EP0373133A2

EP0373133A2 - Method and device in the formation of a paper or board web

Info

Publication number: EP0373133A2
Application number: EP89850423A
Authority: EP
Inventors: Jorma Hujala
Original assignee: Valmet Paper Machinery Inc
Current assignee: Valmet Paper Machinery Inc
Priority date: 1988-12-01
Filing date: 1989-11-29
Publication date: 1990-06-13
Also published as: FI885608A0; CA2004174A1; FI885608A; FI84637B; EP0373133A3; CN1043178A; FI84637C

Abstract

Method and device for the formation of a paper or board web (W) out of a fibrous material in a wire part which consists of a lower-wire loop (20), which is horizontal at the initial end of its upper run, and of an upper-wire loop (10) jointly operative with said lower-wire loop. After the initial dewatering carried out in the single-wire dewatering zone (20a) through the lower wire (20), the fibre layer (W) is passed over a lower-wire (20) support system (30) constituting a direct extension of the single-wire zone (20a), in the area of which said support system (30) dewatering by the effect of gravity through the lower wire (20) is prevented. With web qualities of higher grammage, after the single-wire dewatering zone (20a), the web (W) is passed to an initial part of the twin-wire dewatering zone, which is determined by the above support system (30) placed inside the lower-wire loop (20) and by a suction and foil equipment (40) placed opposite to it inside the upper-wire loop (10). Dewatering is carried out within the initial part through the upper wire (10). When thinner web qualities are being produced, the suction and foil equipment (40) is placed apart from contact with the lower wire (20) placed facing it (Fig. 1). After the above web formation stages, there follows a twin-wire dewatering section, which takes place within an upward short sector (a) of a forming roll (16) fitted inside the upper-wire loop (10), whereupon the twin-wire forming zone is curved downwards, being guided by a forming shoe (26) with a relatively large curve radius (R).

