Method and device for dewatering of a paper web by pressing

Abstract: Method and device in the manufacture of paper or board for dewatering of the paper web (W) that is being manufactured. The paper web (W) is transferred from the forming wire (10) onto the wire (80) in the drying section while constantly on support of a fabric that receives water, a transfer fabric, or of any other, corresponding transfer surface (105′) as a closed draw, at a particularly high speed, which is, as a rule, higher than about 25...30 m/s. Dewatering of the paper web (W) is carried out by means of at least two subsequent press nips (N,NP), of which nips at least one press nip is a so-called extended-nip zone, whose length (z) in the machine direction is larger than z > about 100 mm. The extended-nip zone (NP) is formed in connection with a mobile flexible press-band loop (201; 301). The distribution of the compression pressure employed within said extended-nip press zone (NP) is regulated and/or selected both in the transverse direction of the web (W) and in the machine direction so as to set or to control the different profiles of properties of the web.

Description: The invention concerns a method for dewatering a paper or board web according to the preamble of claim 1, which is based on DE-A-3 515 575, and a paper making machine according to the preamble of claim 10, which is also based on DE-A-3 515 575. One of the most important quality requirements of all paper and board qualities is uniformity of the structure both microscopically and macroscopically. The structure of paper, in particular of printing paper, must also be symmetric. The good printing properties required from printing paper mean good smoothness, evenness and certain absorption properties at both faces. The properties of paper, in particular the symmetry of density, are affected considerably by the operation of the press section in a paper machine, which has also a decisive significance for the evenness of the transverse profiles and longitudinal profiles of the paper. Increased running speeds of paper machines provide new problems, which are mostly related to the running quality of the machine. At present, running speeds of up to about 1400 m/min. are employed. At these speeds, so-called closed press sections, which comprise a compact combination of press rolls fitted around a smooth-faced centre roll, usually operate satisfactorily. As examples of such press sections should be mentioned the applicant's "Sym-Press II" and "Sym-Press O" press sections (" " = trade marks). One item that requires development is the centre roll in the compact press sections and the material of said roll, which has commonly been rock, which, however, being a natural material, has certain drawbacks. Dewatering by means of pressing is energy-economically preferable to dewatering by evaporation. This is why attempts should be made to remove a maximum proportion of water out of a paper web by pressing in order that the proportion of water that must be removed by evaporation could be made as low as possible. The increased running speeds of paper machines, however, provide new, so far unsolved problems expressly in the dewatering taking place by pressing, because the press impulse cannot be increased sufficiently by the means known in prior art, above all because, at high speeds, the nip times remain unduly short and, on the other hand, the peak pressure of compression cannot be increased beyond a certain limit without destruction of the structure of the web. When running speeds of paper machines are increased, the problems of running quality of paper machines are also manifested with increased emphasis, because a watery web of low strength cannot withstand an excessively high and sudden impulse of compression pressure or the dynamic forces produced by high speeds, but web breaks and other disturbance in operation are produced with resulting standstills. With a modern printing paper machine, the cost of a break standstill is at present about 40,000 FIM per hour. Further drawbacks of the prior-art press sections include the requirement of suction energy of the suction rolls commonly employed in them as well as the noise problems arising from the suction rolls. Also, the suction rolls with their perforated mantles, interior suction boxes, and other suction systems are components that are expensive and require repeated servicing. Further problems which are manifested with more emphasis at high speeds of paper machines and for which, at least not for all of them, satisfactory solutions have not yet been found, include the quality problems related to the requirements of evenness of the longitudinal and transverse property profiles of the paper web. The evenness of the web that is produced also affects the running quality of the whole paper machine, and it is also an important quality factor of finished paper, which is emphasized in respect of copying and printing papers when the requirements on the speeds of copying and printing machines and on the uniformity of the printing result are increased. The property profiles of the paper that is produced in the machine direction are also affected significantly by oscillations of the press section, the transverse variations of properties by the transverse profiles of the nip pressures in the press nips, and with increasing running speeds of the machine these profile problems tend to be increased remarkably. Recently, running speeds even as high as about 40 m/s = 2400 m/min have been contemplated as running speeds of paper machines. The realization of such high speeds, in particular in wide machines, creates ever more serious problems to be solved, of which problems some of the most important ones are the running quality of the machine and adequate dewatering capacity at high speeds. In respect of the prior art most closely related to the invention, reference is made to the US Patents Nos. 4,483,745 (Beloit Corp.), 4,526,655 (Valmet Oy), 4,561,939 (Beloit Corp.) as well as to the published patent applications WO-85/00841 (J. M. Voith GmbH), DE-OS-3742848 (Sulzer-Escher Wyss GmbH), and to the FI Patent Applications 842114 (Valmet Oy), 842115 (Valmet Oy), and 850665 (Valmet Oy). Thus, the object of the present invention is to provide novel solutions for the problems