CA2123820A1 - Gap former in a paper machine - Google Patents

Abstract

ABSTRACT
A gap former for a paper machine having a twin-wire zone including a combination of the following web-forming and dewatering units in the given sequence in the running direction of the web (W) that is formed:
(a) a forming gap defined at one side by one of the wires which runs over an open face of a first forming roll, and at an opposite side by the substantially straight run of the opposite wire starting from a guide roll to the next forming member;
(b) a forming shoe having a curved ribbed deck whose curve radius is from about 2 m to about 8 m, the tangential direction of the inlet side of the ribbed deck of the forming shoe substantially coincides with the principal direction of the forming gap;
(c) at least one MB unit including a dewatering unit and a press and support unit placed one opposite to the other, sets of ribs in the dewatering and press and support units pressing the web being formed between the wires; and (d) a large-diameter second forming roll which includes suction zones, the covering wire is separated by means of the guide roll from the web in or after the area of the suction zones and the web is passed further on the carrying wire to the press section.

Description

GAP FORMER IN ~ PAPER MACHINE

BACKGRO~ND OF THE INVENTION
The present invention relates to a gap former in a paper machine in which a twin~wire zone is formed by a loop of a carrying wire and a loop of a covering wire. The twin-wire zone starts from the forming gap into which the slice part of a headbox feeds a pulp suspension jet. After the twin-wire zone, the web is separated from the covering wire and is transferred on the carrying wire onto a pick-up fabric to be carried into the press section of the paper machine. The present invention also relates to a method for dewatering a web in a gap former.
With increased running speeds of paper machines, several problems in the web formation have been manifested with increased emphasis. In the former section of a paper machine, the phenomena that act upon the fiber mesh and upon the water, which is still -relatively free in connection with the fiber mesh, such as centrifugal forces, are increased in proportion to the second power of the web speed. The highest web speeds of existing newsprint machines are of an order of about 1500 meters per minute. However, newsprint machines are being planned in which a web speed of up to about 2000 m/min is aimed at.
The present invention is also related to further development of the formers described in the assignee's Finnish Patent Application No. 751774 (corresponding to the assignee's U.S. Patent No. 3,996,068, the specification of which is hereby incorporated by reference herein) and Finnish Patent Application Nos. 851650, 852662, and 902283, which formers are marketed by the assignee under the trademark "Speed-Former". The "Speed-Former" formers are characterized as gap-roll-shoe formers. In the "Speed-Former" web former, there has arisen a need for further development in particular in the case of papers made of slowly draining pulp .
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qualities, such as SC-paper, and with particularly high machine speeds. In view of satisfying this need of development so that a sufficiently high dry solids content after the wire part can be guaranteed with all web grades and web speeds in use, a former has been developed that is described in the assignee's Finnish Patent Application No. 902283, in whose gap-roll-shoe-roll former a considerable proportion of dewatering is carried out on the last (second) forming roll. Also, in connection with this second forminq roll, the necessary dewatering equipment is arranged so that, on the last forming roll, the dry solids content of the web is still raised by about 3 per cent to about 5 per cent.
With respect to the prior art related to the present invention, reference is also made to U.S. Patent No. 4,769,11 of Messrs. A. Ahlstrom Corporation, to the assignee's Finnish Patent Application No. 885609, as well as to the Finnish Patent Application Nos. 885606 and 885607 of Messrs. Valmet-Ahlstrom Inc., in which formers marketed under the trademark "MB-former" are described.
In the roll-gap formers mentioned above, the pulp suspension jet is passed from the headbox into a gap formed by a forming roll and by two wires. After the gap, the two wires and the pulp layer placed between them follow the curve form of the forming roll, whereby most of the water being drained is removed through the wires. The necessary dewatering pressure is produced by means of the tension of the outer wire, which tension produces a pressure in the pulp layer which is proportional to the wire tension and inversely proportional to the curve radius of the forming roll. In the gap, part of the kinetic energy of the pulp jet is converted to pressure energy, and the jet speed is lowered accordingly. The dewatering through the outer wire away from the forming roll is intensified by the centrifugal force. Dewatering towards the forming roll can be intensified by means of negative pressure in a suction zone of the forming roll.
With respect to additional prior art related to the present invention, reference is further made to the assignee's Finnish Patent Application Nos. 904489 and 920863. In FI '~89, a roll-gap former is described and the application is primarily directed to the novelty that the dewatering unit or units comprise(s) a stationary press-support unit which guides the wire that enters into contact with the unit as a substantially straight run, and that the dewatering unit or units comprise(s) a dewatering equipment placed facing press and support unit and provided with a suction and foil equipment. The dewatering equipment removes a substantial amount of water out of the web.
One basic type of a twin-wire former is the so-called ribbed gap former in which the discharge jet of the headbox is passed into the gap formed by two wires. The gap becomes narrower either as a substantially straight dewatering area formed by means of dewatering ribs placed at both sides of the wires transversely to their running direction or as one or two successive curved dewatering areas in which the ribs are placed at the side of the curve center of the wires. In respect of these formers, reference is made to U.S. Patent Nos. 3,578,558, 3,944,464, 4,125,428, to German Published Patent Application No. DE 21 13 014, and to Finnish Published Patent No. 50,647.
In addition, Finni~h Patent Application No. 913480, which was originally applied for by Messrs. Tampella Papertech Oy, describes a gap former wherein it has been considered novel that the breast roll that guides the first wire is open, and the narrowed discharge jet meets the first wire in the area of contact with the open roll and meets the second wire after the breast roll that guides the second wire. The distance between the wire placed on the open breast roll, at the point of separation of the first wire from the breast roll, and the second wire, which is placed between its own ., . , . ~ .
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2123~20 breast roll and a guide member, is smaller than the thickness of the narrowed discharge jet, preferably only from about 1 mm to about 4 mm smaller.