Description

The present invention concerns a method for the formation of a paper or board web out of a fibrous material, which said method is applied in the wire part of a paper machine or equivalent, which said wire part consists of a lower-wire loop, which is substantially horizontal at least at the initial end of its upper run, and of an upper-wire loop jointly operative with said lower-wire loop, and in which said method the fibre suspension arriving from the head box of the paper machine is fed onto the initial part of the upper run of the lower-wire loop, said initial part constituting the first single-wire dewatering zone, whereupon the partly dewatered fibre layer is passed to the second dewatering zone, within whose area the upper-wire loop is made to cover said partly dewatered fibre layer so that the dewatering of the fibre layer goes on in the area of the second dewatering zone, whereupon the upper-wire loop is detached from the fibre web formed, which is passed further to follow along with the run of the lower-wire loop.
Further, the invention concerns a web forming device intended for carrying out the method of the invention, comprising a lower-wire loop that has a substantially horizontal upper run and an upper-wire unit jointly operative with said lower-wire loop and provided with an upper-wire loop, as well as a head box, which is arranged to feed the fibre suspension jet onto the single-wire initial part of the upper run of the lower-wire loop, whereinafter a twin-wire forming zone is fitted, which is defined between the joint runs of the lower wire and the upper wire, and within which said twin-wire forming zone there are various members and groups of members affecting the dewatering and placed inside the lower-wire loop and the upper-wire loop.
In the oldest methods of formation of a paper or board web the web formation takes place on a horizontal so-called fourdrinier wire part, wherein water is drained out of the fibre suspension over the entire length of the wire part in one direction only, downwards. In such a case the properties of the upper face and the lower face of the paper produced differ from each other so that the upper face of the paper is smoother than the lower face, the latter showing a more or less clear pattern produced by the forming wire, i.e. a so-called wire marking. The upper face and the lower face of the paper also differ from each other in respect of the fibre composition, so that the upper face of the web contains a considerably larger amount of fine and short fibres and fillers than the lower face does, from which a considerable proportion of this fine material of the paper has been washed off during, and as a result of, the one-way draining of water. In papers intended for printing it is important that both faces of the paper are as identical as possible. The difference between the faces of paper is called unequal-sidedness.
In prior art, paper machine types are known by means of which it is possible to reduce the unequal-sidedness of paper. Among these it is possible to distinguish between two main groups, viz. twin-wire formers proper and so-called hybrid formers. In twin-wire formers proper, the formation of the web takes place from the beginning to the end between two wires. In hybrid formers the web is formed initially on one wire, whereupon this partly formed web is passed in between two wires for ultimate solidification of the relative positions of the fibres.
One of the advantages of hybrid formers is that they can be produced from existing fourdrinier wire parts by means of relatively simple modifications by fitting an upper-wire loop on the middle or final portion of the upper run of the lower wire as jointly operative with the latter. In addition to an improvement of the quality of the paper produced, it is also possible to intensify the dewatering taking place in the wire part, and thereby to increase the running speed of the paper machine. One hybrid former is described in the applicant's FI Pat. Appl. No. 820742, which is intended mainly for the production of newsprint and similar qualities of printing paper at running speeds of an order of 900 m/min and even higher.
As a rule, the prior-art hybrid formers are not suitable for the manufacture of thick paper and board qualities, because in the beginning of the twin-wire portion, which follows after the first single-wire dewatering zone, the run of the wires and of the fibre layer that is placed between the wires and that has been partly converted to a web is immediately guided relatively steeply and over a long distance to be curved either over a revolving roll or over the face of a stationary so-called forming shoe. The curve formation produces an inner shear effect in the web, which becomes more extensive when the thickness of the web is increased. Thereby a considerably high dewatering pressure is applied to the fibre layer between the wires, said pressure being proportional to the tensioning of the outer wire and inversely proportional to the curve radius of the guide face concerned.
It results from problems of space and from other constructional factors that in prior-art hybrid formers, e.g. in that described in the FI Patent 75,375 (corresponds to US Pat. 4,614,566), the curve radius of the member that alters the directions of the wires is so small that the relatively sudden compression effect that is applied to the web that is being formed at this stage is all too high in the case that relatively thick paper and board qualities are produced by means of the former concerned. Thereby an excessively intensive compression causes damage to the fibre layers and deteriorates the properties of the finished product, in particular its strength, but also, e.g., the printing quality. In the worst case an excessive compression results in so-called "crushing" of the web and causes a break in the production.
In respect of the prior art related to the present invention, reference is made further to the US Patent 4,769,111 of Messrs. A. Ahlstrom Corporation as well as to the applicant's FI Patent Application No. 885609 filed together with the present application and to the FI Patent Applications Nos. 885606 and 885607 of Messrs. Valmet-Ahlstrom Inc.
The principal object of the present invention is to provide a multi-quality former by means of which it is possible to cover a wide range of grammages and speeds so that, by means of the same former, it is possible to manufacture papers and boards even within a range of grammage as wide as 30 g/m² to 500 g/m² and within a speed range as wide as 100 m/min to 1200 m/min.
One of the starting points of the present invention has been the applicant's "Sym-Former R" (TM) former (said FI Pat. 75,375), which has been found in practice to operate best with relatively low grammages and high speeds. Thus, the object of the present invention is further development of the applicant's "Sym-Former R" former so that its range of operation can be extended to include thick paper qualities and boards so that lower speeds can also be concerned.
When attempts have been made to reach relatively high grammages with formers of the type of said "Sym-Former", it has been noticed that the quality of the paper, in particular its formation and smoothness, suffer. An objective of the present invention is also to avoid this drawback and to provide the paper and board to be manufactured, within a wide range of speeds and grammages, with good quality factors, above all formation and symmetry of the paper.
A further object of the present invention is to provide a former some of whose embodiments are particularly well suitable for rebuilds of existing fourdrinier wire parts.
An additional object of the present invention is to provide a method and a former wherein a microturbulence that improves the formation and the smoothness can be applied to the web at a suitable stage of dewatering.