discussed above so that the above drawbacks in prior art and the drawbacks that will come out later are substantially avoided. An object of the invention is to provide a method for dewatering of a paper web by pressing at high speeds, in particular at speeds of about 25...40 m/s, so that the adjustability of the press section is versatile, the properties of quality of the web produced can be kept high, and the web is not subjected to excessive dynamic forces that produce breaks. Further, by means of the invention, one of the aims is that the overall construction of the press section, in particular its frame construction, should be such that the replacement of press rolls and press fabrics can be accomplished quickly so as to minimize the standstill times. In view of achieving the objectives stated above and those that will come out later, the method of the invention has been given the features in the characterizing clause of claim 1. On the other hand, the press section in accordance with the invention has been given the features in the characterizing clause of claim 10. Dependent claims 2 to 9 and 11 to 18 define further features of the method according to claim 1 and of the paper making machine according to claim 10. In various embodiments the invention, the paper web is not passed through the press section on one press fabric, but, to guarantee an adequate dewatering capacity, an arrangement of fabrics is employed in which the web is transferred from the pick-up point on the first upper fabric through the first press zone, preferably an extended-nip zone, through which zone the first lower fabric runs, onto which the web is transferred after said nip zone, and from said first lower fabric the web is transferred onto the second upper fabric, which carries the web into the second nip zone, which consists of a roll nip or preferably of an extended-nip zone, after which the web is transferred onto the second lower fabric, which runs through said nip zone and carries the web on its upper face as a closed draw onto the drying wire or into the next nip zone. In the present invention and in its various embodiments, it has been successfully possible to combine certain component solutions in a novel and inventive way, some of which solutions are in themselves known in paper machine technology, so that the problems discussed above, which are of different natures, have been brought under control and been solved by means of a novel overall concept. The most important objective achieved by means of the invention is a satisfactory running quality of the paper machine even at speeds as high as about 30...40 m/s. This has been achieved because of a "linear" draw of the web and of a nip arrangement that provides sufficiently long nip times. Preferably, the closed draw in accordance with the invention has been accomplished so that one and the same fabric does not carry the web through the whole press zone, but in at least two subsequent press zones two pairs of press fabrics are employed, the web being transferred onto the first upper fabric at the pick-up point, and after the first nip zone the web is transferred from the first lower fabric onto the second upper fabric, and after the second nip zone on the second lower fabric onto the drying wire or into the next nip zone, whose lower fabric carries the web onto the drying wire as a closed draw. Thus, it has been possible to accomplish a sufficiently high dry solids content in the dewatering taking place by pressing, and the running quality remains at a good level. The method and the paper making machine in accordance with the invention are intended for use above all with thin paper qualities, whose grammage is lower than 120 g/m 2 and with which a closed draw of the web is indispensable at the high web speeds meant in the invention. The invention also achieves a sufficiently cautious and gentle start of the dewatering, which is important because at high speeds the water contents in the web after the former also tend to be higher. According to the invention, when extended-nip presses accomplished by means of hose rolls or band rolls and provided with a number of different possibilities of setting or active regulation are employed, it is also possible to control the profiles of properties of the web both in the machine direction and in the transverse direction. In the most advantageous embodiment of the invention, a new extended-nip press, which has been developed by the applicant and is marketed by the applicant under the trade mark "Sym-Belt Press" and which is based on the use of a so-called hose roll, is utilized in a novel way. When fitted in its environment in accordance with the invention, said "Sym-Belt Press" provides several advantages of synergism, of which should be mentioned that said press produces practically no oscillations at all, for which reason it is well suitable also for very high speeds, said press permits keeping of the nip loads at a sufficiently low level in particular in the initial part of the press section and makes it possible to keep the nip times at a reasonable level even at very high speeds (30...40 m/s). A further aspect of the invention is the use of two press fabrics and their joint operation so that the web is transferred from the pick-up point onto the first upper fabric, after the first nip zone onto the first lower fabric, from the first lower fabric by means of a transfer-suction roll or equivalent onto the second upper fabric, and on it further after the second nip zone onto the second lower fabric, on which the web is transferred as a closed draw onto the drying wire or into the next press zone, in whose connection there is a pair of press and transfer fabrics similar to those described above. Further, said "Sym-Belt Press" provides entirely novel possibilities to control and to regulate the distribution of the nip pressures in the extended-nip zone both in the machine direction and in the transverse direction. Further advantages include low power consumption, elimination of difficulties of oil treatment, reduced wear of the mantle of the hose roll, and reasonable dry solids content of the web even at high speeds (30...40 m/s). In respect of the details of the constructions of said hose rolls, reference is made to the US Patent No. 4,584,059 as well as to