03JECTS AND SUMM~RY OF THE INVENTION
It is an object of the present invention to further develop the prior art web formers described above so that a novel gap former is provided by whose means both good formation of the web and good retention are achieved.
It is a further object of the present invention to provide a former whose dewatering capacity is sufficient at high web speeds and with thicker paper grades.
It is a further object of the present invention to provide a former in which the advantages of the gap part of the former described in the above-mentioned FI Pat. Appl. 913480 are accomplished and in which a dewatering unit of the MD type can be made to operate in an optimal range of dry solids content of the web.
Further, it is an object of the present invention to provide a former in which the ratio of tensile strength machine direction / cross direction MD/CD of the paper produced can be made lower while, nevertheless, maintaining good formation and retention. In this regard, an object of the invention is to provide a former in which, if necessary, it is possible to achieve a ratio of tensile strength of the paper MD/CD less than or equal to 2.5.
In view of achieving the objects stated above, those that will come out later, and others, the web former of the present invention includes a twin-wire zone comprising a combination of the following web-forming and dewatering units, which are placed in the given sequence in the running direction of the web that is formed and which are known in themselves:
(a) a forming gap defined at one side by one of the wires . . . ~

2123~20 which runs over the open face of a first forming roll, and at an opposite side by the substantially straight run of the opposite wire starting from its guide roll to the next forming member;
(b) a forming shoe provided with a curved ribbed deck whose curve radius is Rl > 2 m to about 8 m, the tangential direction of the inlet side of the ribbed deck of the forming shoe substantially coincides with the principal direction of the forming gap;
(c) at least one MB unit comprising a dewatering unit and a press and support unit placed one opposite to the other, the sets of ribs in the dewatering and press and support units press the web to be formed between the wires; and (d) a large-diameter second forming roll which includes at least one suction zone in or after the area of which the covering wire is separated by means of the guide roll from the web, which is passed further on the carrying wire to the press section of the paper machine.
After the MB unit or units and before the large-diameter second forming roll, preferably a suction box is placed either inside the carrying wire or inside the covering wire, preferably inside the loop of the carrying wire.
This specific combination of press elements and dewatering elements enables the press section to run at higher speeds when processing different pulp qualities and also facilitates a more symmetric dewatering of the web. That is, substantially equal amounts of water are removed from both sides of the web through the covering wire and the carrying wire. This more symmetrical dewatering is achieved as a result of the important feature that, in the invention, water is removed on the forming roll in conjunction with the forming gap to a considerably lower extent than in the "Speed-Former" web formers. This lower amount of water removal via the forming roll contributes to an improved retention.