An object of the present invention is to provide a multi-quality former wherein it is possible, at least in the initial portion of the web formation, to avoid excessive dewatering pressures arising from tensioning of the web, said pressures being not employed until at the final stage of the web formation after the web has had time to reach an adequate degree of couching.
A non-indispensable additional object of the invention is to provide such a multi-purpose method and web former wherein the twin-wire dewatering zone does not go down to a level lower than the plane of the lower wire, which is of advantage both in view of the web formation and in the respect that thereby it is possible to make use of the frame constructions of an existing fourdrinier wire part in the case of renewals.
A non-indispensable additional object of the present invention is to provide such a method and multi-quality former wherein it is possible, if necessary, to employ a secondary head box by whose means a second pulp quality or the same pulp quality is fed onto the fibre layer that is being formed.
In view of achieving the objectives stated above and those that will come out later, the method in accordance with the invention is mainly characterized in
that, after the initial dewatering carried out in the single-wire dewatering zone through the lower wire, the fibre layer that is being formed is passed over a lower-wire support system constituting a direct extension of said single-wire zone, in the area of which said support system dewatering by the effect of gravity through the lower wire is prevented to a substantial extent,
that the mode of web formation is chosen in accordance with the grammage of the quality that is being produced so that, when web qualities of higher grammage are being produced, after the single-wire dewatering zone the web is passed to an initial part of the twin-wire dewatering zone, which is determined by the above support system placed inside the lower-wire loop and by a suction and foil equipment placed opposite to it inside the upper-wire loop, the dewatering being carried out within said initial part mainly through the upper wire, and that, in a second, alternative mode of web formation, when thinner web qualities are being produced, said suction and foil equipment is placed apart from contact with the lower wire placed facing it, and
that, after the above web formation stages, there follows a twin-wire dewatering section, taking place within a zone that is guided by a forming roll or equivalent fitted inside the upper-wire loop and curved upwards within a relatively short sector, whereupon the twin-wire forming zone is guided to be curved downwards, being guided by a forming shoe or equivalent with a relatively large curve radius, and that, after the twin-wire forming zone, the web is guided to follow one of the wires, most appropriately the lower wire.
On the other hand, the device in accordance with the invention is mainly characterized in that, in its substantial and relatively long initial part, the twin-wire second dewatering zone is plane and constitutes a straight, direct extension of the single-wire initial part, whereupon the twin-wire zone has been guided to be curved upwards, being guided by a short sector of a forming roll fitted inside the upper-wire loop and provided with a hollow face, and hereinafter a forming shoe is fitted inside the lower-wire loop, whose guide face, which is preferably provided with a ribbed deck and has a relatively large curve radius, guides the twin-wire zone to be curved downwards, and that inside the initial end of the upper-wire loop, a suction and foil equipment is arranged, which is arranged so that it can be raised by means of actuators from a lower operating position, which is used for the production of thick qualities, to an upper position when the former is adjusted for the production of thinner web qualities, as a rule within the grammage range of 30 to 120 g/m².
At high running speeds and in ranges of low grammage (as a rule, below 120 g/m²), the former in accordance with the invention is used as a hybrid former of the Sym-Former type with the suction and foil unit of the upper-wire unit raised in the upper position. Thereby, as a rule, the operation is in a speed range of 800...1200 m/min.
With the same paper machine, when it becomes necessary to produce thicker paper qualities or boards, e.g., within a grammage range of 100...500 g/m² and even higher while thereby using lower running speeds, the former in accordance with the invention is regulated to operate so that the suction and foil unit in the upper-wire unit is pivoted onto a self-adjusted plane set of support beams placed inside the lower-wire unit, whereby, after the single-wire initial part, the dewatering can be reversed to take place mostly through the upper wire by making use of the suction and foil unit, while the dewatering through the lower wire is substantially prevented by means of the set of wire-support beams placed facing the suction and foil unit underneath the lower wire. Hereupon, after most of the water has been drained, the final dewatering is carried out by making use of a suitably curved twin-wire portion of the "Sym-Former" type, which is arranged so that the relatively thick, partly couched web is not subjected to excessively sharp or long changes in direction or to shear forces resulting from such changes.
In the invention there is a need, after the single-wire initial dewatering zone, to direct the dewatering so that it takes place predominantly through the upper-wire loop, and in that connection it is possible to utilize, e.g., a dewatering device in accordance with the applicant's SE Patent Application 8703468 placed inside the upper-wire loop, said device comprising first a suction-aided water doctor in combination with a corresponding water collecting chamber, which is advantageously connected with a source of suction. This dewatering device additionally comprises at least one water collecting trough consisting of several foils and communicating with a source of suction. Additionally, facing this dewatering device placed inside the upper-wire loop, inside the lower-wire loop there is a series of wire-support beams, whose function is to reduce or substantially to prevent the dewatering taking place through the lower wire, on one hand, and, in this area, to apply a pressure that intensifies the upwardly directed dewatering and an appropriate pulsation of said pressure to the partly formed web below the upper wire, on the other hand, the formation of the web that is being formed being improved by means of said pressure and pulsation. Each of these wire-support beams is loaded against the lower-wire loop, e.g., by means of compressed-air hoses. Thus, the compression applied to the web is self-adjusted.
In the invention, if necessary, it is also possible to employ a secondary head box or boxes, by whose means, before the twin-wire forming zone, a jet of the same pulp quality or of a different pulp quality is fed onto the web that is being formed, whereby it is possible to control the overall process of web formation and, if necessary, to produce different lamellar structures, e.g., when board is produced. By means of a set of wire-support beams, i.e. wire board, used in accordance with the invention, which is placed inside the lower-wire loop as a straight and direct extension of the single-wire dewatering zone, microturbulence is produced, which improves the web formation and smoothness and, moreover, a self-adjusting gap area is produced in the initial end of the twin-wire portion, which also provides the advantage that, in this sensitive phase of dewatering and web formation, pressures resulting from the tension of the wires are not applied to the web, but the gap area between the wires is adjusted and shaped quite freely in the form determined by the water drained.
In the following the invention will be described in detail with reference to some exemplifying embodiments of the invention illustrated in the figures in the accompanying drawing, the invention being in no way strictly confined to the details of said embodiments.