the applicant's FI Patent No. 66,932 and FI Patent Applications Nos. 892517 and 892518. One possibility to accomplish the extended-nip zone employed in the invention is the press solution described in the applicant's FI Pat. Appl. No. 891380 ( corresponding to EP-A-0 389 458), in which the press band loop is relatively short and has a run guided by a press shoe and a leading roll or an equivalent guide member, and in which solution the ends of the band have been sealed in a novel way. Thus, there is no risk of oil splashes, and the distributions of the nip pressures both in the machine direction and in the transverse direction are adjustable. In the following, the invention will be described in detail with reference to the accompanying drawing. Figure 1 shows a press section provided with two subsequent "Sym-Belt Presses" (" " = applicant's trade mark), of which extended nips one press fabric is employed in the latter nip, embodiment which does not fall within the scope of the appended claims. Figure 2 shows such a variation of a press section as shown in Fig. 1 in which, in the second "Sym-Belt" extended nip, a lower press and transfer fabric is also employed, which contributes to the formation of the closed draw to the drying section, embodiment which does not fall within the scope of the appended claims. Figure 3 shows a press section in which two subsequent "Sym-Belt" extended nips are employed which operate in opposite directions, as compared with one another, embodiment which does not fall within the scope of the appended claims. Figure 4 shows a press section in which the first nip is a "Sym-Belt" extended nip and the second nip a two-fabric roll nip, embodiment which does not fall within the scope of the appended claims. Figure 5 shows a press section that has a high dewatering capacity and in which three subsequent press nips are employed, of which the first and the last nip are roll nips and the middle nip is a "Sym-Belt" extended nip, embodiment which does not fall within the scope of the appended claims. Figure 6 shows an embodiment of the invention that is intended for particularly high speeds and in which the first nip operates as a so-called wire press, which is followed by an extended nip, and the last nip is a roll nip. Figure 6A shows an alternative embodiment of a wire press. Figure 6B shows a second alternative embodiment of a wire press. Figure 6C shows a third alternative embodiment of a wire press. Figure 7 is an axonometric, partly sectional view of a hose roll of a "Sym-Belt Press" applicable in the invention. Figure 8 is an axonometric view of a press shoe that can be loaded and profiled in a versatile way and that is fitted inside the hose roll and employed in a press as shown in Fig. 7. Figure 9 shows an alternative construction that accomplishes an extended nip and that is provided with a closed glide belt loop. To begin with, the common features of construction of the press geometries as shown in Figs. 1 to 4 will be described. As is shown in Figs. 1 to 4, with a closed draw of the web in a paper or board machine, the press section comprises a first upper fabric 20 that receives water, onto which fabric the web W is transferred on the suction zone 21a of the pick-up roll 21 at the pick-up point P from the forming wire 10, whose return run starts from the wire drive roll 12. According to Figs. 1 to 4, the press has two subsequent press nips, which remove water from the web W efficiently and between which the web W has a fully closed draw so that it is constantly supported by a fabric. In Figs. 1, 2 and 3, both nips NP 1 and NP 2 are so-called extended nips, whose press zone is substantially longer than that in a normal sharp roll nip. The more detailed embodiments of the extended nips will be returned to later, mainly in connection with the descriptions related to Figs. 7, 8 and 9. In Fig. 4, the first nip NP 1 is an extended nip, and the second nip a sharp roll nip N 2 formed between hollow-faced rolls. In Figs. 2 to 6, all the nips may be provided with two press fabrics that receive water, so that the water is removed from them through both faces of the web W. It is also possible to use one or several transfer fabrics that do substantially not receive water, instead of said press fabric. In Figs. 1 to 4, the first upper fabric is guided by alignment, tensioning and guide rolls 22 and conditioned by conditioning devices 23. The first extended nip NP 1 includes a lower fabric 30 that receives water, being guided by alignment, tensioning and guide rolls 32 and conditioned by conditioning devices 33. The first extended nip NP 1 and so also the second extended nip NP 2 are accomplished in a "Sym-Belt Press" of the applicant, and the details of the construction of said press will be discussed later. In respect of its main features, the construction of the press is such that the extended nip NP 1 is composed of a flexible hose mantle and of a backup roll. Inside the hose mantle, there is a hydrostatically and/or hydrodynamically lubricated glide shoe 210, the hydraulic loading means fitted in connection with said shoe pressing the shoe 210 against the hollow-faced backup roll 35,55. The backup roll 35,55 is a hollow-faced press roll, for example the applicant's adjustable-crown "Sym-Z Roll" (" " = trade mark). According to Figs. 1 to 4, the press section includes a second upper fabric 40, onto which the web W is transferred as a closed draw by means of the suction zone 41a of the suction roll 41. After the first nip NP 1 , it is ensured that the web W follows the first lower fabric 30 by means of a suction box 36 or a corresponding foil arrangement. The second upper fabric 40 is guided by alignment, tensioning and guide rolls 42 and conditioned by conditioning devices 43. According to Fig. 1, the second extended nip NP 2 is also formed in connection with a hose roll 45, which is similar to the hose roll at the extended nip NP 1 . Tne lower press member at the nip NP 2 is a smooth-faced 105' press roll 105, in connection with whose lower sector a doctor 107 operates, which doctors the web W o going to broke handling and the leader band to the broke handling arrangement placed below (not shown). The smooth face 105' of the press roll 105 makes sure that, after the extended nip NP 2 , the web W follows the