Web former for a paper machine

Abstract: The invention concerns a twin-wire web former for a paper machine, whose wires (10,20) form a twin-wire forming zone (V₁-V₂) between them, on which zone water is drained out of the web (W) through both of said wires (10,20). After the twin-wire zone the web (W) is separated from the covering wire (20) and is transferred on the carrying wire (10) to its pick-up point. In hybrid formers (Fig. 1), after the single-wire forming zone (10a), and in gap formers (Fig. 2), after the curved forming zone (a₀) placed directly after the forming gap (G), there is a forming shoe (30;30A) provided with a ribbed deck (31) and fitted inside one of the wire loops (10/20). This forming shoe (30;30A) is followed by dewatering and web forming units which include forming ribs and are placed inside both of the wire loops (10,20), of which units at least one is loaded by means of a pressure-hose arrangement.

Description: The invention concerns a twin-wire web former for a paper machine, comprising a carrying wire and a covering wire, which form a twin-wire forming zone between them, on which zone water is drained out of the web through both of said wires, after which twin-wire zone the web is separated from the covering wire and is transferred on the carrying wire to its pick-up point. In web formers of paper machines, a number of different forming members are used. The principal function of these members is to produce pressure pulsation in the fibre layer that is being formed, which pressure pulsation promotes the dewatering of the web that is being formed, while its formation is also improved. Moreover, in prior art, a number of different forming shoes are known, which are usually provided with a curved ribbed deck and over which the forming wires, which are placed one on the other, and the web placed between said wires are curved. In the area of these forming shoes, water is removed mainly through the wire placed at the side of the outside curve, because of its tensioning pressure, said dewatering being also aided by a field of centrifugal force. The ribbed deck of the forming shoe produces pressure pulsation, which promotes the dewatering and improves the formation of the web. In this connection, it should be emphasized that the scope of the invention includes both hybrid formers, which have a single-wire initial part, and gap formers, in which the pulp suspension jet is fed directly into the forming gap defined by the wires. From the applicant's FI Patent No. 75,375 (corresp. US Pat. 4,744,866), a hybrid former is known in which the twin-wire forming zone is placed substantially completely above the level defined by the single-wire initial part, from which level the twin-wire forming zone is curved upwards on a sector of the first forming roll, and, after the initial dewatering has taken place to a suitable extent through the lower wire in the single-wire initial part, the dewatering takes place in the twin-wire dewatering zone first on said sector of the first open forming roll in two directions through both of the wires, after which, in the area of the next forming shoe, the dewatering takes place primarily upwards through the upper wire, and thereupon the dewatering pressure is increased further in the area of the second forming roll, while the dewatering continues to take place primarily through the upper wire. DE-U-9105797 discloses a paper machine having a twin-wire web former, which comprises a carrying wire and a covering wire which form a twin-wire forming zone between them. Further, the machine has a hybrid former, a forming shoe and two dewatering and web forming units. WO-91/02842 discloses a twin-wire former having two screens, together forming a zone which is divided into three sections. A non-deflectable loading device is loaded towards the wire by changing the position of the loading device in a direction perpendicular to the wire. GB-A- 2 045 827 discloses a device for providing pressure pulses and located adjacent a primary drainage of a paper machine. The device comprises an adjustable bar. One object of the present invention is further development of the hybrid formers described in said FI Pat. 75,375 and of equivalent hybrid formers, in which need of development has been noticed in particular with higher grammages of an order of about 100 g/sq.m in view of obtaining a sufficiently good formation and a symmetric structure of the web. It is a particular object of the present invention to provide a hybrid former which is suitable for modernizations of the former described in the applicant's FI Pat. 75,375 or of other, corresponding formers in view of improving their dewatering capacity and the formation so that existing constructions can be utilized as efficiently as possible. The principal object of the present invention is to provide a twin-wire web former by whose means an improved formation of the web is obtained. A second principal object is to provide a dewatering that can be regulated within sufficiently wide limits, so that the distribution of fillers and fines in the z-direction of the paper, discussed above, can be affected. For this purpose, the aim is that it should be possible, within wider limits than in prior art, to regulate the extent of dewatering taking place in the initial part of the twin-wire forming