Figure 1 shows a first embodiment of the invention as adjusted to the "Sym-Former" mode of operation, wherein, by means of the former, papers of relatively low grammage are produced at a high speed.
Figure 2 shows the same former as is shown in Fig. 1, adjusted to the mode of operation by means of which paper or board qualities of high grammage can be produced even at a low speed.
Figure 3 shows a modification of the invention as adjusted to the mode of operation that produces qualities of relatively high grammage.
Figure 4 shows a third alternative mode of operation of the invention, wherein two foil and/or suction boxes are used in the upper-wire unit.
Figure 5 shows such an embodiment of the invention wherein a secondary head box is used and by means of which relatively thick board qualities are manufactured.
Figure 6 is a detailed vertical sectional view of a set of foil and suction boxes in accordance with the invention as well as of a wire-support and blocking equipment placed facing the foil and suction boxes inside the lower-wire loop.

The web former that is shown schematically in Figures 1 to 5 comprises a head box 19 and a lower-wire loop 20, whose run is guided primarily by the breast roll 21, the wire guide rolls 22, the forming shoe 26, the couch roll 28, and by the drive roll 29. Moreover, inside the lower-wire loop 20, there are, within the first dewatering zone 20a, a group of dewatering members 23 and, in the area of the second dewatering zone, the wire-support equipment 30 as well as suction boxes 27, which are placed on the run of the lower wire 20 between the forming shoe 26 and roll 35 (Fig. 3) and the couch roll 28.
The web former that is shown in Figures 1 to 5 further comprises an upper-wire loop 10, which, being guided by the guide rolls 11,11a and 11b, covers the lower-wire loop in the area of the twin-wire dewatering zone. Moreover, inside the upper-wire loop 10, there is dewatering equipment 40, which is illustrated in more detail in Fig. 6. As the first unit, the equipment 40 comprises a dewatering doctor of the autoslice type, which is preferably connected with a suction chamber. The dewatering equipment 40 further includes two or three water collecting troughs, whose bottom part, which is placed against the upper wire 10, consists of foils.
In the following, the relative differences between the various formers illustrated in Figs. 1 to 5 will be described.
Figures 1 and 2 illustrate the same former in two different modes of operation, in Fig. 1 in the "Sym-Former" mode of operation, wherein relatively thin, as a rule about 30...120 g/m², paper qualities are produced at a high speed. Fig. 2 shows the same former in a mode of operation in which thicker qualities, about 120...500 g/m², are produced, as a rule at a lower speed.
As is shown in Figs. 1 and 2, within the twin-wire forming zone, after the hollow-faced 16′ forming roll 16, there is one forming shoe 26 provided with a ribbed deck 26′ , said deck having a relatively large curve radius R, as a rule R = 5...9 m. The shoe 26 is followed by a number of suction flatboxes 27. In Fig. 1, the twin-wire zone is formed beginning from the initial line A₁ of the sector a of the roll 16 and ends on the line B. In Fig. 2, the twin-wire zone is formed beginning from the line A₀ placed facing the guide roll 11a and ends on said line B.
In Fig. 3, the forming shoe 26 in Figs. 1 and 2 is substituted for by a shorter forming shoe 26A and by a subsequent forming roll 25, which is most appropriately smooth- and solid-faced. The radius of the forming roll 25 is substantially smaller than the curve radius R_A of the ribbed deck 26A′ of the shoe 26A, said radius being, as a rule, within a range of R_A = 0.8...1.5 m. In the other respects the construction is similar to that shown above in Figs. 1 and 2.
The construction shown in Fig. 4 is similar to that shown in Figs. 1 and 2 except that above the forming shoe 26 a suction and foil equipment 40A is fitted, whose construction and operation are similar to the preceding dewatering equipment 40 with the difference that the contour of the foil system that defines the lower side of the latter equipment 40A has a curve form corresponding to the shape of the ribbed deck 26′ of the forming shoe 26. The forming shoe 26 is followed by a suction flatbox 27a inside the lower-wire loop 20, and after said flatbox 27a by a second forming roll 16a, which is placed inside the upper-wire loop 10 and which is most appropriately smooth- and solid-faced.
The construction shown in Fig. 5 is similar to that shown in Figs. 1 and 2 with the difference that in Fig. 5 a secondary head box 60 is employed, through whose slice cone 61 a secondary pulp jet J₁ is fed, before the twin-wire forming zone, onto the fibre layer that has been formed on the single-wire fourdrinier wire part 20a. The pulp qualities of the primary jet J and the secondary jet J₁ may be the same or different from each other. In the latter case, for example, board qualities of lamellar structure are produced.
For the production of thick qualities (120 g/m² to 500 g/m²) the web former shown in Figs. 2 to 5 operates as follows. The head box 19 feeds the pulp jet J onto the horizontal initial part 20a of the lower-wire loop 20, i.e. onto the first dewatering zone, in whose area the dewatering takes place exclusively downwards through the lower wire 20 gently intensified by suitable dewatering members 23, e.g. foil boxes. The fibre layer formed on the face of the lower-wire loop 20 continues its run to the second dewatering zone, which is formed when the upper-wire loop 10 is guided to the proximity of the lower-wire loop 20 at a small angle d (Fig. 6), whose magnitude is most appropriately 2...5^o, and it is regulated by adjusting the position of the upper-wire 10 alignment roll 11a in the vertical direction by means of the devices 18 (Fig. 1).
In Figs. 2 to 5 the second, twin-wire dewatering zone is divided into several sections. The dewatering equipment 40 placed inside the upper-wire loop 10 and the wire-support equipment 30, which is placed inside the lower-wire loop 20 and which is jointly operative with said dewatering equipment 40, form the first section of the second dewatering zone. The first dewatering zone 20a and the relatively long first section of the second dewatering zone are at the same horizontal level up to the forming roll 16, which turns the runs of the wires 10,20 to take place upwards at a small angle, which is equal to the magnitude of the sector a on which the joint run of the wires 10 and 20 is in contact with the forming roll 16. The sector a forms the second section in the twin-wire dewatering zone. The compression pressure (p = T/R) between the wires 10,20 removes more water from the web W that is formed, said water being thrown by the effect of centrifugal force into the water collecting trough 13.
