Press section of a paper machine, which press section is provided with two separate press nips

Abstract: The invention concerns a press section of a paper machine, which press section comprises at least two press nips (NP 1 ,NP 2 ,NP 10 ,N 2 ,N 3 ) placed one after the other. Of these nips, at least the first nip is an extended nip (NP 1 ) or an at least to some extent extended roll nip (NP 10 ). The last nip (N 2 ;NP 2 ;N 3 ) in the press section is placed on a level higher (H) than the preceding nip (NP 1 ;NP 2 ;NP 10 ). The first nip (NP 1 ;NP 10 ) and/or, when more than two nips are employed, the nip (NP 2 ) that precedes the last nip (N 3 ) is/are provided with two press fabrics (16,17;27,28) that receive water. The paper web (W) is transferred on the lower fabric of said press fabrics onto the upper fabric (38;38A) of the last nip (N 2 ;NP 2 ;N 3 ). On the lower face of said fabric, the web (W) is transferred into the last nip (N 2 ; NP 2 ;N 3 ). After the web (W) transfer point, the upper fabric (38;38A) of the last nip (N 2 ;NP 2 ;N 3 ) has a relatively short (L 0 ) upwards inclined run. After this, the upper fabric (38;38B) is turned, being guided by the lower roll (40;31) of the last nip (N 2 ;NP 2 ;N 3 ), over a considerable sector (a 1 ). The last nip (N 2 ;NP 2 ;N 3 ) is placed after, or at the vicinity of, the topmost point of the lower roll (40;31) that forms said nip. The point of transfer (TR) of the web (W) onto the drying wire (45) of the first group (R 1 ) in the dryer section following after the press section is placed underneath the level of the last nip (N 2 ;NP 2 ;N 3 ).

Description: The invention concerns a paper machine according to the preamble of claim 1. Such a paper machine is known from EP-A-0 509 199. One of the most important quality requirements of all paper and board grades is uniformity of the structure both on the micro scale and on the macro scale. 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 of both faces. The properties of paper, in particular the symmetry of density, is affected to a considerable extent by the operation of the press section of the paper machine, which operation has also a decisive significance for the uniformity of the profiles of the paper in the cross direction and in the machine direction. Increased running speeds of paper machines create new problems to be solved, which problems are mostly related to the runnability of the machine. Currently, running speeds of up to about 1500 metres per minute 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 0™ press sections. From the point of view of energy economy, dewatering taking place by pressing is preferable to dewatering taking place by evaporation. This is why attempts should be made to remove a maximal amount of water out of the paper web by pressing in order that the proportion of water to be removed by evaporation could be made as little as possible. Increased running speeds of paper machines, however, create new problems expressly for the dewatering taking place by pressing, because the press impulses in roll presses cannot be increased sufficiently, above all because at high speeds the nip times remain inadequately short and, on the other hand, the peak pressure of pressing cannot be increased beyond a certain limit without destruction of the structure of the web. With increasing running speeds of paper machines, the problems of runnability of a paper machine are also manifested with further emphasis, because a web with a high water content and low strength does not withstand an excessively high and sudden impulse of compression pressure or the dynamic forces produced by high speeds and changes in direction, but web breaks and other disturbance of operation arise which result in standstills. A further drawback of the prior-art press sections is the need of suction energy of suction rolls, which are used commonly in said press sections, and the drawback of noise arising from the suction rolls. Further, the suction rolls, with their perforated mantles, inside suction boxes and other suction systems, are expensive components that require repeated servicing. With respect to the prior art related to the present invention, reference is made to the applicant's FI Patent Application 905798 (equivalent to EP publication 0487483 A1), in which a method is described which comprises a combination of the following steps: the paper web is transferred from the forming wire onto the wire in the dryer section while constantly on support of a fabric that receives water, a