zone and thereby to permit a sufficiently large proportion of dewatering through the upper wire upwards, whereby attempts are made to reduce the anisotropy in the web. In hybrid-former applications of the invention, a further aim is that, if necessary, it is possible to drain even up to 50 % of the water upwards through the upper wire. An object of the invention is to provide a former by whose means the extent of upward dewatering can be regulated better than in prior art. In view of achieving the objectives stated above and those that will come out later, the invention is mainly characterized in that, in hybrid formers, after the single-wire forming zone, and in gap formers, after the curved forming zone placed directly after the forming gap, there is a forming shoe provided with a ribbed deck and fitted inside one of the wire loops, and that said forming shoe is followed by dewatering and web forming units which include forming ribs and are placed inside both of the wire loops, of which units at least those placed inside one of the wire loops are loaded, preferably by means of a pressure-medium hose arrangement. By means of a former in accordance with the present invention, it is possible to improve the formation of paper accomplished by means of the prior-art twin-wire formers and the symmetry of the web in the z-direction. Also, in hybrid-former applications, it is favourably possible to regulate the quantity of dewatering taking place upwards. In view of practical runnability, the former construction of the invention is "more gentle" and more stable with respect to the wires, because the wire runs both before and after the loading unit run over stationary units. On the other hand, in the invention, the variations in the tension of the wire can be made lower than in the prior art in corresponding solutions that include MB-units, because the number of the units is low. Variations in tension may produce wrinkle formation and increase the wear of the wires and formation of holes in the wires. The solution can be applied to new upper-wire units or to existing upper-wire units. By means of the spring blade of a wire loading device employed in a preferred embodiment of the invention, an intensive pressure pulse is produced in a relatively short area in the machine direction through the wire into the web that is being formed. The linear load of said pressure pulse is maximally of an order of ∼ 2 kN/m, and the length of the pressure area in the machine direction is ∼ 2-10 mm. An optimal linear load is of an order of 1 KN/m. By means of said pressure pulse, the web formation and the dewatering are promoted most advantageously at the stage of the process at which the dry solids content of the web is in the range of k = 1.5-2.5% , preferably k ≈ 1.5 %. By means of the spring blade of a wire loading device fitted in accordance with the invention, by whose means a pressure pulse is produced in the web that is being formed, it is also advantageously possible to regulate different transverse profiles of the web, such as the transverse profiles of dewatering, distribution of fillers, formation, and/or of retention. The spring blades or blade are/is followed by a loading unit, which is preferably loaded by a hose and through which water can be drained. Said unit with a fixed rib can be substituted for the fixed ribs of the spring blade or blades. It is not necessary to employ a wire loading device provided with a spring blade in all embodiments of the invention. In a preferred exemplifying embodiment of the invention, two forming shoes are used, which are preferably fitted inside the loop of the carrying wire, which are provided with a curved ribbed deck, and between which shoes a draining box is fitted, which is provided with a ribbed deck and in which negative pressure prevails, a hose-loaded web forming and loading unit being fitted facing said draining box inside the opposite wire loop. The boxes of said forming shoes and of said dewatering and web forming units should preferably communicate with sources of negative pressure so that the level of negative pressure in each box can be regulated individually in view of versatile control of the dewatering and of the web symmetry. In the following, the invention will be described in detail with reference to some exemplifying embodiments of the invention illustrated schematically in the figures in the accompanying drawing, the invention being in no way strictly confined to the details of said embodiments. Figure 1 is a schematic side view of the whole of a hybrid former in accordance with the invention. Figure 2 shows the idea of the invention as applied to a gap former. Figure 2A is an enlarged illustration of the detail A indicated in Fig. 2. Figure 3 shows the initial portion of the twin-wire former in a hybrid former as shown in Fig. 1. Figure 4 is an enlarged illustration of the twin-wire forming zone indicated by the rectangles A in Figs. 1 and 3. Figure 5 shows a preferred vertical gap former in accordance with the invention. Figure 6 shows the initial portion of the twin-wire zone in a former as shown in Fig. 5 on an enlarged scale. The hybrid former shown in Fig. 1 comprises a headbox 110, through whose slice 111 the pulp suspension is fed onto the substantially horizontal initial part 10a of the former, which is