The upper wire 10 is detached, by means of the guide roll 11b, from the web W placed on the face of the lower wire 20, said web continuing its run up to the roll 14. The suction boxes 27 are supposed to ensure that the web W does not follow the wire 10, which returns back to the roll 11a. At the proximity of the couch roll 28, a pick-up roll 14 provided with a suction zone 14a is placed together with a detaching felt 15, by means of which the web W is transferred from the wire part to the press section (not shown).
The former shown in Fig. 1 is adjusted for the mode of operation by means of which relatively thin paper qualities, as a rule of a grammage of about 30...120 g/m², are produced. In such a case the dewatering equipment 30 operates as a straight and direct extension of the single-wire initial portion 20a, and by means of the microtur bulence produced by means of the dewatering equipment and by means of the grooves 35 between its support beams (Fig. 6), the dewatering taking place through the lower wire 20 is completed and the formation of the web is affected.
The initial portion 20a of the substantially horizontal single-wire dewatering zone, belonging to the lower-wire unit shown in Fig. 1, is followed by a twin-wire portion between the lines A₁ to B, whereupon the web W follows the lower wire 20. The lower-wire part is, for example, an earlier fourdrinier wire part with its frame construction 100, which existed before modernization and to which a particular upper-wire unit 70 in accordance with the invention was combined in connection with modernization.
On the single-wire initial portion 20 there is, for example, dewatering equipment from the existing fourdrinier wire part, such as foils 23 and/or suction boxes as well as, at the final end of the twin-wire portion, suction boxes 27.
The upper-wire unit 70 comprises a frame part, which includes horizontal and vertical beams 71,72, to which the various parts are attached. The run of the upper-wire loop 10 is guided, from the beginning A₁ of the twin-wire portion, by a hollow-faced 16′ open forming roll 16 and thereupon by a forming shoe 26, which has a ribbed deck 26′ with a curve radius R. The twin-wire dewatering zone ends in the area of the last suction box 27.
According to Fig. 1, the upper-wire unit 70 includes a frame part 75, which is pivotable in the vertical plane and which is attached to the horizontal beam 71 by means of adjusting screws 17a and 17b or equivalent. The first guide roll 11a of the upper wire 10 is mounted on the frame part 75. In Fig. 1, the frame part 75 and the dewatering equipment 40′ mounted on its support and the first guide roll 11a′ are shown in the upwards pivoted position, from which it can be returned to the position shown in Fig. 2 when the paper quality to be manufactured is changed, in which said latter position the equipments 30 and 40 operate jointly one opposite the other.
On the initial portion 20a of the single-wire dewatering zone the dewatering takes place by means of dewatering members 23 placed between the discharge opening of the head box 19 and the equipment 30, such as the forming board and foils. On the single-wire initial portion 20a the dewatering takes place downwards through the lower wire 10 to such an extent that a sufficient proportion remains for the dewatering that takes place upwards. After the line A₁ the joint run of the wires 10 and 20 is curved upwards on a short sector a, whose magnitude is a = 10^o...35^o, most appropriately a =15^o... 25^o.
By means of the grooves 35 in the set of beams in the wire-support equipment 30 (Fig. 6), microturbulence is produced, which finishes and improves the formation of the web W before the beginning of the twin-wire zone.
The short sector a is followed by a very short straight run of the wires 10 and 20, whereupon, in the area of the shoe 26, the dewatering takes place upwards through the upper wire 10 by the effect of the compression between the wires 10 and 20 and of centrifugal forces. The trailing edge of the forming shoe 26 is followed by a straight joint run of the wires 10 and 20.
In the mode of operation in accordance with Fig. 1, the former in accordance with the invention operates for the production of thinner qualities in the way described above as a hybrid-type twin-wire former, in respect of the details of whose construction and operation reference is made to the applicant's said FI Patent 75,375.
Fig. 6 shows the dewatering equipment 40 belonging to the wire part in accordance with Figs. 1 to 5 together with its most important details of construction, as well as the lower-wire support member system jointly operative with said dewatering equipment 40.
The dewatering equipment 40 consists of a combination of, as a rule, two to four (in the figures three) suction and water collecting chambers 46,47,48, wherein the individual chambers are separated from each other by partition walls 47b and 48b. Each chamber 46,47,48 is provided with an air opening (not shown) connected to a source of suction and with a drain water duct 49. The water collecting duct 46a which belongs to the first suction chamber 46 is formed between the frame beam 46b of the dewatering equipment 40 and the guide plate 46c. At the lower end of the duct 46a there is a transverse foil doctor 51 and a rib 52 adjustable by means of adjustment spindles 53, said doctor and rib forming a locally adjustable slot E, which extends across the width of the former and through which the water pressed out of the fibre layer placed between the wires 10 and 20 flows into the first chamber 46.
The foil doctor 51 in the equipment 40 shown in Fig. 6 is followed by a number of similar foils 51′ and 51˝, whose bottom faces are at the same level. The foils 51′ gather the water separated from the fibre structure underneath the first suction chamber 46, said water being guided into the suction chamber 47 through the duct 47a, which is formed between the partition wall 47b and the guide plate 47c. In a corresponding way, the water gathered by the next foils 51˝ is guided into the third suction chamber 48 through the duct 48a, which is formed between the rear wall 48d of the dewatering device and the guide plate 48c.