face 105' of the lower roll 105, from which it is detached by means of a transfer nip N s and is transferred on support of the drying wire 80, which is guided by the guide roll 81, to the drying section, of which the first heated drying cylinders 82 and leading cylinders 83 are shown in the figures. Single-wire draw is employed at least in the first cylinder group in the drying section. Fig. 2 differs from the press section shown in.Fig. 1 in the respect that the second extended nip NP 2 is a two-fabric nip and includes a lower fabric 50, which is guided by the tensioning, alignment and guide rolls 52 and conditioned by the conditioning devices 53. The extended nip NP 2 is formed between the upper hose roll 45, the press fabrics 40,50, and the lower, hollow-faced 155' press roll 155. After the extended nip NP 2 , it is ensured by means of a suction box 56 and/or by means of the surface properties of the fabric 50 that the web W follows the lower fabric 50. From the fabric 50, the web W is transferred as a closed draw onto the drying wire 80 as aided by the suction zone 81a of the suction roll 81, and further in the drying section 82,83, at least at the beginning, as a single-wire draw. Fig. 3 differs from the press section described above in relation to Fig. 2 in the respect that, in the second extended nip NP 2 , the hose roll 55 is placed underneath inside the loop of the press fabric 50, and the upper backup roll is a hollow-faced 145', adjustable-crown press roll 145, which is placed inside the loop of the second upper press fabric 40. The web W is transferred after the second nip NP 2 on the lower fabric 50 to the transfer point S, where the suction zone 81 a of the suction roll 81 is placed, by whose means the web W is transferred as a closed draw onto the drying wire 80. The embodiment of the invention shown in Fig. 4 differs from those shown in Figs. 2 and 3 in the respect that the second nip N 2 is a roll nip provided with two press fabrics 40,50 and formed between two hollow-faced 145' and 155' press rolls 145, 155, which are preferably adjustable-crown rolls. Figs. 5 and 6 show embodiments that are suitable for use at very high paper machine speeds, e.g. in a speed range of about 25...40 m/s, and, if necessary, also with relatively thick paper qualities. In Fig. 5, the first press nip N 1 is a so-called wire press, and therein the first fabric 120 is a relatively loose wire-like press fabric, onto which the web W, which has a very high water content and low strength as yet, is transferred on the pick-up zone 21a of the pick-up roll 21 at the pick-up point P from the forming wire 10 proper. The wire press nip N 1 is formed between two press rolls 125 and 35, both of which press rolls have very open hollow faces 125' and 35'. The lower fabric 30 is a press fabric 30 that receives water, onto which fabric the web W is transferred after the nip N 1 by the effect of the adhesion and surface properties of the suction box 36 and/or of the press fabric 30. From the fabric 30, the web W is guided onto the face of the second upper press fabric 40 by means of the suction zone 41a of the suction roll 41 placed inside the loop of said fabric 40. The second nip proper is an extended nip NP 2 , in which the upper press member consists of the hose roll 45 and the lower press member of the hollow-faced 55' press roll 55. Through the extended nip NP 2 , a press fabric 50 runs, on which the web W is transferred onto the third upper press fabric 60 on the suction zone 61a of the suction roll 61, and further into the third, sharp press nip N3, which is formed between hollow-faced 65' and 75' press rolls 65,75. After the nip N 3 , the web W follows the lower third fabric 70, which is guided by the tensioning, alignment and guide rolls 72 and conditioned by the conditioning devices 73. From the third lower fabric 70 the web W is detached at the transfer point S on the suction zone 81a of the suction roll 81, being transferred onto the drying wire 80, which carries the web W as a single-wire draw through the first drying group in the dryer. Fig. 6 shows an embodiment of the invention that differs from Fig. 5 in the respect that the first wire press nip N o is placed in connection with the wet wire 10 proper so that, before the wire 10 drive roll 12 and the pick-up point P, a suction roll 16 that is provided with an open face and with a suction zone 16a is placed inside the loop of the forming wire 10. Placed against the suction roll 16, there is a press roll 15, which operates inside the loop of the press wire 19, which is provided with a very open mantle face 15', and which is guided by the guide rolls 11. Through the wire press nip N o , a relatively open press fabric 19 runs, which receives water and is well permeable to water. In Fig. 6, the roll 15 may be a hollow-faced 15' steel roll or any other hollow-faced roll of low-weight construction, e.g. a roll with a composite mantle. In Fig. 6, the roll 16 is preferably a wire suction roll. The roll may, however, also be some other hollow-faced roll, and in such a case the wire suction roll is placed after the roll 16 separately. According to Fig. 6, the wire W, which has been pre-pressed in the wire press nip N o , is transferred at the pick-up point P onto the first upper press fabric 20 proper, which carries the web into the first extended nip NP 1 similar to that described above, and from said nip further, on the first lower fabric 30, with the aid of the suction zone 41a, onto the second upper fabric 40, which carries the web W into the two-fabric roll nip N 2 , from which the web W follows the second lower fabric 50 and is transferred on support of said fabric, at the transfer point S, from the suction zone 81a a onto the drying wire 80. In Fig. 6, the dry solids content k o of the web W before the wire press nip N o is of an order of 10 %, and in a press as shown in the figure, the dry solids content k 1 at the pick-up point P is of an order of 20 %. Fig. 6A shows such a variation of the wire press nip as shown in Fig. 6 in which a wet wire 10 and two press fabrics 19 and 19A that operate at opposite sides of the wire 10 are employed. The lower press fabric 19A at the nip N o is placed inside the wire 10 loop and is guided by the guide rolls 11A. According to Fig. 6B, in stead of a