transfer fabric, or of any other, corresponding transfer face as a closed draw, preferably at a speed that is higher than about 25...30 m/s; dewatering of the paper web is carried out by means of at least two successive press nips, of which nips at least one press nip is a so-called extended-nip zone, whose length in the machine direction is larger than z > about 100 mm, and said extended-nip zone is formed in connection with a mobile flexible press-band loop; and the distribution of the compression pressure employed within said extended-nip press zone is regulated and/or selected both in the cross direction of the web and in the machine direction so as to set or to control the different profiles of properties of the web. It is a further essential feature of the method and the device of said FI Pat. Appl. 905798 that the paper web is not passed through the press section on one press fabric, but, in order 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 is a roll nip or preferably an extended-nip zone, after which the web is transferred onto the second lower fabric, which runs through said nip zone and which carries the web on its upper face, as a closed draw, onto the drying wire or into the next nip zone. The object of the present invention is further development of the prior art so that most of the drawbacks that have been mentioned above and that will be described later can be substantially avoided. An object of the present invention is to provide a press-section construction that is quite compact, especially in the machine direction. This is an important objective in particular in such modernizations of paper machines in which it is necessary to fit a new press section, which has a higher dewatering capacity and in which one or several extended nips are used, in the place occupied by an earlier press section consisting of roll nips (such as the applicant's SYM-PRESS II™). It is a further object of the invention to provide a press section in which, in the first nip, a relatively high press load can be employed, which contributes to an adequate dewatering capacity of the press section and to a sufficiently high dry solids content of the web. The latter factor is important, because an increased dry solids content also increases the strength of the web and thereby also secures an undisturbed and reliable transfer of the web through the press section after the first nip. It is a non-indispensable further object of the invention to provide a press section in which it is possible to employ steam boxes, infrared heaters or equivalent for heating the web, by whose means the dewatering can be intensified. With respect to the prior art closely related to the present invention, reference is made further to the applicant's non-public FI Patent Application 935501, filed on December 8, 1993 (equivalent to EP-A-0 657 579). In said patent application, a press section is described in which a combination of the following characteristics has been considered novel: the first nip in the press section is an extended-nip press through whose press zone two opposite press fabrics that receive water have been passed, so that in the first extended-nip press the dewatering takes place in two directions through both faces of the paper web; the upper press fabric in said extended-nip press is a pick-up fabric, which carries the paper web from the forming wire on its lower face; at least two roll nips in the press section have been formed in connection with a smooth-faced centre roll, which centre roll is fitted at a level substantially higher than the level of the extended-nip press, and of which roll nips, in the first roll nip the press fabric consists of said pick-up fabric, and the second roll nip has a press fabric of its own that receives water; and, after said first extended-nip press, the running direction of the paper web has been turned at an angle a, which has been chosen as a ≥ ~45°. As condensed it can be stated that the object of the present invention is further development of the prior art described above and to provide a compact and simple press section of a paper machine, which press section is provided with at least two separate presses and in which press section a closed draw of the web has been accomplished between the first and the second nip so that the runnability is improved and that a draw difference need not be stretched into the web. It is a further object of the invention to provide a press section in which the first extended-nip zone dewaters the web efficiently and in the second