The duct 46a shown in Fig. 6 and the related foil doctor 51 and the adjusting rib 52 form a suction-aided dewatering member. When rather thick and thick board qualities are produced by means of the former at a low speed, it is most appropriate to aid the operation of the autoslice system by means of suction, the vacuum being most appropriately 6 to 8 kPa. The amount of the dewatering taking place upwards at this stage, and partly also the magnitude of the vacuum arising, can be affected by regulating the height of the gap E between the rib 52 and the foils 51.
In Fig. 6, the dewatering effect of the suction-aided dewatering member and of the related first suction chamber 46 is local, being confined to the proximity of the tip of the first foil doctor 51. The dewatering area of the second suction chamber 47 is wider, being determined by the number of the foils 51′, whose number is shown to be seven in Fig. 6, which is an example case. The effect of the foils 51′ is based on joint operation with the wire-support equipment 30 placed inside the lower-wire loop 20. It is an essential feature of the support equipment 30 and of its operation that by its means it is possible, in the area of the dewatering equipment 40, in the desired way, to provide a successively increasing compression by the lower wire 20 applied to the web W that is formed, by the effect of which said compression the dewatering of the water W takes place primarily through the upper-wire loop 10 into the suction duct 47a and through it into the suction chamber 47. The operation of the third suction chamber 48 is analogical to that of the second suction chamber 47.
The negative pressure present in the second and third chamber 47,48 shown in Fig. 6 is most appropriately considerably higher than in the first chamber, viz. about 10...20 kPa in the chamber 47 and about 15...30 kPa in the chamber 48, depending on the web material that is being produced.
In Fig. 4, the water collecting equipment 40A is shown as comprising two suction chambers placed one after the other, and the, for example, two ribs placed underneath said chambers may contact the wire 10. Said ribs define the suction ducts of the equipment 40A at the trailing side of their mouth portions in the direction of running of the wire 10.
In respect of Fig. 4, the essential feature of the invention should be ascertained further that in some embodiments of the invention, in particular in embodiments which are best suitable for rebuilds of fourdrinier wire parts, the forming zone does not at all extend to below the level T-T determined by the single-wire initial portion 20a. According to Fig. 4, after the forming roll 16a, the twin-wire zone, on which there are suction boxes 27 inside the lower wire loop 20, is returned to the level T-T determined by the single-wire initial portion 20a, which is most appropriately horizontal.
In the equipment shown in Fig. 6, when thick qualities are produced by means of a former in accordance with Figs. 2 to 5, the dewatering process takes place as follows. On the first dewatering zone 20a in the former, the fibre layer partly formed on the face of the lower- wire loop 20 enters into the space between the wires 10 and 20, whose gap angle d is determined by the position of the roll 11a, which guides the wire 10 and whose position is adjustable by means of the devices 18. The surface of the fibre mesh meets the upper wire 10 before the rib 52. At this point the portion of the wire-support equipment 30, which supports the lower-wire loop 20, also starts, by whose effect the dewatering of the fibre layer starts taking place primarily upwards.
The beam members 31 in the equipment 30 shown in Fig. 6 rest on longitudinal support beams 33 by the intermediate of rubber hoses 32 pressurized with air, and said support beams 33 are again supported by transverse box beams 34. The pressure effective in the hoses 32 can be regulated so that the loading of the members against the lower wire 20 and the fibre mesh increases successively in the direction of running of the wires 10,20. In the hoses 32, a relatively low pressure is used, e.g. 10...50 cm H₂O, whereby a very gentle compression is applied to the web W that is in the forming stage, and the dewatering pressure is self-adjusted. The surface of the members in the equipment 30 is provided with transverse grooves 35 extending across the entire wire 20 width, which said grooves also permit a little dewatering through the lower wire 20 and by means of which said grooves a microturbulence is produced which improves the formation of the web W.
In Fig. 6, the dewatering process goes on in the area between the contact line of the upper face of the web W and the profile rib 52, where a water layer is formed on the inner face of the upper wire 10, said water layer being gathered in the wedge-shaped space between the wire 10 and the profile rib 52 and in the following gap E between the profile rib 52 and the foil, through which gap the water is forced through the duct 46a into the first chamber 46 in the dewatering device, either by the effect of its kinetic energy and/or by the effect of the vacuum present in the chamber. The rib 52 can be set vertically by means of adjusting means 53, whereby it is possible to regulate the amount of water that enters into the duct 46a and possibly also the amount of air. Naturally, said adjustments both in respect of the angle d of incidence between the wires 10 and 20, in respect of the gap passing into the duct 46a, and in respect of the pressure applied by the support system depend on the paper or board quality produced.
In some cases, the suction-aided system based on the use of an adjusting rib 52, shown in Fig. 6, can be substituted for by a construction wherein the adjusting rib 52 has been replaced by a roll whose speed of rotation as well as its position of height, i.e. its distance from the wire 10, have been arranged adjustable.
In the following the patent claims will be given, whereby the various details of the invention may show variation within the scope of the inventive idea defined in said claims and differ from those described above for the sake of example only.