roll nip N o , an extended nip NP o is used as the wire nip. The construction of the nip NP o corresponds to those of the nips NP 1 and NP 2 , and it is formed between a hose roll 15A provided with a smooth or hollow-faced glide-belt mantle 201 and a suction roll 16. The pressure in the extended-nip zone NP o is, as a rule, in the range of 0.5...3 MPa. The length of the nip zone NP o is, as a rule, in the range of z = 100...300 mm. Fig. 6C shows a variation of the invention in which a belt-tensioned nip NH o is employed. Said nip NH o is formed between a wet wire 10 and a press fabric 19B that run over a suction roll 16. Inside the loop of the press fabric 19B, which is guided by the guide rolls 11B, a smooth-faced or hollow-faced tensioning belt 19C is arranged, which is guided by the guide rolls 11C. The tension T of the tensioning belt 19C produces a compression pressure P = 0.01...0.5 MPa in the press zone a o . The length of the press zone a o is, as a rule, in the range of 100...500 mm. In the other respects the construction is similar to that described above in relation to Figs. 6, 6A and 6B. As comes out from the above, the web W has a closed and supported draw as it moves from the pick-up point P on the forming wire 10 to the point S, at which it is transferred onto the drying wire 80 of the drying section and further as a supported single-wire draw at least through the first drying group. The fact that, after each nip, the web W follows the fabric that is supposed to carry it forwards is ensured by means of various suction or foil devices, covering angles of the press fabrics, and/or adhesion properties of the fabrics. Of these devices, the suction boxes 56 are shown in the figures. From Figs. 1 to 6, it can be concluded directly that the run of the web W to be pressed through the press section is highly linear without major curves. Owing to the linear path of the web, the dynamic forces applied to the web remain sufficiently low in view of minimizing the risk of breaks. In preferred embodiments, the magnitude of the angle a of change in the direction of the web W is in the range of a ≈ 10...30° and, as a rule, a < 15°. An exception from this may be formed by the pick-up roll 21 and its suction zone, at which locally even a high negative pressure may be employed, as well as, in Fig. 1, by the smooth-faced 105' lower press roll 105 and its turning sector b. Out of the reasons stated above, a press geometry as shown in Fig. 1 is not preferable when the maximum speed range (30...40 m/s) is employed. In the press constructions described above, the closed draw is accomplished so that it has been possible to minimize the dynamic forces applied to the web W and the risks of break. Thus, the running quality is satisfactory even at high speeds (30...40 m/s). Moreover, when extended nips NP 1 and NP 2 accomplished by means of hose rolls 200 or band rolls 300 have been employed in a press section in accordance with the invention, it has been possible to ensure a sufficient dewatering capacity and dry solids content even at high speeds without applying compression stages of excessively high peak pressures to the web W. It is a further important property of the extended-nip presses employed in the invention that in them practically no oscillations arise. It is a further important feature of the invention that the length z of the extended-nip zones NP 1 and NP 2 (z is, as a rule, in the range of z = 100...300 mm) in the machine direction is sufficiently large so that sufficiently long nip times are produced at said high speeds (25...40 m/s) as well as a sufficient compression impulse even though the peak pressure of the compression is kept reasonable and such that even a web with a very high water content (for example, k o ≈ 10 %) can be pressed without deterioration of the structure of the web. Said length z of the extended-nip zones NP 1 , and NP 2 in the machine direction is in the invention, as a rule, always z > 100...300 mm, preferably z = 200 mm. In such a case, in the extended nips NP 1 and NP 2 , it is possible to use maximal compression pressures, which are of the order of p = 3...9 MPa, preferably in the range of p = 5...8 MPa. In the roll nips N o ,N 1 , N 2 , N 3 it is, of course, possible also to use higher peak pressures, for example p max = 11 MPa. As a rule, however, a relatively low peak pressure must be used in the first roll nip, in which the water content of the web is high: p max ≈ 2.5...4 MPa. As comes out above from Figs. 1 to 6, the passage of the paper web W through the entire press section is highly "linear" and substantially horizontal. In the following, with reference to Figs. 7, 8 and 9, the hose roll 200 and band roll 300 employed in the embodiments of the extended nips NP used in the press section in accordance with the invention will be described. According to Fig. 7, the hose roll 200 comprises an elastic mantle 201, which is made, e.g., of fabric-reinforced polyurethane, so that the hose mantle 201 is made of rubber-like stretching material, whose maximum elongation is, e.g., about 20...40 %. The thickness of the hose mantle 201 is, e.g., about 2...5 mm. To the hose mantle 201, annular ends 202a and 202b are fixed permanently, the inner parts of said ends being fixed and sealed against revolving axle journals 207a and 207b, which are mounted on the frame parts 110 of the machine by means of fixed bearing supports. The hose roll 200 includes a stationary inner frame 205, around which the hose mantle 201 with its ends 202a,202b revolves on the bearings 206a and 206b. As is shown in Fig. 8, cylinder block sets 203, two sets side by side, are fitted in the inner frame 205. In the bores placed in the sets of cylinder blocks 203, hydraulic support members 206,207 of the glide shoe 210 operate, which members are, thus, placed in two rows, e.g., with a spacing of about 25 cm in the transverse direction one after the other. The two rows of the hydraulic support members 206,207 support a support plate 209, to which a glide shoe 210, e.g., of aluminium is attached, in whose area an extended nip zone NP is formed against a backup roll. The glide shoe 210 is provided with a smooth glide face 211, which operates as a press member against the smooth inner face of the hose mantle 201. The glide shoe 210 has a series