Twin-wire former in a paper machine

Abstract: The invention concerns a twin-wire web former in a paper machine, comprising a carrying wire (20) and a covering wire (10), which together form a twin-wire forming zone. In this forming zone, a forming unit (50) is fitted, which comprises a forming board (30) and a drainage box (51) placed one opposite the other. In the forming board (30), placed facing the drainage box (51), there are a number of transverse loading ribs (33) placed at a distance from each other. Subsequent loading ribs are interconnected by intermediate parts (31), which, together with the loading ribs (33), form ribbed shoes (30A,30B,30C). These shoes can be loaded by means of loading hoses (40a,40b) to produce a dewatering pressure in the web (W) placed between the wires (10,20). In the area of the forming unit (50), the dewatering takes place both through the covering wire (10) and through the carrying wire (20), also towards the forming board (30) through the open spaces (36) placed between its loading ribs (33).

Description: The invention concerns a twin-wire web former in a paper machine, according to the preamble of claim 1. This preamble is based on WO/91/02842. With increasing running speeds of paper machines, several problems in the web formation have been manifested with more emphasis. The phenomena, such as centrifugal forces, which affect the fibre mesh in the former of the paper machine and which also affect the water, which is still relatively free in connection with the fibre mesh, are usually increased in proportion to the second power of the web speed. The highest web speeds of most modern paper machines are of an order of 1200 m/min. However, machines are being planned in which a web speed as high as about 1700 m/min is aimed at. In respect of the prior art related to the present invention, reference is made to the US Patent No. 4,769,111,granted to Messrs. A. Ahlstrom Corp., to the applicant's FI Pat. Appl. No. 885609, as well as to the FI Pat. Appls. Nos. 885606 and 885607 of Messrs. Valmet-Ahlstrom Inc., wherein formers marketed by the applicant under the trade mark "MB-Former" are described. In the prior-art MB-formers, the lower unit consists of a support board, which consists of relatively wide transverse ribs. These ribs are loaded each of them separately by means of a loading hose of its own. The top side of said ribs, which is placed against the inner face of the lower wire, is provided with grooves running across the face in the longitudinal direction of the ribs, the function of the water gathered in said grooves being mainly to lower the friction between the lower wire and the ribs. It is a drawback of this prior-art support board that the draining from the web downwards is almost completely prevented, because water can drain only to a highly limited extent into the narrow and shallow grooves on said ribs, from which grooves the water can flow to the sides only. In particular when a MB-unit or units are fitted, in a high-speed and wide gap former (the applicant's "HHS-former" = trade mark), the narrow and shallow grooves on the ribs in the support board are not adequate to remove the relatively large quantities of water. Thus, in the prior-art MB-units, the draining takes place practically exclusively upwards towards the upper drainage box, aided by suction. Thus, in the prior-art constructions, the formation of the web cannot be affected sufficiently by means of a MB-unit, because the fibrous web must be relatively dry when it arrives in the twin-wire zone in the MB-unit, so that the fibres in the web can no longer be displaced to a sufficient extent in relation to one another. The object of the invention is to provide a former in which the web forming process as a whole is under control better, so that the particular requirements of the paper grade that is being produced can be taken into account better in the process. An object of the present invention is further development of prior-art MB-formers and of their combinations with hybrid and/or gap formers so that the location of the MB-unit is more free, usually so that the MB-unit can be fitted at a point in the web forming process at which the dry solids content of the web is lower than in prior art. A further object of the invention is to provide a former that has an increased draining capacity, which can be utilized as an increased web speed and/or as a thicker grade produced and/or as a lower consistency in the headbox, which in itself already helps to improve many properties of paper. It is a particular object of the invention to provide a former, in particular a high-speed gap former, that is suitable in particular for the production of fine papers whose grammage is higher than about 40 g/m² and when the speed of the paper machine is higher than 1000 m/min, even of an order of 1400...1700 m/min. As is known in prior art, in the manufacture of certain paper grades, a colouring agent is added to the paper pulp, which colouring agent is, however, when the prior-art MB-units are employed, washed along with the water that is drained upwards, to a greater extent to the top face of the web. On the other hand, with prior-art former solutions, when attempts have been made to drain a higher proportion of water downwards to provide a better balance of colour, the consequence has been inferior formation of the paper. In relation to the above, an object of the invention is to provide a former with which the fine paper or printing paper