Gap former in a paper machine

Abstract: A gap former in a paper machine, comprising a slower-wire loop (10) ad an upper-wire loop (20), which together define a substantially horizontal twin-wire forming zone, which comprises the following web forming zones (a), (b) ad (c): (a) In the area of the forming gap (G), a first suction zone (11a) placed inside the first forming roll (11), and immediately thereafter a second suction zone (11b). The suction zones (11a,11b) are placed in the first upper quarter of the first forming roll (11) at the side of the forming gap (G). The levels of negative pressure (P₁,P₂) in the suction zones are adjustable. (b) A forming shoe (12), which is provided with a open guide deck (26), the interior space in said shoe being divided into at least two subsequent suction chambers (12a,12b,12c), the levels of negative pressure (P a ,P b ,P c ) effective In said suction cham­ bers being arranged adjustable, preferably sepa­ rately adjustable. (c) Web-formation ad/or guide members (13a, 13b, 17,25) arranged after said forming shoe (12), said members guiding the twin-wire zone downwards.

Description: The invention concerns a gap former in a paper machine, comprising a lower-wire loop and an upper-wire loop, which together define a substantially horizontal twin-wire forming zone, which starts from the forming gap, which is confined in a space between the upper and the lower wire, said space becoming narrower as wedge-shaped, and in the area of which said forming gap there is, inside the lower-wire loop, a first forming roll, after said roll a forming shoe, after which, inside the lower-wire loop, there are a number of dewatering members, after which the web is arranged to follow along with the lower wire, from which it is detached at the pick-up point, being transferred onto a pick-up fabric or equivalent. In respect of the prior art most closely related to the present invention, reference is made to the US Patents Nos. 3,438,854, 3,996,098, and 4,056,433 as well as to the Published FI Patent Applications Nos. 61,217 and 77,702. Moreover, the prior art related to the present invention includes the former construction marketed by the applicant under the trade mark "SPEED FORMER HHS" (Appendix 1) and the former marketed by Messrs. Escher Wyss AG under the trade mark "TWIN FORMER G", the latter one being described, e.g., in the journal Wochenblatt für Panierfabrikation 11/12, 1989, p. 482, section 3.5.1 (Appendix 2). The present invention expressly concerns a gap former in which the twin-wire forming zone is substantially horizontal, which is, in this connection, understood as meaning that the length of the twin-wire forming zone in the horizontal direction is substantially larger, preferably at least twice as large as the difference in height between the forming gap and the final end of the twin-wire zone. In formers meant for new paper machines, the forming gap and the final end of the twin-wire zone may advantageously be substantially at the same level, whereas in formers in accordance with the invention intended for modernizations the forming gap may be placed at a level considerably higher than the final end of the twin-wire zone, which preferably coincides with the upper plane of the fourdrinier wire to be modernized. A general object of the present invention is further development of the gap formers described in the papers cited above, in particular of the applicant's said "SPEED FORMER HHS" construction. A general object of the invention is to provide a former whose operation can be made such, by means of dimensioning and regulation of various operational parameters, that even with high production rates and with different qualities produced, the operation of the former can be optimized so that the formation and retention of the paper produced as well as the symmetry of its opposite faces and of the whole structure can be made excellent. It is a particular object of the invention to provide a former in which the former roll placed in its gap area is opened upwards, so that there is no risk of the suction chamber placed inside the former roll being filled with water, whereby it is possible to use a suction chamber of full width. An object of the invention is to provide a former whose dewatering capacity can be increased further while, nevertheless, retaining good formation, retention and symmetry of the web. It is a further object of the invention to provide a former which can be applied both to new machines and to modernizations of existing fourdrinier wire parts, such as the former marketed by the applicant under the trade mark "SYM-FORMER R", In respect of which reference is made to the applicant's FI Patent No. 75,375 (corresponding US Pats. Nos. 4,614,566 and 4,744,866). In view of achieving