formed by the lower wire 10. In the initial part 10a, a breast board 12 and foil ribs 13 are placed. The passage of the lower run of the lower wire 10 is guided by guide rolls 19. The former includes an upper-wire unit 50, on whose frame part 51,52 the rolls 21,22,23,24 are mounted, which determine the run of the upper-wire loop 20. The rolls 21 and 22 of the upper-wire unit 50 and the suction boxes 53 and 54 are connected to the frame part 51, which is connected to the rest of the frame part of the upper-wire unit 50 from above the forming shoe 14 by means of horizontal articulated joints 51b and, from the end opposite to said linkages 51b, to the lifting devices 51a, by whose means the frame part 51 of the upper-wire unit, together with the devices mentioned above, can be raised to an upper position, for example, for maintenance work. The twin-wire dewatering zone starts at the line V 1 on the curved box and ends at the line V 2 on the roll 23. According to Fig. 1, after the guide roll 21 of the upper wire 20, in the twin-wire zone, inside the lower-wire loop 10, there is a forming shoe 30, whose ribbed deck 31 is curved downwards with the curve radius R 1 . After the forming shoe 30, inside the upper-wire loop 20 (cf. Fig 3), there is a dewatering foil 55, the draining duct 56 placed in front of the foil communicating with the suction box 53. This is followed by a wire loading device 40 (cf. Fig 4), which is placed inside the lower-wire loop 20 in a web forming unit 32 and whose spring blade 45 produces quite a strong pressure pulse, which removes water and improves the formation, against the inner face of the wire loop 20. Hereupon, there are forming ribs 56a,38a,56b,38b,56c placed in said sequence alternatingly inside the upper-wire and lower-wire loops. The forming ribs 56a,56b, 56c communicate with a second suction box 54. The loads applied to the ribs 38a and 38b are regulated by means of hoses, and the ribs may be interconnected. The unit 32 and the suction box 54 are followed by the sector a of the forming roll 22, on which sector the run between the wires 10 and 20 turns upwards. After this, after a short straight joint run of the wires 10 and 20, there is the forming shoe 14. The shoe 14 is followed by a short straight joint run of the wires 10 and 20, whereupon there is the first forming roll 15 placed inside the lower-wire loop 10. On the sector b of the forming roll 15, the run of the wires 10,20 turns downwards to become a straight run, in whose area, inside the loop of the lower wire 10, there are suction flatboxes 16, by means of whose suction it is partly ensured that the web W follows the lower wire 10. The web W is separated from the lower wire 10 on the run between the rolls 17 and 18 on the suction sector 60a of the pick-up roll 60, being transferred onto the pick-up fabric 61 to be passed to the press section (not shown). In the following, the initial portion of the twin-wire forming zone V 1 -V 2 of a hybrid former as shown in Figs. 1, 3 and 4 will be described, which initial portion involves an essential novelty in the present invention. According to Fig. 3, in the single-wire initial part 10a, the web W 0 has been couched so that its dry solids content k 0 is of an order of k 0 = 1.5 - 2.5 %. The first guide roll 21 placed inside the upper-wire loop 20, which roll is not a forming roll proper, is preferably smooth- and solid-faced. The guide roll 21 is followed by the first forming shoe 30 placed inside the lower-wire loop 10, which shoe has the ribbed deck 31 of a large curve radius R 1 , which deck curves the twin-wire zone V 1 -V 2 downwards. The curve radius R 1 of the shoe 30 is, as a rule, in a range of R 1 = 3-8 m, preferably R 1 ≈ 5 m. In some applications, the forming shoe 30 can be replaced by pre-loading members, such as in a MB-former. After the forming shoe 30 or equivalent, inside the lower-wire loop 10, there is the web forming unit 32, which is attached to the fastening parts 33 of the side frames of the former by means of fastening devices 35 so that the position of the unit 32 can be adjusted substantially in the horizontal direction S-S. Inside the upper-wire unit 50, facing the forming shoe 30 and the forming unit 32, there are two subsequent draining boxes 53 and 54, which communicate with sources of negative pressure by the intermediate of ducts 53b and 54b. At the rear edge of the first draining box 53, there is the foil rib 55, whose tip 55a removes water through the upper wire 20 in the direction of the arrow S 1 denoted in Fig. 4. By the effect of the kinetic energy of the draining water and by the effect of the suction of the negative pressure present in the box 53, the water removed from the web W through the upper wire 20 passes in the direction of the arrow S 1 along an upwards inclined duct 56 into the first suction box 53, from which it is removed to the side of the former through a draining duct 53a. The foil rib 55 is provided with loading hoses 55c and 55d, by whose means the foil rib 55 can be loaded against the inner face of the upper wire 20. The upper hose 55d is fixed between a frame piece 55f and an upper piece 55e. The upper piece 55e is fixed to the foil rib 55 by means of screws 55b, the rear end of said foil rib 55 being attached to the frame piece 55f by means of screws 55g so that it can