produced has an improved balance of colours. It is a further object of the invention to provide an improved construction of the MB-former in which the ratios of the draining taking place up and down can be controlled better than in prior art. The aim is, for example, in view of the above balance of colours and/or in view of the balance of fillers, as a rule, that in the twin-wire zone of the MB-unit water should be removed as substantially equal amounts both upwards and downwards. A further object of the invention is, when necessary, to arrange that the draining is symmetric within the whole former, i.e., for example, when the suction in the forming roll is altered, the balance of draining can be maintained by adjusting the direction of draining by means of the present invention. In view of achieving the objectives stated above and those that will come out later, the invention is characterized by the features defined in the characterizing clause of claim 1. GB-A-1 225 517 discloses a stationary shoe with a plurality of successive ribs placed the single-wire zone, but does not disclose a set of ribs opposite said successive ribs. Three such successive ribs are interconnected and loaded by a pair of hoses. Such a construction in accordance with the invention is particularly advantageous in which the ribbed shoe in the forming board comprising a pair of loading ribs a loading hose is fitted at each of the ribs; into each of which hoses an adjustable pressure can be passed so as to adjust the loading force of the ribbed shoe. Since the wire moving against the loading ribs produces forces on the ribbed shoes, the ribbed shoes must be supported. In the invention, this supporting takes place advantageously so that the support results in a minimal torque that attempts to turn the ribbed shoe. The invention can be applied both to gap formers and to hybrid formers that have a single-wire initial portion of the forming zone, which is followed by a twin-wire zone provided with one or several forming units in accordance with the invention. In the invention, at the forming unit the twin-wire zone is preferably straight, but in some cases the invention may also be accomplished so that said twin-wire zone is curved either towards the drainage box of the forming unit or towards the forming board in accordance with the invention. 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 not confined to the details of said exemplifying embodiments. Figure 1 is a schematic side view of a fourdrinier former provided with a MB-unit in accordance with the invention. Figure 2 shows a high-speed gap former that is provided with two MB-units in accordance with the invention placed one after the other. Figure 3 is a vertical sectional view in the machine direction of a loading member of a forming board in accordance with the invention. Figure 4 is a sectional view taken along the line IV-IV in Fig. 3. Figure 5 shows a second exemplifying embodiment of the invention in a way corresponding to Fig. 3. Figure 6 is a sectional view taken along the line VI-VI in Fig. 5. Figure 7 is a vertical sectional view in the machine direction of a forming board in accordance with the invention. Figure 8 is a sectional view taken along the line VIII-VIII in Fig. 7. Fig. 1 is a schematic side view of the twin-wire part of the former of a paper machine, wherein the upper wire 10 is arranged to run over the guide rolls 16,17,18 and 19 and wherein the lower wire 20 runs underneath the upper wire 10 as substantially parallel to the upper wire 10. The wires 10 and 20 form a wedge-shaped inlet gap K, wherein the web W o placed on the lower wire 20 is constantly pressed between the wires 10 and 20 as they make progress. On the single-wire initial portion 10a of the lower wire 20, the pulp layer that was fed out of the discharge opening of the headbox (not shown in Fig. 1) has had time to be drained so that its dry solids content ka₀ is, as a rule, in a range of ka₀ = 0.7...2.5 %. With the prior-art MB-units, said dry solids content has usually been in a range of ka₀ = 1.5...2.5 %. By means of the MB-unit in accordance with the invention, the aim is to make it possible that the MB-unit can be placed in an area optimal in view of the draining and formation processes, also closer to the lip of the headbox, for example in an area where the dry solids content of the web W o is ka₀ = 0.9...1.5 %. After the wedge-shaped inlet gap K, in the transfer direction F, there is a MB-unit 50, which comprises a drainage box 51. The bottom 52 of the drainage box 51 consists of ribs 53, water being sucked out of the web W o through the gaps placed between said ribs 53 into the drainage box 51 by means of negative pressure. The upper wire 10 rests against said ribs 53 as it runs. Further, Fig. 1 shows several other parts and adjusting members included in the former, which parts and members are in themselves known and will not be described in more detail in this connection. According to Fig. 1, the drainage box 51 comprises three chambers 54a,54b,54c, in which negative pressure prevails, by whose means water is sucked out of the web W o into said chambers. The negative pressure and the removal of the water out of the chambers 54a,54b,54c are produced through the pipes 55a,55b and 55c. In different chambers, negative pressures of different levels are preferably employed. The MB-unit 50 includes a novel forming board 30 in accordance with the invention, which also permits draining downwards, which is