extended-nip zone, partly because of an elevated temperature of the web, efficient dewatering is also achieved. It is a further object of the invention to provide a press section in which, in the second nip, if necessary, it is favourably possible to act upon the smoothness values of the web, because the web tends to become coarse after the first extended nip. The compact and simple construction of the press section in accordance with the invention is associated with the object of providing a particularly low construction, in which the felt cycles are low and simple. It is a further object of the invention to provide a press section in which it is possible to arrange an advantageous removal of broke by the force of gravity into a pulper or onto a broke conveyor placed below the press section. It is a further object of the invention to provide such a concept suitable for modernizations of press sections as can be fitted in the place of an earlier press section even if, in the modernization, the machine speed and/or the dewatering capacity has/have been increased. In view of achieving the objectives stated above and those that will come out later, the invention is mainly characterized by the features defined in the characterizing portion of claim 1. In the invention, it has been successfully possible to provide a novel press section concept, by whose means it is possible to achieve both good quality properties of the paper produced and reliable operation of the press section also at high speeds. An adequate dewatering capacity also at high running speeds has been guaranteed in the invention by employing at least one extended-nip press, the length of the press zone of said extended-nip press in the machine direction being, as a rule, larger than about 100 mm. In the invention, an essential novelty is the arrangement of the last nip, by whose means an undisturbed transfer of the web, an adequate dewatering capacity, equalization of any differences in coarseness of the opposite faces of the web if necessary, and reliable transfer of the web onto the drying wire of the first cylinder group in the dryer section are guaranteed. In the invention, in connection with the preferably smooth-faced lower roll of relatively large diameter, one press nip only has been fitted, as a result of which the construction can be made of low height also in respect of the circulation of the upper fabric in the last nip. In the invention, an advantageous removal of broke by the force of gravity after the first extended nip can also be carried into effect. When a press section in accordance with the present invention is applied to thin printing and writing papers, the first extended nip proper can be substituted for by an extended roll nip, in which the nip length can be increased to the necessary length by using a roll diameter larger than normal in the press rolls, which roll diameter is, in such a case, chosen typically in the range of about 1000...2000 mm. If necessary, the construction can also be carried out with roll diameters smaller than those mentioned above. 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 schematic side view of a first embodiment of the invention with its frame constructions. Figure 2 shows a second embodiment of the invention, in which two successive extended nips are employed, the frame constructions being not shown. Figure 3 shows a third embodiment of the invention without frame constructions, which embodiment differs from the first embodiment as shown in Fig. 1 in respect of the transfer of the web after the second roll nip. Figure 4 shows such a fourth embodiment of the invention, without frame constructions, in which a transfer band that runs around the lower roll in the second roll nip is employed, by means of which transfer band the web is transferred as a closed draw onto the drying wire of the dryer section. Figure 5 shows an embodiment of the invention which is in particular suitable for thicker grades and in which an extended nip is also employed as the last nip. Figure 6 shows such a modification of the press section as shown in Fig. 5 in which the first nip is an extended roll nip. Figure 7 shows such a modification of the press section as shown in Fig. 4 in which an extended roll nip has been used in stead of the first extended nip proper. In the following, to begin with, the common features of the press sections as shown in Figs. 1 to 5 will be described. The paper web W 0 is passed to the press section from the forming wire 10, from which the paper web W 0 is