set forth in Claim 1 and by the twin-wire former according to Claim 5.
The inventor has discovered that substantial improvement in formation upon the manufacture of thick types of paper or cardboard (having a specific basis weight of more than 100 g/m2 and preferably between 200 and 1000 g/m2) can be obtained by the combination of the following features:

SPeL'\123965 1. The drainage path, i.e. the zone of the formation of the web, is drastically reduced as.compared with the hybrid formers previously used for the production of thick webs, since the suspension fed is drained towards both sides from the very start (towards the top and towards the bottom). In this way the microturbulence which is produced in the suspension by the headbox and which keeps the suspension free of flocculation can be used not only for a good lower side of the web but also for a good upper side of the web. This action is, however, intensified from the start by the above-described influence of the in part stationary and in part yieldable drainage strips.

2. In this connection, a short and substantially straight travel path of the lower wire, which is preferably supported by a wire table, is intentionally provided between the exit slot of the headbox and the place where the upper wire comes into contact with the suspension.
To be sure, it is not to be excluded that this wire table has openings through which a downward drainage can already take place. The said short travel path of the lower wire, however, serves essentially only for the transporting of the suspension into the twin-wire zone.
The necessity of feeding the suspension to the twin-wire SPE(~\123965 zone in the form of a free jet, and therefore with relatively high velocity, is thereby avoided.

3. The fact that the two wires, beginning immediately with the start of the twin-wire zone, are conducted over said drainage strips makes use, already at the start of the two-sided drainage, of the possibility of producing shearing forces several times in succession in the suspension in order to keep it as free of flocculation as possible. These shearing forces are produced in the manner that constrictions and, between them, bulges are constantly produced alternately between the two wires at the strips. In this way, the fiber suspension is alternately accelerated and decelerated relative to the wires several times in succession.

The constrictions and bulges concerned here are of very small size so that the substantially linear wire travel paths (converging towards each other) are retained.

In this way, the result is obtained that said alternate accelerating and decelerating of the fiber suspension which is still liquid -- by careful dimensioning of the adjustment forces of the resilient strips -- takes place with great care and therefore not too intensively, since the disturbance again of the fiber layers (forming on SPQC~123965 the two wires) is definitely to be avoided. On the other hand, a premature "sealing" of the fiber layers is avoided, so that they still retain good permeability for water, at least for the time. This contributes substantially to the above-mentioned drastic reduction of the length of the drainage path.

4. In accordance with the invention, it is therefore important that no curved drainage elements which would result in a curved course of the two wires are used in the web-formation region, i.e., until the fibers are immobile. Such curved surfaces would, for instance, be forming shoes, rolls, or the like. By such curved drainage elements, in many cases too high a drainage pressure would namely be produced, due to the wire longitudinal tension which is always necessary. The drainage pressure could therefore not be freely selected. Furthermore, there would be the danger that the oncoming fiber suspension would be dammed up at the entrance into the twin-wire zone, i.e. the twin-wire zone could not receive the large amount of suspension which is necessary in many cases.

s~l~s Further developments of the invention and their advantages will be explained below with reference to the embodiments shown in the drawing.
Each of Figs. 1 and 2 shows a twin-wire former in a diagrammatic side view. Figs. 3 to 6 show different arrangements of the drainage strips.
In Fig. 1, there can be noted a headbox 1 which feeds a fiber suspension to an endless rotating lower wire belt S1.
This wire belt S1, referred to in the following as the "lower wire", travels at the headbox 1 in known manner over a lower breast roll 10 and over a wire table 2 and then over several drainage strips 3. An upper endless wire belt S2, hereinafter referred to as "upper wire", travels in the region of the headbox 1 over an upper breast roller 9 and then over upper drainage strips 6 and 6a. The arrangement is such that, after leaving the upper breast roller 9, the upper wire converges towards the lower wire S1. In the region between the upper breast roll 9 and the first upper drainage strip 6, it comes into contact with the suspension; see contact point K.
In the example shown, the upper drainage strips 6 are developed as stationary strips, while the lower drainage strips 3 are developed as resilient strips each of which rests on a pneumatic hose 4. The inner pressure in each of SPE~123965 -the pneumatic hoses 4 can be individually adjusted so that each of the strips 3 can be pressed with a freely selectable force against the inner side of the lower wire Sl. In other words, each of the lower drainage strips 6 is displaceable in vertical direction relative to a stationary machine frame 5, as indicated by vertical double-ended arrows in Fig. 3. The water which passes downward through the mesh of the lower wire S1 is removed by the drainage strips 3 from the lower wire and flows downward under the force of gravity.