In the following, the invention will be described in detail ~-~

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BRIEF DE5CRIPTION OF ~ 13 DRAWING~;
The following drawings are illustrative of embodiments of the invention and are not meant to limit the scope of the invention as encompassed by the claims.
Figure 1 is a schema~ic side view of a former in accordance with the invention in which there is a vertically rising twin-wire zone and in which the open-faced forming roll in the gap area is placed inside the loop of the covering wire.
Figure 2 shows a twin-wire zone in the other respects similar to Fig. l except that the MB unit is in a position inverse in relation to Fig. 1.
Figure 3 is an illustration corresponding to Fig. 2 of a twin-wire zone in which the open-faced forming roll in the gap area is placed inside the loop of the carrying wire.
Figure 4 shows a twin-wire zone in the other respects similar except that the MB unit is placed in a position inverse in relation to Fig. 3.
Figure 5 shows a former in accordance with the invention in which the principal direction of the twin-wire zone is horizontal and the forming roll in the gap area is placed inside the loop of the upper wire which operates as the covering wire.
Figure 6 shows the initial part of the twin-wire zone and the gap area in a former which is in the other respects similar to that shown in Fig. 5 except that the open forming roll is placed inside the loop of the lower wire, which operates as the carrying wire.
Figura 7 shows an exemplifying embodiment of a MB zone applied in the invention, which zone is in most respects similar to that :; ~ . :. . ~ :

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-` 2~23820 - shown in Fig. 3 except that the forming shoe is placed inside the loop of the carrying wire.
Figure 7A shows the detail A indicated in Fig. 7 on an enlarged scale.

DETAI~ED DESCRIPTION OF THE INVENTION
Referring to the drawings wherein like reference numerals refer to the same elements, the common features of the twin-wire formers illustrated in Figs. 1-6 will be described. The formers generally comprise a twin-wire zone formed by a loop of a covering wire 10 and a loop of a carrying wire 20. Between the lines B-C, the wires 10,20 begin to form the twin-wire zone in which water is removed from a paper web W through both of the wires 10,20. The covering wire 10 is guided by guide rolls 14,14' and by the first forming roll 11 or a corresponding breast roll llA directly. A
forming gap G is defined between the covering wire 10, which is guided by the forming roll 11 or the corresponding breast roll llA, and the carrying wire 20, which is guided by a forming roll 21 or a corresponding breast roll 21A. The forming gap G becomes narrower in a wedge-shape form in the running direction of the web.
It is an important feature of the invention that the wires 10,20 have no common curve sector on the first forming roll 11,21, and that the principal direction of the forming gap G and the joint run of the wires 10,20 following after the gap is straight until the inlet edge of the forming shoe 12,22. The forming gap G is determined by the first forming roll 11,21 having an open-face 11',21' and by the breast roll llA,21A having a smooth-face 11",21". The breast roll llA,21A is placed before the forming roll in the feed direction of the pulp jet J. In such a case, the forming gap G is defined, at one side, by the substantially straight run of the wire 10,20 from the breast roll llA,21A to the inlet edge of the forming shoe 22 and, at the other side, by the ,::: ~ - .

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opposite wire 20,10 curved on the open face 21',11' of the first forming roll 21,11, which opposite wire departs from the open face 21',11' of the forming roll 21,11 in the area of the bottom of the gap G. The tangential direction of the inlet edge or side of the ribbed deck subs~antially coincides with the principal direction of the forming gap. In the area of the gap G, when the wire 10,20 runs on the open face 11',21' of the first forming roll 11,21, dewatering takes place into the open face 11',21' and through the substantially straight run of the opposite wire 20,10. Thus, forming roll 11,21 constitutes a so-called "kissing forming roll"
(11,21).
By means of the gap geometry described above, the controllability and the trans~erse stability of the wires can be improved to some extent in comparison with a case in which the forming gap G would be more open at the forming roll 11,21 than the "kissing forming roll" 11,21 described above.
In the twin-wire zone, after the forming gap G, there is a curved forming shoe 12,22 which has a ribbed deck 12a,22a with a large curve radius R1. The forming shoe 12,22 is followed by an MB
unit 50 in the twin-wire zone. The MB-unit comprises a dewatering unit 40 and a press and support unit 60 operating one opposite to the other so that the wires 10,20 are pressed against one another by means of the press and support unit 60 so as to remove water out of the web W placed between the wires.
There may a variable number of MB units 50, for example two, in the twin-wire zone placed one after the other and preferably arranged so that in the successive units 50, the units 40 and 60 are placed inside different wire loops 10,20.
The MB unit 50 or units is/are followed by a suction box 29 which is provided with a ribbed deck and is placed inside the loop of the carrying wire 20. The suction box 29 is followed by a large-diameter D2 second forming roll 23 which is placed inside the ,~;.,, ' . ` : :' ' : -carrying wire 20. Forming roll 2~ constitutes a suction roll in which there are two successive suction zones 23a and 23b, as shown in Figs. 1-4, or only one suction zone 23a, as shown in Fig. 5.
The diameter D2 of the second forming roll 23 is generally selected in the range of about 1.0 m to about 2.0 m. The covering wire 10 is separated at the line C from the covering wire 20 by means of the guide roll 14' so that the web W follows the carrying wire 20.
As shown in Figs. 1 and 5, the web W is passed further on a suction zone 24a of suction/guide roll 24 and from there further in a downward inclined run to the pick-up point where the web W is transferred onto a pick-up fabric 31 on a suction zone 30a of the pick-up roll 30. After this, the carrying wire 20 continues its run, guided by the guide rolls 24, onto the breast roll 21A in the gap area (Figs. 1, 2 and 5) or onto the forming roll 21 placed in the gap area, which forming roll has an open face 21' which receives water (Figs. 3 and 4).
As shown in Figs. 1-4, the main direction of the run of the twin-wire zone B-C is vertical. The twin-wire zone starts in the forming gap G into which a slice part 32 of the headbox feeds a pulp suspension jet J. The jet J is directed towards the bottom of the straight gap G which becomes narrower in a wedge-shaped form.
As shown in Figs. 1 and 2, inside the loop of the covering wire 10, there is a forming roll that is provided with an open face 11' and that receives water. Breast roll 21A is arranged at a level lower than the level of the forming roll 11 at the distance of the height difference h" inside the loop of the carrying wire.
The breast roll 21A is provided with a smooth solid face 21". The diameters D1 of the rolls 11 and 21A are preferably substantially equal in comparison with one another, being of an order of about 0.5 m to about 1.5 m, preferably from about 0.7 m to about 1.0 m.
With the above roll diameters D1, the difference in height h1 is preferably in the range of about 50 mm to about 300 mm. The .: .. . . ~