transferred before the wire reversing roll 11 onto the upper press fabric 16 by means of the vacuum in the suction zone 13a of the pick-up roll 13. The upper press fabric 16, which is guided by the guide rolls 15 and 15', carries the paper web W on its lower face into the first press nip, which is expressly an extended nip NP 1 . The web W runs through the press zone of the extended nip NP 1 between two press fabrics 16 and 17 that receive water. The upper fabric is the pick-up fabric 16 mentioned above, and the lower fabric is the fabric 17 which receives water, is guided by the guide rolls 18,18', and carries the web W further after the extended nip NP 1 while the upper fabric 16 is separated from the web W in or before the area of the guide roll 15'. The first extended nip NP 1 , which removes water efficiently and from two sides, is formed between a lower hose roll 20 and an upper solid-mantle press roll 21. In the lower hose roll 20, there is a flexible hose mantle 20'. The hose roll 20 may be, for example, similar to that illustrated in Fig. 10 in the applicant's said FI Pat. Appl. 905798 (equivalent to EP publication 0487483 A1). Inside the flexible mantle 20' of the hose roll 20, there are press-glide shoes 22, which come out in more detail from said application and by whose means the compression pressure is produced in the nip zone. The rigid mantle 21' of the upper roll 21 in the extended nip NP 1 may have a smooth outer face or a hollow face without through perforations. The first extended nip NP 1 can also carried into effect so that the upper roll 21 is a hose roll and the lower roll 20 a smooth-faced/hollow-faced rigid press roll. The lower roll 20 may also be a suction roll if a relatively low press load is employed in the extended nip NP 1 . In such a case, the use of a suction roll is also advantageous in the respect that it helps the web W to follow the lower felt 17. The length of the extended-nip zone of the extended nip NP 1 and of possible other extended nips NP 2 , if any, in the machine direction is, as a rule, larger than about 100 mm, preferably in the range of 150...300 mm. In the extended-nip zone NP 1 ;NP 2 , the compression pressure can be arranged so that it can be profiled both in the machine direction and in the cross direction so as to obtain an optimal pressing result. The press roll 21 can be provided with inside crown-variation means 23, so that the compression pressure in the extended nip NP 1 can be profiled at least gently. As was stated above, the first extended nip NP 1 removes water efficiently, for example, so that, while the dry solids content k 0 of the web W 0 is k 0 ≈ 12...25 % before the extended nip NP 1 , said dry solids content k 1 after the extended nip NP 1 is k 1 ≈ 25...50 %. In the following, the frame construction of the press section, which has been sketched in Fig. 1, will be described briefly. The frame construction of the press comprises a front frame 70 and a rear frame 80, which are supported on the floor constructions 100 of the paper machine hall. The front frame 70 comprises vertical parts 71a and 71b, between which the bearing supports of the rolls 20 and 21 of the first extended nip NP 1 are supported by means of horizontal frames 73a and 73b. The first extended nip NP 1 has been arranged to be openable by supporting the bearing supports of the upper roll 21 on the horizontal part 73b of the frame by means of intermediate arms 74 provided with horizontal articulated joints 75a and 75b. The rear frame 80 comprises vertical parts 81a,81b, horizontal parts 82, and a projection part 83 attached to the vertical part 81a. The lower roll 40 of the second nip N 2 is mounted stationarily on the vertical part 81b. Between the frame parts 81b and 81a there is a horizontal part 81c, and the first drying cylinder 47 is mounted on the frame part 81d. In the front frame and the rear frame 70 and 80, at the driving side of the machine, there are openable intermediate pieces 90, after whose opening the fabrics 16, 17 and 38 can be replaced in a way in itself known. Between the front frame and the rear frame 70 and 80 it is possible to arrange a free space T that is open upwards, through which space T the press rolls and the other components can be replaced, if necessary, by lifting straight upwards. For replacement of the upper fabrics 16 and 38, the guide rolls 15 and 37 can be shifted to the inner positions 15V and 37V in the directions of the arrows V. In the following, to begin with, the press section mainly shown in Figs. 1, 3 and 4 