be pivoted around a line 55h by regulating the pressures in the hoses 55c and 55d. According to Figs. 3 and 4, the foil rib 55 is followed by a first fixed rib 37 in the forming unit 32 and thereupon by the wire loading device 40, which is provided with the spring blade 45 and which will be described in more detail later. Said loading device 40 is followed by a unit consisting of two subsequent loading ribs 38a and 38b, which unit is loaded by a pair of transverse loading hoses 39. The loading rib 37, a frame part 41 of the wire loading device 40, and the loading ribs 38a and 38b are attached to fastening ribs 46 by means of their dovetail grooves 47 so that the loading ribs 37 and the device 40 can be exchanged quickly by pulling them to the side of the former in their longitudinal direction and by fitting new parts in their place. According to Figs. 3 and 4, facing the forming unit 32, inside the upper-wire loop 20, the second draining box 54 operates, whose bottom face consists of three to five subsequent loading ribs 56a,56b and 56c, which are attached to their fastening ribs 57 by means of their dovetail grooves 58 so as to be replaceable quickly by pulling to the side of the machine. The first rib 56a operates in the loading area of the spring blade 45 of the device 40, in which area a strong pressure pulse is applied to the web W. The second rib 56b operates between the ribs 38a and 38b, and the last rib 56c operates after the last rib 38b in the unit 32, or there is one more pair of units/ribs. In the area of the ribs 37,38a,38b,56a,56b and 56c, the run of the twin-wire zone is very gently wave-formed with a low amplitude, which promotes the dewatering and the formation and reduces the tendency of wrinkle formation in the wires 10,20. The spaces between the ribs 56a,56b and 56c communicate with the negative pressure present in the suction box 54. The negative pressure present in the first box 53 is in a range of 0-2 mH 2 O, and in the second box 54 the level of negative pressure is higher, being of an order of 0.4-4 mH 2 O. To achieve this, the second box 54 communicates with a suction pump which handles smaller quantities of air than the blower of the first box 53, which blower communicates with the duct 53b. In connection with the dewatering members described above, it is possible to use a number of different regulation parameters, by whose means the dewatering process can be controlled and optimized. These parameters will be described in more detail in the following. The pressure in the loading hose 49 of the loading device 40 is, as a rule, regulated in a range of 0.5-1.5 bars, while the maximal pressure is ∼ 2-5 bars. The length of the pressure pulse of the spring blade 45 in the machine direction is, as a rule, about 2-10 mm, and the magnitude of the pressure pulse in said loading area of the blade 45 is, as a rule, in a range of 0-2 kN/m, while an optimal pressure is ∼ 1 kN/m. The pressure in the loading hoses 39 of the forming ribs 38a and 38b is adjustable, being preferably ∼ 0.5 bar. The positions of the ribs 37,38a, 38b,56a,56b,56c in the machine direction can be arranged adjustable, and so can the position of the spring blade 45 of the loading device 40, in view of regulation of the dewatering and optimization of the formation. The length L 0 of the dragging area of the loading ribs 37,38a,38b,56a,56b,56c in the machine direction is preferably L 0 ≈ 30 mm, and the distance L 1 between the ribs in the machine direction is preferably L 1 ≈ 25 mm. After the loading area of the blade of the loading device 40, the dry solids content of the web W is, at the maximum, k max ≈ 4 %. The values given above just serve as a guideline, and they depend on the raw-material used and on the paper quality that is being manufactured. Before the forming roll 22, which is provided with an open hollow face 22', the dry solids content of the web W placed between the wires 10 and 20 is of an order of k 2 = 3-5 % preferably 3.5 %. In such a situation, in the web W, there is almost no more freely moving water by whose means the formation could be affected. Thus, the formation is produced primarily by means of the single-wire initial part 10a of the dewatering zone together with the dewatering and forming members 30,32,55,56a,56b,56c,53, 54. It is characteristic of the construction described above that by its means it is possible to remove quite a large proportion of water expressly through the upper wire 20 and thereby to compensate for the unequalsidedness of the distribution of fines and fillers that has been produced by the one-sided draining through the lower wire 10 taking place in the single-wire zone 10a. On the sector a of the open face 22' of the forming roll 22 shown in Fig. 1, the dewatering goes on upwards through the upper wire 20 because of the open face 22' of the roll 22 by the effect of the compression between the wires 10 and 20. This draining that takes place upwards goes on on the curved-faced forming shoe 14 as a dewatering that takes place upwards by the effect of the centrifugal force produced by the curve form R 2 of the shoe 14 and by the effect of the tension between the wires 10 and 20. On the sector b of the smooth-faced forming roll 15, the pressure of the dewatering that takes upwards is increased substantially. This is achieved so that