the objectives stated above and those that will come out later, the invention is mainly characterized in that the twin-wire forming zone comprises a combination of the following web forming zones (a), (b) and (c) placed one after the other in the sequence stated below: (a) in the area of the forming gap or immediately after it, a first suction zone placed inside the first forming roll, and immediately thereafter a second suction zone placed inside the same forming roll, said suction zones being placed in the first upper quarter of the first forming roll at the side of the forming gap, and the levels of negative pressure in said suction zones being adjustable, preferably separately; (b) a forming shoe, which curves the twin-wire forming zone downwards at least at its rear end and which is provided with an open guide deck, the interior space in said shoe being divided into at least two subsequent suction chambers, the levels of negative pressure effective in said suction chambers being arranged adjustable, preferably separately adjustable; and (c) web-formation and/or guide members arranged after said forming shoe, said members guiding the twin-wire zone downwards. The former in accordance with the invention is adjustable in a very versatile way, so that the dewatering and formation process is all the time efficiently under control. According to the invention, as the first former roll is placed inside the lower-wire loop and as its suction zones are placed in the upper quarter of the forming roll at the side of the gap, it is achieved that the negative pressure prevailing in the suction zones densifies the lower side of the web that is being formed, in contrast with the above "SPEED FORMER HHS" former. Said lower side of the web is typically less dense than its upper side. Thus, owing to the invention, a reduced unequalsidedness of the web is achieved. The former in accordance with the invention is suitable both for new formers and particularly well also for modernizations of fourdrinier wire parts, whereby it is possible to improve the quality and the rate of production of paper produced by means of fourdrinier wires substantially at relatively low investment costs. Further embodiments of the invention are claimed in claims 2 to 11. 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 by no means strictly confined to the details of said embodiments. Figure 1 is a schematical side view of a version of the invention that is intended for use in new paper machines. Figure 2 shows an embodiment of the invention for use in modernizations of fourdrinier wire parts. Figure 3 shows the initial part of a twin-wire forming zone in accordance with the invention on an enlarged scale. The twin-wire former of a paper machine shown in Figs. 1 and 2 comprises a lower-wire loop 10 and an upper-wire loop 20, which together form a twin-wire web-forming zone. The twin-wire zone starts at the forming gap G and ends at the suction box 13b placed inside the lower-wire loop 10, after which the web W is arranged to follow along with the lower wire 10. Inside the lower-wire loop 10, there is a forming roll 11 in the area of the gap G, and it is followed by a forming shoe 12, suction boxes 13a and 13b, a couch roll 14, and a drive roll 15. On the downwards inclined run of the lower wire 10 between the rolls 14,15 the web W is transferred on the suction zone 18a of the pick-up roll 18 onto the pick-up felt 19, which carries the web W to the press section (not shown) of the paper machine. The return run of the lower wire 10 is guided by guide rolls 16. Inside the upper-wire loop 20, there is a guide roll 21 provided with a smooth solid face 21' in the area of the gap G, and it is followed by the twin-wire forming zone between the wires 10 and 20. The upper wire 20 is separated from the lower wire 10 and from the web W by means of a suction box 13a. The guide roll 23 turns the run of the upper wire 20. The return run of the upper wire 20 is guided by guide rolls 24. Inside the upper-wire loop, there is a water collecting trough 22, which collects the waters drained out of the web through the upper wire 20 especially in the areas of the roll 11 and the forming shoe 12. Through the slice 31 in the headbox 30, the pulp suspension jet J is fed into the wedge-shaped forming gap G between the wires 10 and 20, the bottom of the gap being placed in the area of the first suction zone 10a of the forming roll 11. The forming roll 11 is provided with a perforated mantle 11', in whose interior there are two suction zones 11a and 11b placed one after the other. The suction zones are placed in the upper quarter of the roll 11 at the side of the gap G. It is an essential feature of the invention that the suction zones 11a and 11b in the former roll 11 are opened