will be described, in which press section, after the first extended nip NP 1 , the second press nip is a sharp roll nip N 2 . The second roll nip N 2 is placed at a slightly higher level than the first extended nip NP 1 , so that said difference in height H 0 (Fig. 3) is typically H 0 ≈ 500...2000 mm. In the exemplifying embodiment as shown in Fig. 1, the overall height H of the press section can be made relatively little, and said height H above the machine plane is typically in a range of H ≈ 5...12 m. Thus, a relatively low construction is provided, because the cycles of the upper fabrics 16 and 38 can also be made low and simple. The horizontal distance L between the nips NP 1 and N 2 in the machine direction is, as a rule, dimensioned in a range of L ≈ 5...12 m. As is shown in Figs. 1, 3 and 4, the web W is transferred from the upper face of the lower fabric 17 of the first extended nip NP 1 onto the lower face of the upper fabric 38;38B of the second roll nip N 2 by means of a transfer-suction roll 35 or by means of a reversing roll 25 (Fig. 3) which has a smooth mantle 25'. In the transfer of the web W from the lower fabric 17 to the upper fabric 38;38B, an angle a 0 as little as possible for the change in direction is used, which angle is arranged to be a 0 < 45°, preferably a 0 ≈ 30°. In Figs. 1 and 4, the upper fabric 38 is a press fabric that receives water, in which case a suction-roll transfer is employed and the transfer-suction roll 35 includes a suction zone 35a. In Fig. 3, the upper fabric 38B is a substantially impervious transfer band 38B that does substantially not receive water, in which case the transfer roll 25 is not a suction roll, but the transfer from the fabric 17 to the fabric 38B takes place based on the adhesion properties of the outer face of the transfer band 38B. After the transfer point, there follows a short upwards inclined straight run of the upper fabric 38;38B. The length L 0 of said run (Fig. 3) is typically L 0 ≈ 400...2000 mm. On this run of the fabric 38;38B, outside said fabric loop, a steam box 42 is fitted against the free lower face of the web W, which steam box is provided with a duct 42a for the supply of hot water steam. In the press section as shown in Fig. 3, the last nip N 2 is particularly well suitable for use as an equalizing press nip, by whose means an asymmetry of coarseness of the opposite sides of the web W to be pressed, which asymmetry arose in the first extended nip NP 1 , is equalized. In such a case, the surface properties of the transfer fabric 38B are chosen so that they are suitable for the equalizing-press function. According to Figs. 1 and 4, opposite to the steam box 42, fitted inside the fabric loop 38, there is a suction box 43, preferably a suction box based on air blowings and marketed by the applicant under the trade mark PRESS-RUN™, which suction box is not employed in the embodiment as shown in Fig. 3. By the effect of the steam box 42, a heating effect is applied to the free outer face of the web W, which effect is intensified further by the suction box 43. In Figs. 1 to 4, after the straight run L 0 of the fabric 38;38B, there follows the turning sector a 1 of the smooth-faced 42 lower roll 40 of the second nip N 2 , before the sharp roll-nip zone N 2 . Said turning sector a 1 is, as a rule, chosen in a range of a 1 ≈ 45...130°, preferably in a range of a 1 ≈ 60...100°. The roll nip N 2 is formed between said lower roll 40 and the hollow-faced 41 upper roll 41. In Fig. 3, the face 41" of the upper roll 41 may be smooth. In connection with the smooth face 40' of the lower roll 40 of the roll nip N 2 , a heating device 60 is fitted, in whose treatment gap 61 a heating effect is applied to the roll face 40', e.g., by means of infrared radiation, a magnetic induction effect, and/or by means of hot steam. In this way, the temperature of the roll face is raised to the level T 0 ≈ 60...150°C, and on the sector a 1 the thermal energy is transferred from the roll face 40' to the web W so that, owing to the heating effect jointly with the steam box 42, the temperature level T 1 of the web W before the nip N 3 has been raised considerably. Typically, this temperature level T 1 is in a range of T 1 ≈ 60...110°C. Owing to a sufficiently long turning sector a 1 , an efficient transfer of heat from the heated roll face 40' to the web W is achieved. Owing to the raised temperature level T 1 of the web W, the dewatering is intensified in the nip N 2 ;NP 2 (Fig. 5). The last nip N 2 ;NP 2 ;N 3 is placed after the topmost