the radius of the roll 15 has been chosen substantially smaller than, for example, the curve radius R 2 of the curved forming shoe 14. On the sector b of the roll 15, the pressure of draining of the water through the upper wire 20 has been dimensioned maximal so that, for its part, the radius of the roll 15 determines the dry solids content of the web W. Hereupon, the suction flatboxes 16 are used to the extent that is necessary. However, it is the main principle of the invention that attempts are made to use a minimum number of suction flatboxes 16 or even to omit them completely, because these dewatering members consume a relatively large amount of energy. Figures 2 and 2A show a gap former, whose twin-wire zone runs vertically upwards. The carrying wire 10 and the covering wire 20, which are guided by their guide rolls 11;22A, form a forming gap G between them. Into this forming gap G, the pulp suspension jet J is fed out of the discharge opening 111 of the headbox 110. After the bottom of the forming gap G, the twin-wire zone is curved on the sector a 0 of the forming roll 22A, whose magnitude is, as a rule, in a range of a 0 = 20-45°. After the sector a 0 , the twin-wire zone is separated from the forming roll 22A and continues is run, being guided by the ribbed deck 31 of the forming shoe 30A fitted inside the loop of the carrying wire 10. In Figs. 2 and 2A, after the forming shoe 30A, inside the loop of the covering wire 20, the wire loading device 40 is fitted, whose spring blade 45 produces quite a strong pressure pulse against the inner face of the wire 20. This pressure pulse is effective at the latest at a first loading rib 81 of a MB-unit 80 fitted inside the loop of the wire 10. After the pressure pulse of the spring blade 45 of the loading device 40, there is a MB-unit 70 placed inside the loop of the covering wire 20, whose construction is seen in more detail in Fig. 2A. In the MB-unit 70, there are ceramic loading ribs 71, 72, arranged in pairs, which ribs are interconnected by a support construction 73. The unit 72,73 is loaded against a frame constructions 74 by means of pressure passed into the interior of loading hoses 75. The parts 70 and 80 may also change places with one another. According to Fig. 2A, the ceramic forming ribs 81 of the MB-unit 80 placed inside the loop of the wire 10 are attached to a frame constructions 84 by means of dovetail joints. The ribs 81 are placed alternatingly with respect to the ribs 71,72 of the unit 70, so that the twin-wire zone runs between the units 70,80 along a very gently meandering path. In the area of the MB-units 70,80, the dewatering can be intensified by the effect of the negative pressure prevailing in the gaps between the ribs 71,72,81. In respect of the other details of the constructions of the MB-units 70,80, reference is made to EP-B-0 397 430 . After the MB-units 70,80, the twin-wire zone goes on as a vertical run, on which a suction flatbox 85 has been fitted, water being sucked out of the web W through the carrying wire 10 through the gaps in the ribbed deck 86 of said box 85. The vertical run of the twin-wire zone is curved on the suction zone 15a of the second forming roll 15A, whereupon the covering wire 20 is detached from the web W, which is guided on the carrying wire 10 to the pick-up point (not shown). In the following, mainly with reference to Fig. 4, the details of the construction and the operation of the wire loading device 40 will be described. The loading device 40 comprises the thin plate-like spring blade 45, whose tip 45a may be rounded. The spring blade 45 extends as a unified construction across the entire width of the web W and of the wires 10,20. The area of the tip 45a of the spring blade 45 may be provided with perforations. The spring blade 45 loads and drags against the inner face of the wire 10;20 by means of its wide side. At its opposite edge, the spring blade 45 is attached to the frame part 41 of the loading device 40 by means of a fastening piece 42 and a screw 43. The blade 45 operates as a plate spring, by whose means, when it is loaded by one edge so that it becomes curved, a dragging and loading pressure is produced against the wire 10. The blade 45 is stationary, and it drags "with the fur" against the wire 10 that it loads. The spring blade 45 of the loading device 40 is made of a restorably flexible plate-like spring material. The ratio of the length of the spring blade 45 to the thickness of the plate material of the blade is chosen within the range of L/S = 10-1000. Optimal applications are usually found within the range of L/S = 300-500. Said ratio L/S also depends on the material of the spring blade. As the blade material, preferably wear-resistant spring steel is used, such as stainless steel. Some plastic materials and composite and sandwich structures may also be possible. The spring blade 45 does not necessarily have to be of uniform thickness or of the same material or same construction across its entire length or entire width. When the blade 45 is loaded by means of the loading devices 49, the shape of the blade plate in the machine direction can be deflected with a relatively large curve radius ≈ 200 - 1000 mm depending on the elasticity conditions and loads, and a sufficiently wide dragging area against the wire 10 can be produced. Thus, the