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forming roll 11 may also be a suction roll provided with an inside or interior suction chamber lla, the negative pressure employed in this type of roll being preferably in the range of about 10 kPa to about 20 kPa.
In Fig. 1, the outlet end of the forming gap G is in the horizontal plane T-T determined by the central axis of the first forming roll 11 or near this plane. In the horizontal formers shown in Figs. 5 and 6, the outlet end of the forming gap is arranged in proximity to the vertical plane passing through the center of the forming roll 11.
As shown in Figs. 3 and 4, a water-receiving forming roll 21 provided with an open face 21' is placed inside the loop of the carrying wire 20, and the breast roll llA provided with a smooth face 11" is placed inside the loop of the covering wire 10. For fine adjustment of the gap geometry, the forming roll 11,21 is arranged so that its position can be adjusted substantially in the horizontal plane (arrow E), via displacement means (not shown).
The gap area is substantially immediately followed by the forming shoe 12,22. The forming shoe 12 shown in Figs. 3 and 4 is placed inside the loop of the covering wire 10, and the forming shoe 22 shown in Figs. 1, 2, 5 and 6 is placed inside the loop of the carrying wire 20. The forming shoes 12,22 are provided with a ribbed deck having a large curve radius Rl. The curve radius R1 is usually chosen within the range of about 2 m to about 8 m, preferably in the range of about 3 m to about 5 m. If necessary, ! the box of the forming shoe 12,22 may be connected to a source of negative pressure p3~ which is illustrated by the corresponding arrow in Fig. 1. The negative pressure p3 promotes the draining through the gaps in the ribbed deck 12a,22a. However, in the area of the forming shoe 12,22, the dewatering takes place primarily in a direction away from the shoe by the effect of the pressure PT =
T/R~ produced by the tightening tension T of the wires, which dewatering is aided by centrifugal forces.
The forming shoe 12,22 is substantially immediately followed by the MB unit 50, which, as shown in Fig. 1, comprises two draining chambers 40a and 40b, which are arranged inside the loop of the covering wire 10 and are separated from one another by a partition wall 49. Both draining chambers 40a,40b communicate with different negative pressures P1 and P2. At the side of the chambers 40a and 40b placed against the twin-wire zone, there is a set of ribs 80. The press and support unit 60 is provided with a corresponding set of ribs 70, and the construction and the operation of these sets of ribs 70,80 will be described in more detail later with reference to Figs. 7 and 7A. The box 61 in the press and support unit 60 can also, if necessary, be connected to negative pressure P~. The forming shoe 12a may also be connected directly to the first draining chamber 40a as shown in Fig. 4.
As shown in Fig. 2, the units 40 and 60 operating one opposite to the other in the MB unit 50 are placed in an inverse order, compared with Fig. 1, so that in Fig. 2 the dewatering unit 40 is placed inside the loop of the carrying wire 20 and the press and support unit 60 is placed inside the loop of the covering wire 10.
The MB unit 50 or units is/are followed by a suction box 19 arranged inside the loop of the covering wire 10 to operate against the inner face of the wire 10 and which is provided with a ribbed deck having slots.
In the web former shown in Fig. 3, the dewatering unit 40 of the MB unit 50 is placed inside the loop of the carrying wire 20, and the press and support unit 60 is placed inside the loop of the opposite wire 10, whereas in Fig. 4, compared with Fig. 3, the units 40 and 60 are placed in an inverse order. The principal direction of the twin-wire zone guided by the sets of ribs 80,70 in the units 40 and 60 is straight, but alternatively it is possible to use a ribbed zone between the sets of ribs 70,80 whose principal - ~23~2~
direction is slightly curved, the curve radius of this ribbed zone is denoted by R2 in Figs. 1 and 2. The curve radius R2 is generally selected within the range of about 3 m to about 8 m. In Fig. 1, the center of curvature of the curved ribbed zone is placed at the side of the dewatering unit 40, and in Fig. 2 the center of curvature of the curved ribbed zone is placed at the side of the press and support unit 60.
In the web former shown in Fig. 4, a second MB-unit 40C (shown in phantom lines) may be arranged after the MB unit 40. The second MB unit has a dewatering unit 40A arranged inside a loop of the carrying wire and a press and support unit 40B arranged inside a loop of the covering wire, i.e., in opposite relation to the arrangement of these units in the first MB unit 40.
Figs. 5 and 6 show two horizontal versions of a gap former in accordance with the invention. As shown in Fig. 5, the gap G is determined by the lower breast roll 21A which has a smooth solid face 21" and by the upper forming roll 11 which is provided with an open face 11' that receives water. Between the rolls 21A and 11, there is a horizonal distance v,. The upper forming r~ll 11 has been arranged to be adjustable mainly in the vertical direction (arrow E) in view of fine adjustment of the gap geometry (via displacement means (not shown). In Fig. 6, in the area of the gap G, in the horizontal direction, first there is a breast roll 11 provided with a smooth face 11' and a lower forming roll 21 which is provided with an open face 21'. The horizontal distance between these rolls is vl which is generally in the range of about 50 mm to about 300 mm.
In the web formers shown in Figs. 5 and 6, the forming gap G
is followed by a forming shoe 22 placed inside the loop of the lower wire 20, which forming shoe is provided with a ribbed deck 22a having a curve radius Rl. After the forming shoe 20, the MB
unit 50 is arranged and comprises a first water drain chamber 41 ' " ' . ~ " ' . ', ~ . ~ ,' ~ 2123820 placed above the forming shoe 22 inside the loop of the upper wire 10 in the manner shown in Fig. 6. Drain chamber 41 has a water drain duct 42 arranged preferably at the driving side of the machine only. Underneath the first chamber 41, a gap space 43 remains through which the water is drained, while being aided by negative pressure P1, through a suction-deflector duct 44 in the direction of the arrow V1 into the chamber 41 and from it further through the duct 42 to the side of the paper machine and into the wire pit (not shown). In the direction of the arrow V~, through the suction-deflector duct 44, mainly water that has been separated from the web W through the upper wire 10 in the area of the shoe 22 is drained.
The first water drain chamber 41 is followed directly by a second water drain chamber 45 having a rear wall against which there is a water drain duct 47. Through the duct 47, water is drained in the direction of the arrow V2 through the slot gaps in the sets of ribs 80 in the dewatering unit 40. This amount of water is removed out of the chamber 45 through a water drain duct 46 which is placed preferably at the operating side of the machine only. Against the set of ribs 80, a set of ribs 70 operates which is loaded by means of the pressure of the pressure medium of the press and support unit 60. The construction and the operation of said sets of ribs 70,80 will be described in more detail in the following with reference to Figs. 7 and 7A.
The MB unit 50 or units is/are followed by a suction box 29 which is placed inside the loop of the carrying wire 20 and which is provided with a ribbed deck that includes slots. After the suction box 29, the twin-wire zone is curved in the suction zone 23a of the second forming roll 23 to have a downward inclined run, and the web W is separated from the upper wire 10 and guided to follow the lower wire 20. This separation is aided by the negative pressure present in a ribbed deck 25a of a suction box 25 placed ~.. . . .. .