downwards, so that the water is removed in the sectors a and b of the suction zones 11a,11b with the aid of the wire tension, negative pressure, and gravity downwards, so that there is no risk of the suction chambers placed in the suction zones 11a and 11b being filled. As is shown in Fig. 3, the twin-wire forming zone starts at, or at the proximity of, the foremost sealing rib 37 of the first suction zone 11a, the bottom of the forming gap G being, at the same time, placed at said rib. The joint run of the wires 10 and 20 is separated from the mantle 11' of the forming roll at, or at the proximity of, the rearmost sealing rib 37 of the second suction zone 11b, whereupon a short straight run of the wires 10,20 follows. The diameter 2R o of the roll 11 is, as a rule, within the range of 2R o = 1400...1800 mm. The area of effect of the forming shoe 12 provided with a curved ribbed deck 26 starts thereafter. As is shown in Fig. 3, the forming shoe is divided in three subsequent suction chambers 12a, 12b and 12c, which are defined by partition walls 36, by the outer mantle 29, and by plane ends. The curved ribbed deck 26 of the forming shoe consists of ribs 28, whose longitudinal direction is perpendicular to the direction of running of the wires, and of gaps 27 between said ribs, which gaps 27 are opened into said vacuum chambers 12a,12b and 12c in the shoe. The curve radius of the rib deck 26 placed facing the first chamber 12a is R a , and its length is L a , the curve radius of the deck of the second chamber 12b is R b and its length is L b , and the curve radius of the rib deck of the third chamber 12c is R c and its length is L c . Most appropriately R a ≧ R b ≧ Rc > R o and, in a corresponding way, L a ≧ L b ≧ L c . Advantageously, R o is within the range of R o = 0.7...0.9 m, R a is within the range of R a = 3.0...5.0 m, R b is within the range of R b = 3.0...4.5 m, and R c is within the range of R c = 2.5...3.5 m. In a corresponding way, L a is within the range of L a = 0.4...0.8 m, L b is within the range of L b = 0.35...0.75 m, and L c is within the range of L c = 0.30...0.70 m. The curve radii R a ,R b ,R c of the guide deck 26 of the forming shoe 12 have an effect on the web formation thereby that the curve radius R of the deck 26 determines the change (a) in angle taking place in the area of the shoe 12 in the direction of running of the wires 10,20 at the tip of the rib of each deck 26. Said change in angle produces in the web W a force vector Fc = 2·T·sin a , wherein T = tensioning stress of the upper wire 20. As is well known, centrifugal forces and the negative pressures in the suction chambers of the shoe also act upon said dewatering pressure p. It is an essential feature of the combination of the invention that the twin-wire forming zone rises upwards starting from the gap G, and the summit point K of the twin-wire forming zone is placed at the ribbed deck 26 of the forming shoe, preferably in its middle area and at the middle chamber 12b. In the former shown in Fig. 1, which is intended for new paper machines, the twin-wire forming part is curved downwards after its summit point K placed at the forming shoe 2, and on this downwards inclined (angle c) and substantially straight run there are first three subsequent suction flatboxes 13a and thereupon a suction flatbox 13b at the leading roll 23 of the upper wire 20. Said angle c is, as a rule, within the range of c = 10...40°. The former shown in Fig. 2, which is intended for modernizations of existing fourdrinier wire parts, differs from that shown in Fig. 1 in the respect that after the forming shoe 12, inside the lower-wire loop 10, there is a leading roll 17, in whose area the twin-wire zone is curved steeply downwards onto the leading roll 25 placed inside the upper-wire loop 20, which roll guides the twin-wire zone to the horizontal plane T-T, which is at the same time the original operation plane of the fourdrinier wire to be modernized. The suction flatbox 13b ensures that the web W follows along with the plane part 10T of the lower wire, whose length is denoted with L o . The existing frame parts, dewatering equipment and rolls 14,15,16 placed after the suction box 13b at the fourdrinier wire can, as a rule, be retained as such. The headbox 30 can be placed at least partly in the place of the old headbox and usually on support of its stand constructions. The summit point K of the initial part of the twin-wire zone is placed by the dimension H higher than the plane T-T. Said difference in height H is, as a rule, within the range of H = 1...3 m, preferably within the range of H = 1.5...2.5 m, and the length of the plane part 10T of the wire 10 is, as a rule, within the range of L o = 4...8 m. The summit points of the forming roll 11 and of the leading