material of the spring blade 45 must have suitable spring properties, and permanent deformations must not be produced in it. The spring blade 45 is dimensioned, and the spring properties of its material are, as a rule, chosen so that the elastic constant of the blade deflection per metre of width is in a range of 0.02-0.3 kN/mm, preferably in a range of 0.06-0.12 kN/mm. In particular in composite structures, the elastic constant may be different in the machine direction as compared with the transverse direction. The area of the spring blade 45 that will load and drag against the wire 10,20 can, if necessary, be provided with a wear piece or with a wear-resistant coating, e.g. with a ceramic layer, which is represented by the dashed line and the reference numeral 11k. It should be still repeated in this connection that, by means of the spring blade 45 of the wire loading device 40, a very intensive pressure pulse is produced, whose linear load is, as a rule, in a range of 0.2-3 kN/m, preferably in a range of 0.7-1.2 kN/m. The length of said pressure pulse in the machine direction is relatively short, as a rule 2-10 mm, preferably ∼ 3-7 mm. By means of said pressure pulse, the formation of the web W is improved and a strong dewatering effect is produced through the opposite wire, in hybrid former applications through the upper wire 20. Said pressure pulse has the most advantageous effect on the dewatering and on the formation of the web when the dry solids content of the web is in a range of k = 1.2 - 3% , preferably k = 1.5-2 %. When the distribution of the linear load of the spring blade 45 of the loading device in the transverse direction is regulated, it is also possible to control the transverse profiles of different properties of the web W, such as the transverse profiles of dewatering, distribution of fillers, formation, and/or of retention. Figures 5 and 6 show a former that is in many respects similar to that shown in Figs. 2 and 2A. Thus, in this connection reference is made to the description related to Figs. 2 and 2A, and in the following the concentration will be mainly on the features of the former as shown in Figs. 5 and 6 that differ from those illustrated in Figs. 2 and 2A. The former shown in Figs. 5 and 6 has no wire loading device 40 at all. After the forming roll 22A, the twin-wire zone has a short straight run, which is followed by a forming shoe 30A similar to what has been described above, said shoe being provided with a ribbed deck 31. The box 32 of the forming shoe 30A communicates with a source of negative pressure, such as a suction pump, which is illustrated by the arrow S 0 . The ribbed deck 31 of the forming shoe 30A has a curve radius of R 1 = 3-8 m, preferably R 1 ≈ 5 m. The suction box of the forming shoe 30A is directly connected with a first box 89 in a MB-unit 80A, which box 89 communicates with a source of negative pressure, which is illustrated by the arrow S 2 . The suction box 89 is provided with a ribbed deck 81, which is described in more detail above in connection with Fig. 2 and whose initial portion guides the twin-wire zone along a straight run D 1 up to the forming rib 81A. The forming rib 81A is placed substantially at the middle of the box 89. Hereupon, there are ribs on the ribbed deck 81 as mounted in such a way that, with respect to the first straight run D 1 , the twin-wire zone will be guided along a second straight run D 2 . Between the straight runs D 1 ,D 2 , there is a little angle of a few degrees, whose "tip" is placed facing the rib 81A. The box 89 of the MB-unit 80A is followed immediately by a second forming shoe 30B, whose box 33 communicates with a source of negative pressure, which is illustrated by the arrow S 3 . The ribs 31 on the guide deck of the second forming shoe 30B are placed so that the forming shoe 30B guides the twin-wire zone with the curve radius R 3 , which has the same direction as the curve radius R 1 of the first forming shoe 30A. The curve radius R 3 is chosen substantially equal to, or somewhat larger than, the curve radius R 1 . Opposite to the MB-unit 80A, there is the MB-unit 70A, which is fitted inside the loop of the covering wire 20 and whose box 78 communicates with a source of negative pressure, which is illustrated by the arrow S 4 . In the MB-unit 70A, the ribs 71,72 operate against the inner face of the wire 20, are loaded by pairs of pressure medium hoses 75a,75b, and are interconnected in pairs by means of intermediate parts 73 in the way shown in more detail in Fig. 2A. The operation of the MB-unit 80A,70B is, in the other respects, similar to that described above in relation to Figs. 2 and 2A. According to Fig. 5, the second forming shoe 30B is followed by a suction flatbox 79 fitted inside the loop of the covering wire 20 and, thereupon, by two suction flatboxes 85a and 85b fitted inside the loop of the carrying wire 10, after which the construction and the run of the web W are similar to that described in relation to Fig. 2. In Figs. 5 and 6, the directions and routes of dewatering are illustrated by the arrows WA. In the following, the patent claims will be given, and the various details of the invention may show variation within the scope of the inventive idea defined in said claims and differ from what has been stated above for the sake of example only.