- 2123~20 inside the loop of the lower wire 20.
In Figs. 7 and 7A, the dewatering unit 40 of the MB unit 50 is arranged directly after the curved ribbed deck 22a of the forming shoe 22 and inside the loop of the carrying wire 20. Inside the S loop of the covering wire lo, there is the press and support unit 60 of the MB unit 50, whose construction comes out in more detail from Fig. 7A. The unit 60 includes a set of ribs 70 which comprises ceramic loading ribs 71,72, which are interconnected in pairs by means of support structures 73. The ribs 71,72, and so also back-up ribs 81 in the dewatering unit 40, extend in the transverse direction across the entire width of the web W and the wires 10,20. The set of ribs 70 is loaded by means of pressure Pk passed into loading hoses 75 mounted against stationary frame constructions 74. Into the hoses 75, pressure Ph is applied through pipes 77 from a source 76 of negative pressure, which is illustrated schematically. The units 40 and 60 may also alternate locations with one another, i.e., be situated in the opposite wire loops.
As shown in Fig. 7A, the set of ribs 80 of the dewatering unit 40 placed inside the loop of the carryinq wire 20 comprises forming ribs 81 attached to frame constructions 84 by means of dovetail joints. The ribs 81 are placed as alternating with, i.e., not directly opposite to, the ribs 71,72 in the set of ribs 70 so that, the twin-wire zone runs between the units 40,60 along a very gently meandering path while being guided by the sets of ribs 70,80. The whose principal direction of the twin-wire zone is straight or curved and, if curved, the curve radius is, in the embodiments of Figs. 1 and 2, denoted by R2.
If necessary and~or desired, in the area of the MB unit 50, the dewatering can be intensified by means of the effect of negative pressures P" P2, P4 present in gaps between the ribs 71,72,81. The symmetry of dewatering and of web formation can be 2123~20 controlled by means of regulation of the levels of negative pressure P1,P2, P3, P~ in the units 40 and/or 60. In Fig. 7, the dewatering unit 40 has been divided by a partition wall 49 into two compartments 40a and 40b, each of which communicates separately with a suction pump, so that the compartments 4Oa and 4Ob can be provided with negative pressures P~ and P2 having different levels of suction to promote the dewatering.
In Fig. 1, the dewatering proportions at the different dewatering units are illustrated by the arrows F1,F2,F3,F~,F5,F6,F, and F8. One particularly favorable distribution of the dewatering proportions that is achieved by the particular combination of press members and dewatering elements in a former in accordance with the invention is the following:

F, K 15% F2 ~ 27% F3 ~ 20% F4 ~ 4%
F5 ~ 29% F6 ~ 1~ F7 ~ 3.5% F8 ~ 0-5%

Of the dewatering percentages given above, the proportion of the total dewatering taking place through the covering wire 10 are F3 + Fs + F6 which total about 50%, and the proportion of the total dewatering taking place through the carrying wire 20 are F, + F2 +
F~ + F7 + F8 which totals about 50~. Thus, as comes out from the above, in the invention an extremely symmetrical and equal dewatering is achieved through both of the wires 10 and 20, whereby the symmetry of the web structure is promoted. This more symmetrical dewatering is achieved as a result of the essential feature, and one of the differences of the present web former when compared with the assignee's prior-art "Speed-Former" concept, that in the invention, water is removed on the forming roll 11,21 to a considerably lower extent than in the "Speed-Former". This lower amount of water removal via the forming roll 11,21 contributes to an improved retention.

21~3~20 The consistencies, i.e., dry solids contents, of the web that is being formed, which are indicated for the web former illustrated in Fig. 3, are preferably as follows:
- consistency in the headbox ~ from about 0.5% to about 1.7%;
- consistency before the forming shoe 12,22 (k1) z from about 0.6% to about 1.9%;
- consistency after the forming shoe 12,22 and before the MB
unit 50 (k2) ~ from about 0.90% to about 3.0%;
- consistency after the MB unit 50 (k3) ~ from about 7% to about 12%; and - consistency after the second forming roll 23 (k4) ~ from about 10% to about 14%.
The various parameters in accordance with the invention that affect the web formation, such as the gap G geometry, the diameters of the roll 11,21, the curve radius R~ of the forming shoe 12,22, the pressures Pk,P1,P4 and P4 that act upon the operation of the MB
unit, the diameter of the roll 23 and the negative pressures in its suction zone 23a,23b, and the negative pressure in the suction zone lla, if any, of the forming roll 11, are preferably selected so that the dry solids contents k1,...,k4 defined above and given in the following table will be carried into effect.