roll 17 are preferably substantially on the same horizontal plane. An essential feature of the operation of the former in accordance with the invention is its versatile adjustabillty, attempts having been made to illustrate said adjustabillty in Fig. 3. The perforated mantle 11' of the forming roll 11 is quite open, and its openness, i.e. the percentage R of holes, is preferably R = 70...95 %. The adjustability is contributed to by the fact that immediately after the gap G at least two, sometimes even three, subsequent suction zones 11a,11b are used, whose levels of negative pressure are separately adjustable, which is illustrated in Fig. 3 by separate connecting ducts 32a,32b of negative pressure, which communicate with the suction pump 41 via pressure regulation valves 33a and 33b. In this way it is possible to regulate the negative pressures p a and p b effective in the chambers 11a and 11b independently from each other. Said levels of negative pressure are preferably within the ranges of p a = 0...20 kPa and p b = 0...30 kPa. The magnitudes of the sectors a and b of the suction zones 11a,11b are preferably chosen so that a > b and that a and b are, as a rule, within the ranges of a = 20...60° and b = 10...20°. The versatile adjustability of the former is partly also contributed to by the fact that, behind the ribbed deck 26 of the forming shoe 12, vacuum chambers 12a,12b and 12c are effective, whose negative pressures p a ,p b and p c of different levels are also adjustable. This adjustment is illustrated in Fig. 3 by the suction ducts 34a,34b,34c, which are connected to the pump 42 of negative pressure through regulation valves 35a,35b and 35c so that the negative pressures p a ,p b and p c can be regulated independently from each other. Said regulation is illustrated by the regulation unit 40, to which, e.g., measurement results from the process computer of the paper machine concerning the operation of the former are fed. By, at the beginning, dimensioning the magnitudes of the sectors a and b of the suction zones 11a and 11b, the magnitudes of the curve radii R a ,R b and R c and of the lengths L a ,L b and L c of the forming shoe 12 in a purposeful way and by, during operation of the former, regulating the pressure levels p₁ and p₂ as well as p a ,p b and p c by means of the control system 40,43, the operation of the former with the web speeds that occur and with the paper qualities and pulp qualities produced can be optimized so that the final result is an optimal web formation, retention, and symmetry of the opposite faces and of the whole structure of the web. 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 the details given above for the sake of example only.

Claims: Gap former in a paper machine, comprising a lower-wire loop (10) and an upper-wire loop (20), which together define a substantially horizontal twin-wire forming zone, which starts from the forming gap (G), which is confined in a space between the upper and the lower wire (10,20), said space becoming narrower as wedge-shaped, and in the area of which said forming gap (G) there is, inside the lower-wire loop (10), a first forming roll (11), after said roll a forming shoe (12), after which, inside the lower-wire loop (10), there are a number of dewatering members (13a,13b), after which the web (W) is arranged to follow along with the lower wire (10), from which it is detached at the pick-up point (P), being transferred onto a pick-up fabric (19) or equivalent, characterized in that the twin-wire forming zone comprises a combination of the following web forming zones (a), (b) and (c) placed one after the other in the sequence stated below: (a) in the area of the forming gap (G) or immediately after it, a first suction zone (11a) placed inside the first forming roll (11), and immediately thereafter a second suction zone (11b) placed inside the same forming roll (11), said suction zones (11a,11b) being placed in the first upper quarter of the first forming roll (11) at the side of the forming gap (G), and the levels of negative pressure (p₁ ,p₂) in said suction zones being adjustable, preferably separately; (b) a forming shoe (12), which curves the twin-wire forming zone downwards at least at its rear end and which is provided with an open guide deck (26), the interior space in said shoe being divided into at least two subsequent suction chambers (12a,12b,12c), the levels of negative pressure (p a ,p b ,p c ) effective in said suction chambers being arranged adjustable, preferably separately adjustable; and (c) web-formation and/or guide members (13a,13b,17,25) arranged after said forming shoe (12), said members guiding the twin-wire zone downwards. Gap former as claimed in claim 1, characterized in that the highest summit point (K) of the