The water which penetrates upward through the mesh of the upper wire S2 must, on the other hand, be removed in upward direction against the force of gravity by means of a vacuum. For this purpose, in Fig. 1 each of the upper drainage strips 6 is fastened to a so-called skimmer 7. The latter is a vacuum chamber provided with a suction channel.
In Fig. 1 the lower and upper drainage strips 3 and 6 are arranged alternately in the direction of travel of the wires, and therefore along an imaginary zig-zag line. In the rear region of the twin-wire zone, a plurality of stationary strips 6a are provided at a substantially smaller distance apart on the bottom side of a common vacuum drainage box 7a.
A stationary supporting structure 8 is provided for the upper wire S2. Supports for the upper breast roll 9, the SPE~123965 skimmers 7 with the upper drainage strips 6, as well as the vacuum box 7a with the drainage strips 6a are fastened to it.
The supports for wire guide rolls, for instance 9a, which return the endless upper wire 2 to the breast roll 9, can also be fastened thereon. As indicated by double-ended arrows at 14, each of the ends of the supporting structure 8 can be adjusted individually in height in order thereby to be able to adjust, if desired, the distance between the two wires and their angle of convergence, and thus also the position of the contact point K. At the end of the twin-wire zone, a separating suction box 13 is arranged which sees to it that the fiber web formed separates from the upper wire S2 and travels along with the lower wire S1. A system of conduits 11 and a fan 12 produce the necessary vacuum in the skimmers 7 and in the vacuum wire box 7a. A possible variant for Fig. 1 in which -- at least in a partial region of the twin-wire zone -- stationary strips are provided on the lower wire S1 while strips which can be pressed on resiliently with selectable force are provided on the upper wire S2, has not been shown.
Fig. 2 differs from Fig. 1 essentially in the manner that a relatively large vacuum box 7' which is divided into several chambers is provided for all stationary drainage strips 6' in the upper wire S2. This vacuum box could also, SPEC\1~3965 differing from Fig. 2, be divided into two or three boxes.
Opposite one space between two lower resilient drainage strips 3, two upper stationary drainage strips 6' are in each case arranged. The wire table 2' (which is developed closed in Fig. 1) can have drainage slots, as shown in Fig. 2.
The action of the arrangement of the strips in accordance with Fig. 1 can be noted somewhat more clearly from Fig. 3. A slightly undulated course of the two wires Sl and S2 has been shown in exaggerated manner in Fig. 3. The two wires are bulged out somewhat in each case in the center between a lower strip 3 and an upper strip 6. The vertical distance there between the two wires is designated "a". At each of strips 3 or 6 the wire in question is somewhat constricted, so that the vertical distance between the two wires, designated "b", is somewhat smaller there than the distance "a". Therefore, as already explained above, there are alternate constrictions and bulges between the two wires.
How much the vertical distance between the two wires changes in this connection can be determined by a different horizontal positioning of the strips with respect to each other, for instance by horizontal displacement of the lower resilient strips 3, as indicated diagrammatically by horizontal double-ended arrows. Fig. 4 shows an extreme example, in which each of the lower strips 3 is arranged SPEC\123965 relatively precisely opposite an upper strip 6. In this way a substantially larger difference is obtained between the vertical wire spacings a' and b', and therefore substantially more pronounced bulges and constrictions.
In accordance with Figs. 5 and 6, twice as many resilient strips 3 are provided in a part of the twin-wire zone (preferably at the starting region thereof) on the lower wire S1 as there are stationary strips 6 in the upper wire S2. Every second lower strip is, for instance, arranged directly opposite an upper strip. Every lower strip, in its turn, can be pressed with a freely selectable force from below against the lower wire S1. In this connection, as shown in Fig. 5, the pressing forces at all lower strips 3 are set to approximately the same values. In this way, there is produced a substantially parallel undulated course of the two wires S1 and S2. On the other hand, in Fig. 6 the lower strips 3 which are present at spaces between two upper strips 6 are acted with a smaller pressing force so that here clear bulges are produced between the two wire which, similar to what is shown in Fig. 4, clearly differ from the constrictions in the region of the upper strips 6.