Table Dry solids content Widest range of dry Preferabledry solids of the web, (%) solids content, (%) content, (%) kl 0.6% to 1.9% about 1.5%

k2 0.9% to 3.0% about 2.2%
k3 7% to 12% about 10%
k~ 10% to 14% about 13%

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3~20 The examples provided above are not meant to be exclusive.
Many other variations of the present invention would be obvious to those skilled in the art, and are contemplated to be within the scope of the appended claims.

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Claims (20)

1. Gap former for a paper machine having a twin-wire zone formed by a loop of a carrying wire and a loop of a covering wire, a pulp suspension jet being fed from a slice part of a headbox into a forming gap at a beginning of said twin-wire zone to form a paper web, the web being separated from the covering wire after said twin-wire zone and being transferred on the carrying wire onto a pick-up fabric, said gap former comprising a combination of the following web-forming and dewatering units arranged in the following sequence in the running direction of the web:
(a) a forming gap defined between a first one of said wires which runs over an open face of a first forming roll and a substantially straight run of a second one of said wires from a guide roll to a next, following web forming unit;
(b) a forming shoe comprising a curved ribbed deck having a radius of curvature greater than about 2 m to about 8 m, said ribbed deck having an inlet side at which said twin-wire zone first contacts said ribbed deck, the tangential direction of said inlet side of said ribbed deck substantially coinciding with the direction of said substantially straight run;
(c) at least one MB unit comprising a dewatering unit and a press and support unit arranged opposite said dewatering unit, said dewatering unit and said press and support unit comprising respective sets of ribs arranged to press the web between said wires; and (d) a large-diameter second forming roll comprising at least one suction zone, one of said first and said second wires being separated from the web in proximity to said at least one suction zone by means of a guide roll, the web remaining on the other of said first and said second wires.

2. Gap former of claim 1, further comprising a suction box arranged in said twin-wire zone after said at least one MB unit.

3. Gap former of claim 1, wherein said guide roll determines the geometry of said forming gap, said guide roll being arranged before said first forming roll in the direction of feed of the pulp suspension jet.

4. Gap former of claim 3, wherein said forming gap has an outlet end arranged in vertical formers in proximity to a horizontal plane passing through the center of said first forming roll and, arranged in horizontal formers in proximity to a vertical plane passing through the center of said first forming roll.

5. Gap former of claim 1, wherein said forming gap has an outlet end arranged in vertical formers in proximity to a horizontal plane passing through the center of said first forming roll and, arranged in horizontal formers in proximity to a vertical plane passing through the center of said first forming roll.

6. Gap former of claim 1, wherein said twin-wire zone is substantially vertical and runs in an upward direction, the center of said first forming roll being arranged at a first level, said guide roll being arranged at a second level, said second level being spaced at a distance from said first level in the range of about 50 mm to about 300 mm, said guide roll and said first forming roll having diameter in the range from about 0.5 m to about 1.5 m.

7. Gap former of claim 1, wherein said twin-wire zone is straight in said at least one MB unit.

8. Gap former of claim 1, wherein said twin-wire zone in said at least one MB unit is curved and has a radius of curvature greater than or equal to 3 m to about 8 m.

9. Gap former of claim 1, wherein said second forming roll has a diameter in the range from about 1 m to about 2 m, said second forming roll comprising at least two successive suction zones.

10. Gap former of claim 1, wherein said twin-wire zone has a principal running direction which is substantially horizontal, said first and second wires comprising a lower wire and an upper wire, said forming shoe being arranged in said twin-wire zone after said forming gap inside a loop of said lower wire, said second forming roll being arranged inside a loop of said lower wire, the gap former further comprising a suction device for separating the web from said upper wire and guiding the web to follow said lower wire.

11. Gap former of claim 10, wherein said MB unit comprises a dewatering unit arranged inside a loop of said upper wire, said dewatering unit comprising at least two successive dewatering chambers, said forming shoe being arranged underneath a first one of said dewatering chambers, and a press and support unit arranged inside said loop of said lower wire and comprising a set of ribs loaded by a pressure medium against said lower wire.