twin-wire forming zone is arranged on the forming shoe (12), preferably in its middle area. Gap former as claimed in claim 1 or 2, characterized in that at, or in the area of, the latter sealing rib (37) of the latter suction zone (11b) in the first forming roll (11), the twin-wire run is arranged as upwards inclined at a small angle to be directed to the front edge of the forming shoe (12). Gap former as claimed in any of the claims 1 to 3, characterized in that the forming shoe (12) includes at least two, preferably three, suction chambers (12a, 12b, 12c) placed directly one after the other and each of them separately connected to a negative pressure (p a ,p b ,p c ) of adjustable level, so that the level of negative pressure that acts through the holes or slots (27) in the guide deck of the forming shoe (12) upon the web that is being formed is adjustable. Gap former as claimed in any of the claims 1 to 4, characterized in that the forming gap (G) is defined from above by the upper wire (10), which runs over a guide roll (21) provided with a smooth solid face (21') and which runs, in the area of the gap (G), as a straight run onto, or to the area of, the foremost sealing rib (37) of the first suction zone (11a) in the first forming roll (11). Gap former as claimed in any of the claims 1 to 5, characterized in that the sector (a) of the foremost forming zone (11a) in the first forming roll (11) is substantially larger than the sector (b) of the directly following second suction zone (11b), said sectors (a,b) being preferably within the ranges of a = 20...60°, b = 10...20°. Gap former as claimed in any of the claims 1 to 6, characterized in that the forming shoe (12) includes three subsequent suction chambers (12a,12b,12c), whose lengths are within the ranges of, the length of the first chamber L a = 0.4...0.8 m, the length of the second chamber L b = 0.35... 0.75 m, and the length of the third chamber L c = 0.30...0.70 m, and that the curve radii of the ribbed guide decks (26) or equivalent placed facing said suction chambers (12a,12b,12c) are within the ranges of, the radius of the first deck R a = 3.0...5.0 m, of the second deck R b = 3.0...4.5 m, and the radius of the third deck R c = 2.5...3.5 m. Gap former as claimed in any of the claims 1 to 7, characterized in that, after said forming shoe (12), the run of the twin-wire zone is directed as a substantially straight run as downwards inclined at an angle (c), which is c = 10...40°, and on which straight run, inside the lower-wire loop, there are dewatering members, preferably suction flatboxes (13a, 13b), of which the latter one (13b) ensures that the web (W) follows along with the lower wire (10) when the upper wire (20), guided by its leading roll (23), is separated from the web (Fig. 1). Gap former as claimed in any of the claims 1 to 7, intended in particular for modernizations of fourdrinier wire parts, characterized in that after said forming shoe (12), inside the lower-wire loop (10), there is a leading roll (17), which guides the twin-wire run and which turns the joint run of the wires (10,20) steeply downwards, whereupon there is a leading roll (25) placed inside the upper-wire loop (20), said roll (25) guiding the run of the wires (10,20) to the plane (T-T) of the upper run of the existing fourdrinier wire part or to the proximity of said plane (Fig. 2). Gap former as claimed in claim 9, characterized in that the summit point of said leading roll (17), which is placed inside the lower-wire loop (10), is substantially at the same level as the summit point of the first forming roll (11) and that the difference in height (H) between the summit point (K) of the twin-wire zone and said plane (T-T) of the lower wire is H = 1.0...3.0 m, preferably H = 1.5...2.5 m (Fig. 2). Gap former as claimed in any of the claims 1 to 10, characterized in that the levels of negative pressure in each of the suction zones (11a,11b) in the first forming roll are arranged adjustable so that the level of negative pressure p₁ in the former suction zone (11a) is within the range of p₁ =0...20 kPa and the level of negative pressure p₂ in the latter suction zone (11b) is within the range of p₂ = 0...30 kPa, and that the levels of negative pressure p a ,p b ,p c in the various chambers (12a,12b,12c) in said forming shoe (12) are arranged adjustable in the ranges, the negative pressure in the first chamber (12a) within the range of p a = 0...5 kPa, that in the second chamber (12b) within the range of p b = 0...10 kPa, and/or the negative pressure in the last chamber (12c) within the range of p c = 0...15 kPa.

Inventors: KOIVURANTA MAURI, KOIVURANTA, MAURI
Applicants: VALMET PAPER MACHINERY INC, VALMET PAPER MACHINERY INC.
Application year: 1990
Main classification: D21F9
IPC codes: D21F9