point of the lower roll 40 on the first upper quarter of the roll 40. After the last nip N 2 ; NP 2 ;N 3 , there follows a downwards inclined run of the web W, after which the web W is transferred onto the drying wire 45 of the dryer section. The arrangement of the last nip N 2 ;NP 2 ;N 3 is also characterized in that, in connection with the large-diameter D ≈ 1000...1700 mm lower roll 40, just one nip, the last nip in the press section, is fitted, which is placed slightly after the topmost point of said lower roll 40. According to Fig. 1, after the second nip N 2 , the web W is transferred on the smooth face 40' of the lower roll 40, at the transfer-suction roll 44, as a short free draw WD onto the drying wire 45, to which the web W is made to adhere by means of suction boxes 46. On the drying wire 45 the web W is transferred onto the first drying cylinder 47, which is placed at a level lower than the normal position of the upper cylinders in the first group R 1 . After this, there are the reversing suction cylinders 50 of the dryer section, for example the applicant's VAC™ rolls. Further, in Fig. 1, the first contact-drying cylinder 48 placed at the normal level and the blow-suction boxes 49, such as the applicant's UNO RUN BLOW BOXES™, are shown. In Fig. 3, after the second roll nip N 2 , the web W follows the upper transfer band 38B and enters on support of its straight downwards inclined run to the transfer point TR, where the web W is passed on the suction zone 51a of the transfer-suction roll 51 onto the face of the drying wire 45 of the cylinder group R 1 . In Fig. 3, the first cylinder group R 1 is an inverted group, in which the heated contact-drying cylinders 48A are placed in the lower row and the reversing suction cylinders 50A in the upper row. In Fig. 4, a transfer band 38A that runs over the lower cylinder 40 of the roll nip N 2 is employed, the web W being transferred on the upper face of the downwards inclined straight run of said band 38A, after the second roll nip N 2 , onto the drying wire 45 of the first cylinder group R 1 at the transfer point TR by means of the vacuum present in the suction zone 44a of the transfer-suction roll 44B. After the transfer point TR, the transfer band 38A is guided by the roll 47A. When a transfer band 38A is used, it is not always favourable to use a roll 40 heating device 60, but, if necessary, the heating effect can be applied directly to the band 38A, which is illustrated schematically by the heating device 60A denoted by the dashed line. Fig. 2 shows an embodiment of the invention in which, after the pick-up point P of the web W, two successive extended nips NP 1 and NP 2 are employed. The first extended nip NP 1 is similar to that described above in relation to Figs. 1, 3 and 4. After the reversing roll 15' of the first upper fabric 16 that receives water, the web W follows the lower fabric 17 that receives water, on whose downwards inclined run between the guide rolls 18 the web W is transferred onto the second upper fabric 27, which is guided by guide rolls 26, on the suction zone 25a of the transfer-suction roll 25. On this sector, the direction of the web W is changed over said little angle a 0 . On the upper face of the second upper fabric 27 that receives water, the web W is passed into the second extended nip NP 2 , in which there is a lower extended-nip roll 30 provided with a hose mantle 30' and an upper rigid press roll 31, which has a smooth-faced or hollow-faced mantle 31'. In the rolls 30 and 31, there are press-glide shoes 32 and 33 corresponding to the shoes 22 and 23 in the first nip NP 1 . The embodiment as shown in Fig. 2 is particularly well suitable for thicker paper grades whose grammages are typically in a range of 60...300 g/m 2 . In such a case, the dry solids content k 1 of the web W after the first extended nip NP 1 is typically k 1 ≈ 30...50 %, and after the second extended nip NP 2 k 2 is typically k 2 ≈ 45...55 %. After the second upper fabric 27, the web W is transferred on the lower fabric 28 that receives water of the second extended nip NP 2 , to be transferred on the suction zone 35a of the transfer-suction roll 35 onto the upper fabric 38 of the third roll nip N 3 . The arrangement of the third roll nip N 3 with its various devices is similar to the arrangement of the corresponding second roll nip N 2 shown in Fig. 1. In Fig. 2, the upper fabric 38 of the last roll nip N 3 can be substituted for by a transfer band 38B as shown in Fig. 3, in which case the last nip N 3 is particularly well suitable