12. Gap former of claim 11, wherein said dewatering unit further comprises a suction-deflector duct coupled to said first chamber and arranged to pass water into said first chamber from a gap space adjoining said first chamber and said ribbed deck, said gap space containing water removed from the web through slot gaps in said ribbed deck, said dewatering unit further comprising a second one of said dewatering chambers directly following said first dewatering chamber, and a fixed set of ribs arranged on a bottom side of said second dewatering chamber, said fixed set of ribs defining slot gaps therebetween through which water is removed through a water drain duct leading to said second dewatering chamber, said first and second dewatering chambers are connected to the same or different negative pressure sources, and said set of ribs in said press support unit operate against said set of ribs in said dewatering unit.

13. Gap former of claim 1, wherein said first wire is a carrying wire and said second wire is a covering wire, said second forming roll being arranged inside a loop of said carrying wire, said first forming roll being arranged inside a loop of said covering wire or said loop of said carrying wire, said dewatering unit being arranged inside said loop of said covering wire or said loop of said carrying wire, and said press and support unit being arranged inside the opposite wire loop from said dewatering unit.

14. Gap former of claim 1, wherein the dry solids content of the web after said forming gap and before said first forming shoe is from about 0.6% to about 1.9%, the dry solids content of the web after said forming shoe and before said at least one MB unit is from about 0.9% to about 3.0%, the dry solids content of the web after said at least one MB unit and before said second forming roll is from about 7% to about 12%, and the dry solids content of the web after said second forming roll is from about 10% to about 14%.

15. Gap former of claim 1, wherein the dry solids content of the web after said forming gap and before said first forming shoe is about 1.5%, the dry solids content of the web after said forming shoe and before said at least one MB unit is about 2.2%, the dry solids content of the web after said at least one MB unit and before said second forming roll is about 10%, and the dry solids content of the web after said second forming roll is about 13%.

16. Gap former of claim 1, wherein said first forming roll, said forming shoe, said at least one MB unit and said second forming roll are arranged such that the proportion of total dewatering of the web through said first wire and through said second wire are substantially equal.

17. Gap former of claim 1, wherein said first forming roll, said forming shoe, said at least one MB unit and said second forming roll are arranged to provide a symmetric dewatering of the web, such that the proportion of total dewatering of the web via said first forming roll about 15%, the proportion of total dewatering of the web via said forming shoe in a direction toward an interior of said forming shoe is about 27% and the proportion of total dewatering of the web via said forming shoe in a direction outward away from said forming shoe is about 20%, the proportion of total dewatering of the web via said press and support unit is about 4% and the proportion of total dewatering of the web via said dewatering unit is about 29%, the proportion of total dewatering of the web via said second forming roll in a direction outward from said second forming roll is about 1%, the proportion of total dewatering of the web via said second forming roll in a direction toward said second forming roll is about 3.5%, and the proportion of total dewatering of the web in proximity to a suction roll arranged in a loop of the wire on which the web is carried to the press section is about 0.5%.

18. Gap former of claim 1, further comprising two of said MB
units, a first one of said two MB units comprising a dewatering unit arranged inside a loop of said covering wire and a press and support unit arranged inside a loop of said carrying wire, a second one of said two MB units comprising a dewatering unit arranged inside said loop of said carrying wire and a press and support unit arranged inside said loop of said covering wire.

19. Gap former of claim 1, wherein said next, following web forming unit comprises said forming shoe.

20. A method for dewatering a web in a gap former of a paper machine having a twin-wire zone formed by a loop of a carrying wire and a loop of a covering wire, a pulp suspension jet being fed from a slice part of a headbox into a forming gap at a beginning of said twin-wire zone to form a paper web, the web being separated from the covering wire after said twin-wire zone and being transferred on the carrying wire onto a pick-up fabric, comprising the steps of:
(a) depositing the pulp suspension jet in a forming gap defined between a first one of said wires which runs over an open face of a first forming roll and a substantially straight run of a second one of said wires from a guide roll to a next, following web forming unit;
(b) carrying the forming web in said substantially straight run to a forming shoe comprising a curved ribbed deck having a radius of curvature greater than about 2 m to about 8 m, said-ribbed deck having an inlet side at which said twin-wire zone first contacts said ribbed deck, the tangential direction of said inlet side of said ribbed deck substantially coinciding with the direction of said substantially straight run;
(c) carrying the forming web from said forming shoe into at least one MB unit comprising a dewatering unit and a press and support unit arranged opposite said dewatering unit, said dewatering unit and said press and support unit comprising respective sets of ribs arranged to press the web between said wires; and (d) separating the forming web from one of said first and second wires in proximity to at least one suction zone arranged on a large-diameter second forming roll by means of a guide roll, the web remaining on the other of said first and said second wires.