Claims

1. Method for the formation of a paper or board web (W) out of a fibrous material, which said method is applied in the wire part of a paper machine or equivalent, which said wire part consists of a lower-wire loop (20), which is substantially horizontal at least at the initial end of its upper run, and of an upper-wire loop (10) jointly operative with said lower-wire loop, and in which said method the fibre suspension jet (J) arriving from the head box (19) of the paper machine is fed onto the initial part (20a) of the upper run of the lower-wire loop (20), said initial part constituting the first single-wire dewatering zone, whereupon the partly dewatered fibre layer is passed to the second dewatering zone, within whose area the upper-wire loop (10) is made to cover said partly dewatered fibre layer so that the dewatering of the fibre layer goes on in the area of the second dewatering zone, whereupon the upper-wire loop (10) is detached from the fibre web (W) formed, which is passed further to follow along with the run of the lower-wire loop (20) for the subsequent stages of treatment of the web (W), characterized in
that, after the initial dewatering carried out in the single-wire dewatering zone (20a) through the lower wire (20), the fibre layer (W) that is being formed is passed over a lower-wire (20) support system (30) constituting a direct extension of said single-wire zone (20a), in the area of which said support system (30) dewatering by the effect of gravity through the lower wire (20) is prevented to a substantial extent,
that the mode of web formation is chosen in accordance with the grammage of the quality that is being produced so that, when web qualities of higher grammage are being produced, after the single-wire dewatering zone (20a) the web (W) is passed to an initial part of the twin-wire dewatering zone, which is determined by the above support system (30) placed inside the lower-wire loop (20) and by a suction and foil equipment (40) placed opposite to it inside the upper-wire loop (10), the dewatering being carried out within said initial part mainly through the upper wire (10), and that, in a sec ond, alternative mode of web formation, when thinner web qualities are being produced, said suction and foil equipment (40) is placed apart from contact with the lower wire (20) placed facing it (Fig. 1), and
that, after the above web formation stages, there follows a twin-wire dewatering section, taking place within a zone that is guided by a forming roll (16) or equivalent fitted inside the upper-wire loop (10) and curved upwards within a relatively short sector (a), whereupon the twin-wire forming zone is guided to be curved downwards, being guided by a forming shoe (26) or equivalent with a relatively large curve radius (A), and that, after the twin-wire forming zone, the web (W) is guided to follow one of the wires, most appropriately the lower wire (20).

2. Method as claimed in claim 1, characterized in that the zone that follows after the single-wire initial dewatering zone (20a) on the upper run of the lower wire is, in respect of its relatively long initial part, plane and constitutes a straight direct extension for the initial run (20a) of the lower wire of the former, and that on said initial part microturbulence is produced in the web (W) by means of grooves (35) or equivalent provided in the wire-support members (31).

3. Method as claimed in claim 1 or 2, characterized in that the set of suction and foil boxes (40) placed inside the upper-wire loop (10) is, together with at least the foremost guide roll (11a) of the upper wire (10), raised to an upper position (40′) when the former has been adjusted to operate in the range of lower grammages, as a rule 30...120 g/m² (Fig. 1), so that the twin-wire web forming zone starts on a short sector (a) of the hollow-faced (16′) forming roll (16) placed inside the upper-wire loop (10), and that when thicker paper qualities, as a rule within a grammage range of about 120...500 g/m², are produced, said part (75) of the upper-wire unit is pivoted to the lower position so that the wire support equipment (30) placed inside the lower-wire loop (20) and the suction and foil equipment (40), which is placed inside the upper-wire loop and which has been pivoted down (Fig. 2), for an initial section of the twin-wire dewatering zone, on which the dewatering is carried out primarily through the upper wire with the aid of said suction and foil equipment (40).