for use as an equalizing press nip, by whose means it is possible to equalize the asymmetry of coarseness of the opposite faces of the web W that arose in the preceding extended nips NP 1 and NP 2 . Fig. 5 shows such an embodiment of the invention in which the second roll nip N 2 as shown in Fig. 1 has been substituted for by a corresponding extended nip NP 2 . The second extended nip NP 2 is formed by an upper extended-nip roll 30 provided with a flexible hose mantle 30', and the lower press roll is a smooth-faced 31' rigid press roll 31, which is provided with inside glide shoes 33 for crown variation. After the extended-nip zone NP 2 , the web W follows the smooth face 31' of the lower roll 31, from which the web W is separated as a short free draw WD and transferred onto the drying wire 45 of the first cylinder group R 1 in the dryer section. The arrangement in accordance with the present invention of the second nip N 2 ;NP 2 or of the third nip N 3 is also advantageous in the respect that the smooth-faced 40'; 31' lower roll 40;31 of the nip concerned can be doctored readily, and there is an adequate space available for the doctor devices in the arrangement. In Fig. 1, said doctors 63 are shown, from which there is a straight and direct connection to the pulper (not shown) placed below, the broke web passing into said pulper being denoted with the reference WA in Fig. 1. Figs. 6 and 7 show an embodiment of the invention in which, as the first nip NP 10 , in stead of an extended nip NP 1 proper, an extended roll nip has been applied. The press section as shown in Fig. 6 is in the other respects similar to that shown in Fig. 5, and the press section shown in Fig. 7 is in the other respects similar to that shown in Fig. 4. According to Fig. 6, the first extended roll nip NP 10 is provided with two press fabrics 16 and 17 and formed between an upper press roll 210 and a lower press roll 200. Both of the press rolls 200 and 210 are provided with a hollow face 201;211, which hollow face has been produced by means of grooves and/or blind-drilled bores. The first extended roll nip NP 10 in Fig. 7 has been formed in a corresponding way. The extending of the roll nip NP 10 has been accomplished by using a press-roll 200,210 diameter D 0 larger than normal. As a rule, a sufficient extension of the roll nip NP 10 is obtained with roll diameters of D 0 ≈ 1000...2000 mm. Within the scope of the present invention, it is not necessary to use a roll diameter D 0 larger than normal in the nip NP 10 . In such a case, the extending of the roll nip NP 10 , if it is necessary, can be accomplished by choosing press fabrics 16 and/or 17 thicker than normal. The press section as shown in Figs. 6 and 7 is particularly well suitable for use with thin printing and writing papers. An advantage of the (extended) roll nip NP 10 in comparison with an extended nip proper and with hose rolls is the substantially lower cost of the construction. In the following, typical and preferred exemplifying embodiments of the linear loads in the various nips in the press section in accordance with the present invention will be given while, nevertheless, not confining the invention to said embodiments. First extended nip NP 1 ≈ 1200 kN/m, preferably ~ 1000 kN/m, Second extended nip (Fig. 2) NP 2 ≈ 1200 kN/m, preferably ~ 1000 kN/m, Second extended nip (Fig. 5) NP 2 ≈ 1200 kN/m, preferably ~ 1000 kN/m, Second roll nip N 2 ≈ 200 kN/m, preferably ~ 150 kN/m, Third roll nip N 3 ≈ 200 kN/m, preferably ~ 150 kN/m. In the extended nips NP 1 ,NP 2 and in the extended roll nip NP 10 it is preferable to use press felts that are slightly heavier and-thicker than normal, because in them the amount of water removed is larger, and a high press impulse produces a marking of the fabric or of the hollow face in the paper more readily. The hose mantle 20',30' of the extended-nip rolls 20,30 is preferably hollow-faced, such as grooved, blind-drilled or provided with other recesses. According to the invention, a particularly compact press section is produced so that, for example, in modernizations of paper machines, in which the dewatering capacity of the press section is increased, for example, because of increased running speed of the paper machine, the press section can be placed in the place of an existing, for example, three-nip or four-nip press section that comprises exclusively roll nips, for example in the place of the applicant's SYM-PRESS II™ press. A number of different variations of the details described above are possible within