of hydrostatic chambers 212 placed one after the other, which chambers contribute to the formation of a hydrostatic loading pressure and to oil lubrication of the glide face 211. Each of the subsequent cylinder blocks 203 communicates with a pipe connector 214, to which pipes 213 of loading medium pass so that a separately adjustable pressure can be passed into each individual block in the series of cylinder blocks 203. In this way, the pressure profile in an extended-nip zone NP can be regulated and controlled precisely and in a versatile way both in the machine direction and in the transverse direction. The pressure ratio p 2 /p 1 of the two different rows of support members 206,207 is, as a rule, chosen invariably as p 2 /p 1 = 1.5...2, whereas the pressure passed into each block is freely adjustable within certain limits. An example of the distribution of the nip pressure in an extended-nip zone NP is such a distribution in the machine direction in which the nip pressure (the pressure applied to the web W) at the front edge of the shoe 210 rises, owing to the hydrodynamically generated pressure, to about 40 bars, whereupon the pressure remains at this value as invariable, and in the trailing area of the shoe there is still an increase in the pressure, while the peak pressure is about 70 bars, from which value the pressure goes abruptly to zero at the trailing edge of the shoe 210. As was stated, said distribution of pressure can be varied so as to obtain an optimal pressing result. In any case, the compression pressure at the hose roll 200 and the distribution of said pressure in the machine direction can be arranged such that the start of the dewatering, while the dry solids content of the web W is still relatively low, can be carried out so gently that the fibre structure of the web W is not deteriorated. In Fig. 7, a regulation system related to the invention is sketched, by whose means the pressure profiles of the extended nip NP in the transverse direction and in the machine direction can be controlled. The regulation system is illustrated by the block 250, from which a series of regulation signals c 1 is given which regulate the hydraulic pressures fed through the pipes 213. To the regulation system 250, a feedback signal is received from separate wirings 214, which is illustrated by the series of signals c 2 . Further, the system 250 communicates with a measurement arrangement 260, by whose means the different profiles of the paper web W produced, such as moisture or thickness profiles, are measured, and this provides a series of feedback signals c 3 for the regulation system 250, which produces the series of regulation signals c 1 . The hose roll 200 is oil-tight, and the interior of the hose 201 can be arranged as slightly pressurized. From the glide faces 211 of the glide shoes 210, a slight leakage of oil takes place, which oil is collected from inside the hose mantle 201 and passed through the pipe 215 back to the oil circulation. The hose roll 200 shown in Figs. 7 and 8 is preferably mounted on fixed bearing supports, in which case the extended nip NP must be opened by means of a movement of the backup roll. This is necessary, because a play of, e.g., about 40 mm for movement of the glide shoes 210 of the hose roll is not sufficient for opening the nip NP sufficiently, e.g., for replacement of the fabrics. Fig. 9 shows an embodiment of a band roll 300. Therein a band 301 loop is used that is longer than the circular hose mantle 201. Said band 301 is guided from inside and the extended nip NP is loaded by a hydrostatically and hydrodynamically loaded glide shoe 310, which is fitted inside the band 301 loop and which has a hydrostatically loadable series of pressure fluid chambers 312 in the area of the extended nip NP. Inside the band loop 301, a beam 305 is fitted, which is provided with a series of hydraulic loading members 306 and 307, by whose means the glide shoe 310 can be loaded in a controlled way. The band loop 301 is guided by a leading roll 311, in whose connection a spreader roll 312 is operative. Both ends of the band loop 301 are closed by means of end pieces so as to prevent oil leakages and splashes, of which end pieces one piece 312a is shown in Fig. 9. The more detailed embodiment of the band roll shown in Fig. 9 comes out, e.g., from the applicant's FI Pat. Appl. No. 891380 (corresp. EP-A-0 389 458). The backup roll used in an extended nip NP as shown in Fig. 9 is an adjustable-crown roll 160, e.g. an adjustable-crown roll marketed by the applicant under the trade mark "Sym-Z Roll", which forms an extended nip NP by means of its sector C with the band roll 300. A corresponding roll can be used together with the hose roll 200. The roll 160 has a cylinder mantle 161, against whose smooth inner face 162 a series of glide shoes 165 operates, which is provided with hydraulic lubrication and loading chambers 166. The series of shoes 165 is loaded by means of a series of hydraulic actuators 164. If the backup roll 160 is employed together with the fabric 60 as a member that receives water, the outer face of the mantle 160 is provided with a hollow face. On the other hand, if the principal purpose of the roll 160 is to heat the web 60, e.g., by means of induction heating devices 170, a smooth mantle face is employed on the roll. According to Fig. 9, in connection with the mantle 161 of the roil 160, a heating device is provided, e.g. an inductive heating device 170, by whose means the temperature profile of the roll mantle, and thereby the profile and the dewatering capacity of the extended nip, can be affected. The roll 160 can also be used so that it has a smooth outer face and that by its means the web W is pressed directly, in which case there is no fabric 60 in between, and in this way the web W can be heated directly, thereby affecting the viscosity of the water present in the web and the elastic properties of the web W, thus promoting the dewatering and the transverse profile of the dry solids content. The dry solids content k out of the web as it departs from the press section in accordance with the invention is, as a rule, in the range k out = 35...65, preferably in the range k out = 40...55. 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 which have been stated by way of example only.