Claims: Twin-wire web former for a paper machine, comprising a carrying wire (10) and a covering wire (20) forming each one a loop, wherein said wires (10,20) form a twin-wire forming zone (V 1 -V 2 ) between them, on which zone water is drained out of the web (W) through both of said wires (10, 20), after which twin-wire zone the web (W) is separated from the covering wire (20) and is transferred on the carrying wire (10) to its pick-up point, and wherein, in hybrid formers, after the single-wire forming zone (10a), and in gap formers , after the curved forming zone (a 0 ) placed directly after the forming gap (G), there is a forming shoe (30; 30A) provided with a ribbed deck (31) and fitted inside one of the wire loops (10, 20), and said forming shoe is followed by dewatering and web forming units (70, 80; 70A, 80A) which include forming ribs (81, 81A, 71, 72) and are placed inside both of the wire loops (10, 20), of which units at least those placed inside one of the wire loops (10/20) are loaded, preferably by means of pressure-medium hose arrangements (75; 75a, 75b), characterized in that, after said forming shoe (30; 30A), inside one of the wire loops (10/20), a wire loading device (40) is fitted, which is provided with a spring blade (45), loaded by a variable fluid pressure in a loading hose (49), and by which an intensive pressure pulse can be produced in the web (W) that is being formed in the twin-wire forming zone (V 1 -V 2 ). Web former as claimed in claim 1, characterized in that, in the twin-wire zone, substantially immediately after the dewatering and web forming unit (70A, 80A), a second forming shoe (30B) is fitted, which is provided with a ribbed deck (31), which guides the twin-wire zone along a curved (R 3 ) path , and that the ribs on the ribbed deck (81) of a draining box (80A) placed between the first and the second forming shoe (30A, 30B) are fitted in such a way in relation to one another that, in the middle area of said ribbed deck (81), the direction of the twin-wire zone is changed at a small angle. Web former as claimed in claim 2, characterized in that the linear load of the pressure pulse produced by means of the spring blade (45) of the wire loading device (40) is arranged in a range of 0.2-3 kN/m, preferably in a range of 0.7-1.2 kN/m, and/or that the length of said pressure pulse in the machine direction is arranged in a range of 2-10 mm, preferably in a range of 3-7 mm. Web former as claimed in claim 2 or 3, characterized in that, by means of the spring blade (45) of the wire loading device (40), a pressure pulse is produced, by whose means the formation of the web (W) is improved and the dewatering is promoted in an area in which the dry solids content k of the web (W) is in a range of k = 1,2-3%, preferably in a range of k = 1.5-2%. Web former as claimed in any of claims 2 to 4, characterized in that the linear load of the pressure pulse of the spring blade (45) of said wire loading device (40) in the transverse direction is arranged adjustable in view of the control of the transverse profiles of the dewatering of the web, the distribution of fillers, the formation, and/or of the retention. Web former as claimed in any of claims 1 to 5, characterized in that said forming shoe (30; 30A) is a forming shoe provided with a curved ribbed deck (31), whose curve radius R, is in a range of R 1 = 3-8 m, preferably R 1 ≅ 5 m. Web former as claimed in any of claims 1 to 5, characterized in that said forming shoe (30) and the subsequent wire loading device (40) are, both of them, placed inside the loop of the carrying wire (10). Web former as claimed in any of claims 1 to 7, characterized in that said forming shoe (30A) is placed inside the loop of the carrying wire (10), and said wire loading device (40) is placed inside the loop of the opposite covering wire (20). Web former as claimed in any of claims 2-8, characterized in that the ribbed deck (31) of the second forming shoe (30B) is arranged to guide the twin-wire zone so as to be curved (R 3 ) in the same direction as the preceding first forming shoe (30A) guides it. Web former as claimed in claim 2 or 9, characterized in that the suction box (32) of the first forming shoe (30A), the suction boxes (89, 78) of the dewatering and web forming units (70A, 80A), and/or the suction box (33) of the second forming shoe (30B) communicate (S 0 ,S 1 , S 2 , S 3 , S 4 ) with sources of negative pressure preferably so that the levels of negative pressure in said boxes (32, 33, 89, 78) are arranged adjustable in view of controlling of the dewatering. Web former as claimed in any of claims 1 to 10, comprising a lower-wire (10) loop placed in connection with the headbox of the paper machine, which wire loop forms the single-wire, preferably substantially horizontal initial part (10a) of the dewatering zone, in which part water is drained by means of dewatering members (12, 13) through the lower wire (10) out of the web (W) that is being formed, and which web former comprises an upper-wire unit (50), in which there is an upper wire (20) guided by the guide and web-forming rolls (21, 22, 23, 24), which upper wire (20), together with the run of the lower wire (10), forms the twin-wire dewatering zone, which starts on the guide roll (21) fitted inside the loop (20) of the upper wire, characterized in that, at the beginning of the twin-wire forming zone (V 1 -V 4 ), inside the lower-wire loop (10), there is a forming shoe (30), which is provided with the ribbed deck (31), after which, inside the upper-wire loop, there is a foil rib (55) communicating with a suction box (53) through a drain duct (56), which foil rib removes water through the upper wire (10), and that said foil rib (55) is followed by the wire loading device (40) placed inside the lower-wire loop, by means of whose spring blade (45) the intensive pressure pulse is applied to the web through the lower wire loop (10), that said wire loading device (40) is followed, inside the lower-wire loop, by a forming rib or ribs (38a, 38b), which is/are preferably loaded by the loading hoses (39), that, inside the upper-wire loop (20), there are loading ribs (56a, 56b, 56c) operating against said wire, of which ribs the first one operates in the area of the loading zone of the spring blade (45) of the wire loading device (40), and that the gaps between said forming ribs (56a, 56b, 56c) communicate with a suction box (54), which communicates with a suction pump through a suction duct (54b). Web former as claimed in claim 11, characterized in that said suction boxes (53, 54) and said sets of loading ribs are followed by an open-faced (22') forming roll (22) fitted inside the upper-wire loop (20), the run of the twin-wire forming zone being curved upwards on the sector (a) of said forming roll (22), which is followed by a curved-faced (R 2 ) forming shoe (14) placed inside the lower-wire loop (10), which shoe is followed, further inside the lower-wire loop (10), by a forming roll (15), in whose area the twin-wire forming zone is curved downwards, being followed by a straight run of the twin-wire forming zone onto the guide roll (23) of the upper wire (20), in whose area the web (W) is separated from the upper wire (20) and is passed on the lower wire (10) onto a pick-up fabric (61). Web former as claimed in claim 11 or 12, characterized in that, after said set of dewatering ribs (38a, 38b, 56a, 56b, 56c), before the first hollow-faced (22') forming roll (22), on which the twin-wire zone is curved upwards, the dry solids content k 2 of the web (W) is in a range of k 2 = 3-5%, preferably k 2 ≅ 3,5%. Web former as claimed in any of claims 11 to 13, characterized in that said first suction box (53) communicates with a blower through its suction duct (53b) so that the negative pressure prevailing in said first box is at a level of 0-2 mH 2 O, and that said second suction box (54) communicates with a vacuum pump through its duct (54b) so that the level of negative pressure prevailing in the second suction box (54) is of an order of 0.4-4 mH 2 O. Web former as claimed in any of claims 1 to 10, characterized in that the twin-wire forming zone is substantially vertical and that, after said forming shoe (30A) and said wire loading device (40), MB-units (70, 80) placed one opposite to the other have been arranged on the twin-wire zone. Web former as claimed in claim 15, characterized in that, before said forming shoe (30A), the carrying wire (10) and the covering wire (20), guided by the guide rolls (11, 22A), for a forming gap (G) between them, into which gap the pulp suspension jet (J) is fed out of the discharge opening (111) of the head box (110), that, after the bottom of the forming gap (G), the twin-wire zone is curved on the sector (a 0 ) of the forming roll (22A), the magnitude of said sector being preferably in a range of a 0 = 20-45°, and that, after said sector (a 0 ), the twin-wire zone is separated from the forming zone (22A) and continues its run as guided by the ribbed deck (31) of said forming shoe (30A).

Inventors: AULA JOUKO, EVAESOJA PEKKA, JAAKKOLA JYRKI, ODELL MICHAEL, AULA, JOUKO, EVAESOJA, PEKKA, JAAKKOLA, JYRKI, ODELL, MICHAEL, Eväsoja, Pekka
Applicants: VALMET CORP, VALMET CORPORATION
Application year: 1993
Main classification: D21F9
IPC codes: D21F9, D21F1