4. Method as claimed in claims 1 to 3, characterized in that on the forming roll (16) placed inside the upper-wire loop, the twin-wire zone is curved upwards within a short sector (a), whose magnitude is chosen within a range of a = 10^o...45^o, most appropriately within a range of a = 15^o...25^o, whereupon the twin-wire zone is curved downwards on a curved stationary shoe member (26), whose curve radius (R) is substantially larger than the radius of said forming roll (16).

5. Method as claimed in any of the claims 1 to 4, characterized in that after the zone (a) on the hollow-faced (16′) forming roll (16) placed inside the upper-wire loop, the twin-wire zone is curved downwards on a relatively short forming shoe (26A) with a relatively large curve radius (R_A), followed by a steeper downwardly curved zone, which is guided by means of a second forming roll (25) placed inside the lower-wire loop (20), and that hereupon the twin-wire zone is passed across one or several suction flatboxes (27) or equivalent placed inside the lower-wire loop (20) (Fig. 3).

6. Method as claimed in any of the claims 1 to 5, characterized in that in the method, after the upwardly curved zone occurring on the forming roll (16) on the twin-wire forming zone, in the dewatering taking place through the upper wire (10), a water collecting equipment (40A) fitted against the forming shoe (26) is employed (Fig. 4).

7. Method as claimed in any of the claims 1 to 6, characterized in that, when thicker web qualities are produced, before the beginning of the twin-wire forming zone, secondary pulp (J₁) is fed out of a secondary head box (60) to above the single-wire dewatering zone (20a) (Fig. 5).

8. Web forming device intended for carrying out the method as claimed in any of the claims 1 to 7, comprising a lower-wire loop (20) that has a substantially horizontal upper run and an upper-wire unit (70) jointly operative with said lower-wire loop and provided with an upper-wire loop (10), as well as a head box (19), which is arranged to feed the fibre suspension jet (J) onto the single-wire initial part (20a) of the upper run of the lower-wire loop, whereinafter a twin-wire forming zone is fitted, which is defined between the joint runs of the lower wire (20) and the upper wire (10), and within which said twin-wire forming zone there are various members and groups of members affecting the dewatering and placed inside the lower-wire loop (20) and the upper-wire loop (10), characterized in that, in its substantial and relatively long initial part, the twin-wire second dewatering zone is plane and constitutes a straight, direct extension of the single-wire initial part (20a), whereupon the twin-wire zone has been guided to be curved upwards, being guided by a short sector (a) of a forming roll (16) fitted inside the upper-wire loop and provided with a hollow face (16′), and hereinafter a forming shoe (26) is fitted inside the lower-wire loop (20), whose guide face, which is preferably provided with a ribbed deck (26′) and has a relatively large curve radius (R), guides the twin-wire zone to be curved downwards, and that inside the initial end of the upper-wire loop (10), a suction and foil equipment (40) is arranged, which is arranged so that it can be raised by means of actuators (17a,17b) from a lower operating position, which is used for the production of thick qualities, to an upper position (40′) when the former is adjusted for the production of thinner web qualities, as a rule within the grammage range of 30 to 120 g/m² (Fig. 1).

9. Device as claimed in claim 8, characterized in that on said straight initial section of the twin-wire dewatering zone, a forming board (30) is fitted, which comprises self-adjusted support beams (31) for the lower wire, which are placed inside the lower-wire loop (20) and which are, for their part, also fitted to act as means to prevent dewatering, and that, facing the forming board (30) and placed inside the upper-wire loop, a suction and foil equipment (40) is fitted.

10. Device as claimed in claim 8 or 9, characterized in that the forming board placed inside the lower-wire loop (20) consists of self-adjusted transverse beams (31) supported by loading hoses (32) or equivalent, said beams extending across the area facing the two or three suction chambers and their foil equipment (51,51′, 51˝) that guides the upper wire (10) in the dewatering equipment placed opposite to said transverse beams (31).

11. Device as claimed in any of the claims 8 to 10, characterized in that the suction and foil equipment (40) placed inside the upper-wire loop (10) consists of a set of boxes provided with at least two, most appropriately three, subsequent suction chambers (46,47,48), the foils (55,51′,51˝) placed in the bottom part of said set of boxes being fitted to guide the twin-wire dewatering zone as a straight and direct extension of the single-wire initial portion (20a).

12. Device as claimed in any of the claims 8 to 11, characterized in that before the suction and foil equipment (40), there is a suction-aided equipment whose upwardly inclined duct (46a) communicates with a first suction chamber (46), the mouth portion of said duct (46a) being provided with a profile rib (52) adjustable in relation to the upper wire (10).