Claims: Method for dewatering a paper or board web (W) in a paper machine, the paper web (W) having been drained in the former of the paper machine, wherein the dewatering is effected by passing the paper web (W) on water receiving fabrics (20,30,40,50) through a press section having at least two subsequent dewatering press nips (NP1, N2) so that, by the effect of the compression pressure, water is transferred out of the fibre mesh of the paper web (W) into the spaces in said fabrics as well as into the spaces in hollow faces of mibile dewatering members, such as press rolls (35, 145, 155), that apply pressure to the web (W), the paper web being subsequently transferred to the drying section of the paper machine, wherein the paper web (W) is transferred from the pick-up point (P) on the forming wire (10) of the former to the transfer point (S) to the drying wire (80) of the drying section in a closed draw, being constantly supported by a water receving fabric (20, 30, 40, 50) a transfer fabric or any other corresponding transfer surface, at least one of said two subsequent press nips being an extended press nip (NP1), the length of which extended press nip in the machine direction being larger than about 100 mm, characterized in that a draining pressing is carried out on the web (W) forming wire (10) in a pre-pressing zone (No, NHo, NPo) preceding said pick-up point (P), in which a relatively open water receiving fabric (19,19B) is placed against the forming wire (10), that the extended press nip (NP1) is formed by a hose roll (200) or a band roll (300) and an opposite press roll (35), that the distribution of the compression pressure employed within said extended press nip (NP1) is regulated and/or selected both in the transverse direction of the web (W) and in the machine direction so as to set or to control the different profiles of properties of the web, and that said transfer from the forming wire (10) to the drying wire (80) is carried out at a speed that is higher than about 25-30 m/s. Method as claimed in claim 1, characterized in that, in said pre-pressing zone, the relatively open press fabric (19) and the forming wire (10) are passed through a roll nip (No) optionally together with another, opposite press fabric (19A). Method as claimed in claim 1, characterized in that, in said pre-pressing zone, the relatively open press fabric (19B) and the forming wire (10) are passed through a belt-tensioned nip (NHo), in which the compression pressure is preferably in the range of 0,01-0.5 MPa and whose length is preferably in the range of 100-500 mm. Method as claimed in claim 1, characterized in that, in said pre-pressing zone, the relatively open press fabric (19) and the forming wire (10) are passed through an extended nip (NPo) in which a compression pressure of preferably 0,5-3 MPa is employed and whose length is preferably in the range of 100-300 mm. Method as claimed in any of the claims 1-4, characterized in that the paper web is transferred at the pick-up point (P) onto the lower face of a first upper water receiving fabrics (20) which is passed through the first press nip (NP1) in the press section together with a first lower fabric (30), that on the upper face of the first lower fabric (30) the web (W) is transferred to a suction roll (41) at which the web is transferred onto the lower face of second upper fabric (40), that the web is passed passed through the second, susequent press nip (N2) in the press section between the second upper fabric and a second lower receiving fabric (50), and that the web is transferred on support of the second lower fabric to said transfer point (S) on the drying wire (80), both press nips preferably being extended press nips. Method ad claimed in any of the claims 1-4, characterized in that the paper web is transferred at the pick-up point onto the lower face of a first water receiving fabric which is passed through the first press nip in the press section together with a first lower fabric, that on the upper face of the first lower fabric the web is transferred to a suction roll, at which the web is transferred onto the lower face of a second upper fabric, that the web is passed through a second press nip in the press section between the second upper fabric and a second lower water receiving fabric on whose support the web is transferred onto a third upper fabric, that the web is passed through a third press nip, preferably a roll nip, between the third upper fabric and a third lower fabric, and that the web is transferred on support of the third lower fabric to the transfer point on the drying wire. Method as claimed in any of the claims 1-6, characterized in that the maximum compression pressure used in the extended press nip is adjusted to the range pmax = 3-9 MPa, preferably pmax = 5-8 MPa, and that the distribution of pressure is fitted preferably such that in the initial part of the extended press nip the compression pressure is increased steeply, whereupon the compression pressure is kept substantially invariable, and in the rear end of the extended press nip the compression pressure is regulated such that it is higher than said invariable compression pressure. Method as claimed in any of the claims 1-7, characterized in that the dewatering pressing of the web (W) is started when the dry solids content ko of the web (W) is = 10%, and water is removed out of the web so that after the press section its dry solids content is kout = 35...65%, preferably kout = 40...55%. Method as claimed in any of the claims 1-8, characterized in that the web (W) is guided through the different pressing stages in the press section as a substantially straight run, such that the angle (a) of change in the direction of the web (W) as the web moves through the press nips and from one fabric onto the other is a < 30°, preferably a ≈15°. Paper making machine, in which a paper web (W) to be dewatered by pressing is passed from a former section to a press section and subsequently into a drying section, which press section comprises at least two subsequent press nip zones (NP1,N2), two water receiving press fabrics (20, 30) passing through at least the first one (NP1) of said press nip zones with the web (W) therebetween, the press-section comprising: press and transfer fabrics (20, 30, 40, 50) fitted in such a way that the paper web (W) to be dewatered has a closed draw from the pick-up point (P) on the forming wire (10) of the forming section to the transfer point (S) to the drying section, without free, unsupported draws, the web being constantly supported between said pick-up point (P) and said transfer (S) by a press fabric (20, 30, 40, 50), a transfer fabric or any other corresponding transfer surface, an arrangement of press fabrics (20, 30, 40, 50) and press rolls (25,35,145,155), which forms said at least two subsequent press nip zones, at least one of said two subsequent press nip zones being an extended press nip zone (NP1), the length of which extended press nip zone (NP1) in the machine direction being larger than 100 mm, characterized in that the extended press nip zone (NP1) is formed by a hose roll (200) or a band roll (300) and an opposite press roll (35), and that a pre-pressing zone preceding said pick-up point (P) is formed by passing the forming wire (10) and an opposite. open press fabric (19, 19B) with the web (W) therebetween through a press nip (No, NPo, NHo). Paper making machine as claimed in claim 10, characterized in that the press nip (No) of the pre-pressing zone is formed by a suction roll (16) and a hollow-faced roll (15). Paper making machine as claimed in claim 10, characterized in that the press nip of the pre-pressing zone is an extended nip (NPo). Paper making machine as claimed in claim 12, characterized in that said extended nip (NPo) is formed by a suction roll (16) and a hose roll (15A) provided with a hollow-faced slide belt mantle (201). Paper making machine as claimed in claim 10, characterized in that the press nip (NHo) of the pre-pressing zone is formed by a suction roll (16) and a tensioned belt (19C). Paper making machine as claimed in any of the claims 10 to 12, characterized in that the first press nip zone (NP1) in the press section ccmprises a first upper pick-up and press fabric (20) and a first lower press fabric (20), that the second press nip zone in the press section comprises a second upper press fabric (40), that a suction roll (41) is arranged between the two subsequent press nip zones for transferring the web (W) from the first lower fabric (30) to the second upper fabric (40), that the second press nip zone (N2) in the press section comprises a second lower press fabric (50), and that both press nip zones preferably are extended press nip zones. Paper making machine as claimed in any of the claims 10 to 12, characterized in that the press section comprises three subsequent press nip zones, of which is at least one is an extended press nip zone and the others are provided by roll nips. Paper making machine as claimed in any of the claims 10 to 16, characterized in that the extended press nip zone (NP1) is formed in connection with a hose roll (200) which comprises a thin, elastic hose of substantially circular section, to which hose (201) closed ends (202a, 202b) are fixed permanently, said roll (200) being provided with a stationary inner frame (205), to which an adjustable glide shoe (210) is attached, which is loaded in a controlled way by hydraulic loading members, preferably hydraulic loading members (206, 207) placed in two separate rows, so that the compression pressure in the extended press nip zone can be controlled both in the machine direction and in the transverse direction. Paper making machine as claimed in any of the claims 10 to 16, characterized in that said opposite press roll (160) in the extended press nip zone is preferably an adjustable-crown press roll, and that the extended press nip zone is formed in connection with a band roll (300) which is provided by a flexible band loop (301), a glide shoe (310) that forms the nip zone being placed inside the loop (301), said band loop (310) being guided by a leading roll (311) or an equivalent guide member provided in the band loop from the opposite side of the glide shoe (310), whereby in view of prevention of oil splashes, both ends of said band loop are closed.

Inventors: HIRSIMAEKI MARTTI, KARVINEN MIKKO, KIVIMAA JUHA, LAAPOTTI JORMA, PAJULA JUHANI, TASKINEN PEKKA, HIRSIMAEKI, MARTTI, KARVINEN, MIKKO, KIVIMAA, JUHA, LAAPOTTI, JORMA, PAJULA, JUHANI, TASKINEN, PEKKA, Hirsimäki, Martti
Applicants: VALMET CORP, VALMET CORPORATION
Application year: 1991
Main classification: D21F3
IPC codes: D21F3