shown in Fig. 1 just schematically, and whose construction will be described in detail later with reference to Figures 3 to 8. The high-speed gap former shown in Fig. 2 comprises the loop of the covering wire 10 and the loop of the carrying wire 20. The wires 10,20 have a joint run between the lines A and B, said lines limiting the twin-wire forming zone of the former. The web W follows the carrying wire 20 after the twin-wire forming zone A-B. The slice part 60 of the headbox feeds a pulp jet J into the gap G between the wires 10,20, which gap is, at one side, defined substantially by the run of the wire 10 from the roll 16a to the line A, where the wire 10 meets the other wire 20 (the pulp layer is placed in between). At the other side, the forming gap G is defined by the wire 20 which runs over the first forming roll 21. According to Fig. 2, the first forming roll 21 is a forming roll which is provided with an open face 21', which has a relatively large diameter D₁, and which is provided with a suction box 21a. On the sector a of the first forming roll 21, the draining takes place substantially away from the forming roll 21 in the direction of the arrow F₁ and to some extent into the open face 21' of the roll 21. As an important part, the twin-wire former shown in Fig. 2 includes two MB-units 50A and 50B placed one after the other. The MB-units 50A and 50B comprise drainage boxes 51 and novel forming boards 30 in accordance with the invention, which boards operate as press and drainage units, the wires 10 and 20 and the web W placed between them running between said box 51 and board 30. In the first MB-unit 50A, the drainage box 51 is placed above, inside the loop of the upper wire 10, and the forming board 30 is placed below. In the next MB-unit 50B, the parts 51 and 30 are placed in the reversed order. The twin-wire zone runs through the units 50A and 50B as a straight run so that drainage takes place in both MB-units 50A and 50B both through the upper wire 10 and through the lower wire 20. The units 50A and 50B may also be arranged so that the twin-wire zone is curved in the area of said units or unit. The units 50A and 50B may also be substituted for by one unit 50, in which case the drainage box 51 is preferably placed above, inside the loop of the covering wire 10. According to Fig. 2, the MB-unit or units 50A,50B is/are followed by a second forming roll 24, which is placed inside the loop of the carrying wire 20 and on whose sector b the run of the wires 10,20 is turned to be curved towards the pick-up point. After the second forming roll 24, the web W proceeds to the line P, at which it is separated from the wire 20 by means of the pick-up roll 60b and its suction zone 60a, being transferred onto the pick-up fabric 61, which carries the web W further to the press section (not shown) of the paper machine. The loading member of the forming board 30 of the MB-unit 50 shown in Fig. 3 comprises transverse frame parts 30a and 30b, on support of whose top sides the ribbed shoe is supported by means of pressure-loaded hoses 40a and 40b. The ribbed shoe comprises a number of ribs 31 in the machine direction, openings 36 remaining between said ribs. To said ribs 31, transverse loading ribs 33 have been attached by means of dovetail joints 32 or equivalent, and the plane top sides of said loading ribs 33 drag, while water operates as the lubricant fluid in the former as shown in Fig. 1, against the inner face of the loop of the lower wire 20 and load said inner face of the wire upwards with the force determined by the pressures p, passed into the pair of hoses 40a,40b. The top front edges of the loading ribs 33 are provided with tip edges 35, which doctor water to be drained from the web W off the lower face of the wire 20. The ribbed shoe 31,33 is supported in its place of operation by means of a number of vertical arms 37, which are attached to the frame part 30a by means of joints 38. The top ends 37a of the arms 37 are supported on the rear edge of the front rib 33, in the direction of transfer F of the wires 10,20. The arms 37 keep the ribbed shoe 31,33 in its position. The support point of the arms 37 is placed very high near the bottom face of the lower wire 20, for which reason, from the support, a minimal torque that attempts to turn the shoe is produced on the ribbed shoe 31,33. In Fig. 3, only one upper rib 53 of the drainage box 51 in the MB-unit 50 is shown, a wear piece 57 being attached to the bottom side of said rib 53 by means of a joint 56, which wear piece 57 is provided with a tip edge 58 that doctors water from the inner face of the upper wire 10. The ribbed-shoe construction shown in Figs. 5 and 6 differs from that shown in Figs. 3 and 4 in the respect only that underneath the ribbed shoe 31,33, a suction box 41 is placed, which is attached to the intermediate ribs 31 in the shoe by means of its flanges 43. The suction box 41 communicates with a source of negative pressure through a flexible suction hose 42 so that, by the intermediate of the suction box 41, negative pressure p can be applied through the openings 36 between the ribs 31 to the space between the ribs 33, preferably from the side, by means of which negative pressure the drainage down through the lower wire 20 is promoted. According to Figs. 5 and 6, the trailing edge of the rearmost rib 33 of the shoe 31,33 is supported on the frame part 30b by means of a series of arms 37 so that the support produces a minimum torque on the ribbed shoe 31,33 