SPEC\123965

Claims

1. A method for the forming of a fiber web, particularly a thick paper or cardboard web, from a fiber suspension, having the following method steps:

a) a stream of suspension is conducted by means of a headbox (1) onto a substantially horizontal travel path of an endless, continuously rotating lower wire (S1);

b) directly thereafter, an endless (also continuously rotating) upper wire (S2) is brought in contact with the suspension (which travels with the lower wire), namely at a contact point (K);

c) the two wires (with the suspension to be drained present between them) are conducted further through an at least approximately straight and also substantially horizontal twin-wire zone;

d) starting directly behind the contact point (K), a drainage pressure is produced in the suspension in the manner that, in the twin-wire zone against the inner side of each of the two wires (traveling in substantially straight paths), several strips are applied one after the other, they extending transversely to the direction of travel of the web, namely stationary strips (6) on the one wire (S2) and resilient strips (3) which can be applied with an individually adjustable force on the other wire (S1), whereby, in addition to the discharge of the water from the wires -- shearing forces are at the same time produced several times one behind the other in the suspension which is still present.

2. A method according to Claim 1 or 2, in which the application of the strips takes place alternately one after the other on the upper wire and the lower wire.

3. A method according to Claim 1 or 2, in which the frequency of the strips increases with increasing formation of web.

4. A method according to Claim 1, 2 or 3, in which the uniformity of the distribution of the fibers ("formation") in the web to be produced is controlled not only by varying the strip application force but also by changing the position of individual strips in the direction of travel of the web.

5. A twin-wire former for the forming of a fiber web, in particular a thick paper or cardboard web, from a fiber suspension, having the following properties:

a) a headbox (1) is arranged at the start of a substantially horizontal travel path of an endless lower wire (S1) in order to feed a stream of suspension to it;

b) guide elements (9, 6, 6a, 9a) for an endless upper wire (S2) are so arranged that the upper wire comes into contact (contact point K) with the suspension (present on the traveling lower wire) at a slight distance from the headbox (1);

c) the two wires (S1, S2) form with each other an at least approximately straight and substantially horizontal twin-wire zone;

d) starting directly behind the contact point (K), there are provided -- for drainage of the suspension and for the production of shearing forces in the suspension -- in the twin-wire zone on the inner side of each of the two wires (traveling on substantially straight paths) several strips one after the other which extend transverse to the direction of travel of the web, namely stationary strips (6) on the one wire (S2) and resilient wires (3) which can be applied with an adjustable force, on the other wire (S1).

6. A twin-wire former according to Claim 5, characterized by the fact that at least one of the strips (3 or 6) of the one wire is arranged opposite a space between two strips of the other wire.

7. A twin-wire former according to Claim 5 or 6, characterized by the fact that, in each case, two or three strips of the strips (6a or 6') of the upper wire (S2) form at least one group which is arranged opposite a space between two strips (3) of the lower wire (S1).

8. A twin-wire former according to Claim 5, 6 or 7, characterized by the following features:

a) in the initial region of the twin-wire zone, at least part of the upper strips (6) is located opposite a space between two lower strips (3);

b) in the end region of the twin-wire zone, there are groups of two or three upper strips (6a) opposite a space between two lower strips (3).

9. A twin-wire former according to one of Claims 5 to 8, characterized by the fact that the position of individual strips, preferably lower strips (3), in the direction of travel of the web is variable.

10. A twin-wire former according to Claim 9, characterized by the fact that the position of individual strips (3) is variable in the direction of travel of the web only in the initial region of the twin-wire zone, while in a region of the twin-wire zone adjoining same, substantially all strips (3, 6, 6a,) are fixed in position relative to the direction of travel of the web.

11. A twin-wire former according to any of Claims 5 to 9, characterized by the following features:

a) in the initial region of the twin-wire zone, the pitch (distance from center to center) of the resilient strips (3) is only one-half (or one-third) of the pitch of the stationary strips;

b) in the initial region of the twin-wire zone, the position of the resilient strips relative to the direction of travel of the web is variable.