when a torque is applied by the movement of the lower wire 20 to said ribbed shoe. In Figs. 7 and 8, a number of ribbed shoes 30A,30B and 30C placed one after the other are shown, the suction boxes 41 described above in connection with Figs. 5 and 6 being attached below said shoes. The ribbed shoes 30A,30B and 30C are separated from one another by a narrow transverse gap 46. Adjacent ribbed shoes are interconnected from underneath the gaps 46 between them by means of the suction boxes 44, the flanges 34 at the top side of said suction boxes being provided with resilient seals 45 that permit a "play" of adjacent ribbed shoes 30A in relation to one another. By means of said suction boxes 44, an effect of negative pressure is applied to the web W through the lower wire 20 in the same way and for the same purpose as in the case of the fixed suction boxes 41. Moreover, in Fig. 7, facing the last ribbed shoe 30C, three subsequent ribs 53a,53b,53c of the drainage box 51 are seen, which are placed with relatively short mutual gaps in the machine direction. The first rib 53a is placed facing the rib 33 of the lower ribbed shoe, and the last rib 53c is placed facing the next lower rib 33, and the middle rib 53b is placed in the middle area between said ribs 33. The number of said ribs 53 may also be some other suitable number different from three, placed one after the other. In the following, the principal features of the operation of the ribbed shoes illustrated above in Figs. 3 to 8 will be described. From the point of view of the drainage and formation process, the MB-unit 50 in accordance with the invention operates substantially in the same way as the prior-art MB-unit. An essential difference consists of improved control and wider ranges of regulation of the drainage directions and water quantities. In the invention, the loading of the ribbed shoes takes place by means of two or more hoses 40a,40b, between which there remain openings 36, through which water can be allowed to drain downwards by the force of gravity while, if necessary, intensifying the drainage by means of negative pressure. The forces parallel to the wires 10,20 are received by means of small support arms 37, which are placed as high as possible near the wire 20 in order that turning of the unit should be minimized. Through the openings 36 mentioned above, quite large quantities of water can be sucked down, in which case the MB-unit 50 can be placed at a location that is optimal in view of the manufacture of paper. In this connection, it should be emphasized that, even though, above, exclusively such formers have been described in which the run of the twin-wire zone at the MB-unit 50 is straight, the invention can also be accomplished so that the ribbed shoes 31,33 of the forming board 30 are not in the same plane, but the guide planes of subsequent ribbed shoes are at a small angle in relation to one another, so that the run of the twin-wire zone is curved with the shape of a broken line with a relatively large curve radius. The ribs 53 of the drainage box 51 must also be arranged accordingly. In such a case, the curve radius of the twin-wire zone may be either at the side of the drainage box 51 or at the side of the forming board 30. Owing to the novel forming board 30 in accordance with the invention, the water can drain down equally well as up. By means of the hoses 40a,40b, the open ribbed shoe is loaded with a pressure of 0.2...0.4 bar. The drainage upwards is accomplished in the same way as in the prior-art MB-formers. The loading hoses 40a and 40b produce a pressure between the ribbed shoe 31,33 and the frame part 30a,30b, which pressure is applied to the inner face of the lower wire 20 by the intermediate of the top sides of the loading ribs 33. By means of this pressure, the web W is pressed against the lower face of the upper wire 10, and the back-up members of the upper wire 10 consist of the stationary ribs 53 of the drainage box 51. In this way, a dewatering compression is produced on the web W between the wires 10,20 even if the runs of the wires 10,20 were straight, in which case the pressure produced by tensioning of the wires 10,20 cannot be converted to dewatering pressure. Underneath, the water drains through the openings 36 into the suction box 41,44, whose dimension is, e.g., equal to that of two ribs 33. The ribbed shoes 31,33 may have a length larger than two ribs 33 in the running direction F of the wires, in which case a change in the thickness of the web W may require that a somewhat resilient material, e.g. a suitable durable plastic, is used for the ribbed shoes. The negative pressure is produced for the suction boxes 41,44 from the side by means of a flexible hose 42, in which case the forces produced by the negative pressure are almost in balance, whereby the loading force of the ribs 33 can be controlled irrespective of the negative pressure, and, thus, the drainage can be altered from upward drainage to predominantly downward drainage by just regulating the levels of negative pressure in the suction boxes 41,44. The control of the loading forces in the construction is quite easy. The drainage can be controlled by varying the pressure in the hoses 40a,40b, by adjusting the levels of negative pressure, and by means of the wire 10,20 tensions when the twin-wire zone is curved. 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 of the invention defined in said claims and differ from what has been stated above by way of example only.

Claims: Twin-wire web former in a paper machine, comprising a carrying wire (20) and a covering wire (10), which together form a twin-wire forming zone, in which a forming unit (50) is fitted, which comprises a forming board (30) and a drainage box (51) placed one opposite the other, which drainage box comprises a number of ribs (53), water being drained out of the web (W) through the spaces between said ribs to a significant extent by the effect of negative pressure into the drainage box (51), in which forming board (30), placed facing the drainage-box (51), there are a number of transverse loading ribs (33) laced at a considerable distance from each other in the machine direction, and in the area of said forming unit (50), the dewatering can be arranged as taking place both through the covering wire (10) and through the carrying wire (20), also towards the forming board (30) through the open spaces (36) placed between its loading ribs (33), characterized in that successive loading ribs (33) are interconnected in pairs by intermediate parts (31), which, together with the loading ribs (33) attached to them, form ribbed shoes (30A,30B,30C), which can be loaded by means of loading hoses (40a,40b) to produce a dewatering pressure in the web (W) placed between the wires (10,20), while the ribs (53) on said drainage box (51) operate as back-up members for the loading forces in a manner known in itself. Web former as claimed in claim 1, characterized in that the frame part of the ribbed shoe in the forming board (30) is provided with two transverse frame beams (30a,30b), on which a pair of said loading hoses (40a,40b) is fitted, by whose means the ribbed shoe can be loaded, preferably with an adjustable loading force, and that the pair of said loading ribs (33) is linked from the side of one of the loading ribs (33) at the proximity of the plane of the wire (20) running at its proximity so that, from this linkage, a minimal torque arises that turns the ribbed shoe (31,33). Web former as claimed in claim 1 or 2, characterized in that, between the loading ribs (33) in the ribbed shoe (31,33), a suction box (41) is provided, which is connected to a flexible suction hose (42) or an equivalent suction duct, and that, by means of the negative pressure passed to the suction box (41), the drainage towards the forming board (30) is promoted. Web former as claimed in any of the claims 1 to 3, characterized in that the spaces between subsequent ribbed shoes (30A,30B,30C) are resiliently (45) connected with suction boxes (44), which are connected to a flexible suction duct (42), by whose means, into said intermediate spaces, a negative pressure can be introduced which promotes the drainage of water towards the forming board (30). Web former as claimed in any of the claims 1 to 4, characterized in that the loading ribs (33) of the ribbed shoe (31,33) are placed so that the loading ribs (53,53b) of the drainage box (51) placed facing said ribbed shoe are located in the middle between the loading ribs (33) of the forming board (30). Web former as claimed in any of the claims 1 to 5, characterized in that the loading ribs (53a,53c) of said drainage box (51) are placed partly or fully opposite to the loading ribs (33) of the forming board (30). Web former as claimed in any of the claims 1 to 6, characterized in that, in the area of the forming unit (50), the run of the twin-wire zone is substantially straight. Web former as claimed in any of the claims 1 to 6, characterized in that, in the area of the forming unit (50), the run of the twin-wire zone is curved so that the centre of curvature is either at the side of the drainage box (51) of the forming unit (50) or at the side of the opposite forming board (30). Web former as claimed in any of the claims 1 to 8, characterized in that the former is a so-called gap former, which comprises a first forming roll (21) and, after it, one or two forming units (50;50A,50B), which are followed by a second forming roll (24) or by an equivalent stationary forming member. Web former as claimed in any of the claims 1 to 8, characterized in that the former is a so-called hybrid former, which comprises a single-wire initial forming zone (10a), onto which the pulp suspension jet is fed out of the discharge opening of the headbox and which is followed by a twin-wire zone, in which there are several forming units (50;50A,50B).

Inventors: JAAKKOLA JYRKI, JAAKKOLA, JYRKI
Applicants: VALMET PAPER MACHINERY INC, VALMET PAPER MACHINERY INC.
Application year: 1992
Main classification: D21F9
IPC codes: D21F1, D21F9