the scope of the present invention. For example, the means 60 for heating the lower roll 40 in the last nip N 2 ;NP 2 ;N 3 can also be fitted inside the roll, for example, by using hot water steam fed from the roll axles as a heating medium in a way similar to a drying cylinder. Also, if necessary, the temperature of the roll 40 face 40' can be arranged so that it can be profiled in the axial direction of the roll 40, i.e. in the cross direction of the web W, in view of controlling the different property profiles of the web W. As a coating on the roll 40;31, it is possible to use the applicant's VALROK™ or DYNAROK™ coating or equivalent. The general geometry of the press section is preferably arranged such that the level of the first extended nip NP 1 is placed substantially at the same level as the level at which the web W is transferred at the transfer point TR or as a free draw WD onto the drying wire 45. The last nip N 2 ;NP 2 ;N 3 is placed at a level higher than said level, with said difference in height H 0 ≈ 600...2000 mm. In view of securing an optimal utilization of space and the removal of broke as well as the transfer of the web W, it is preferable that the last nip N 2 ;NP 2 ;N 3 is placed after the topmost point of the lower roll 40;31 in which case the transfer of the web W from the last nip onto the drying wire 45 takes place so that its substantial direction is a gently downwards inclined run. In the following, the patent claims will be given, and the various details of the invention may show variation within the scope of the inventive idea defined in said claims and differ from what has been stated above by way of example only.

Claims: A paper machine comprising a drying section and a press section, which press section comprises one last nip (N 2 ; N 3 ; NP 2 ) formed between a lower roll (40; 31) and an upper press member (41), said last nip being provided with an upper press fabric (38; 38B), and at least one further nip (NP 1 ; NP 2 ; NP 10 ) that precedes the last nip and is provided with one lower press fabric (17, 28) and one upper press fabric (16, 27) that receive water from a paper web (W) conveyed therebetween, said paper web (W) being transferred at a web transfer point from the lower fabric of the preceding nip onto the upper press fabric (38; 38B) of the last nip (N 2 ; NP 2 ; N 3 ), on whose lower face the web (W) is transferred into the last nip, wherein after the web transfer point, the upper fabric (38; 38B) of the last nip (N 2 ; NP 2 ; N 3 ) has a relatively short upwards inclined run, after which said upper fabric (38; 38B) is turned, being guided by said lower roll (40; 31) over a considerable sector (a 1 ), and wherein the point of transfer (TR) of the web (W) onto the drying wire (45) of a first group (R 1 ) in the dryer section following after the press section is placed underneath the level of the last nip (N 2 ; NP 2 ; N 3 ), characterized in that the last nip (N 2 ; NP 3 ; N 3 ) is placed after the topmost point of the lower roll (40; 31) on a level higher (H) than said preceding nip (NP 1 ; NP 2 ; NP 10 ). A press section as claimed in claim 1, characterized in that , on said upwards inclined run of the upper fabric (38;38B) of the last nip (N 2 ;NP 2 ;N 3 ), at the vicinity of the free lower face of the web (W), there is a heating device, preferably a steam box (42), which raises the temperature level (T 1 ) of the web (W) before the last nip (N 2 ;NP 2 ;N 3 ). A press section as claimed in claim 1 or 2, characterized in that , in connection with the lower roll (40;31) of the last nip (N 2 ;NP 2 ;N 3 ), there is a heating device (60) which heats the mantle (40';31') of said lower roll (40;31) and by whose means the temperature level (T 0 ) of said roll face (40';31') is raised so that, on the turning sector (a 1 ) that precedes the last nip (N 2 ;NP 2 ;N 3 ), the temperature level (T 1 ) of the web (W) can be raised before the last nip (N 2 ;NP 2 ;N 3 ). A press section as claimed in any of the claims 1 to 3, characterized in that , inside the loop of the upper fabric (38) of the last nip (N 2 ;NP 2 ;N 3 ), on said straight run of the fabric, there is a suction box (43), by whose means the holding of the web (W) on the face of said upper fabric (38) and possibly the penetration of the heating effect of the opposite steam box (42) into the web (W) are promoted. A press section as claimed in any of the claims 1 to 4, characterized in that , at the transfer point at which the web (W) is transferred onto the upper fabric (38;38B) of the last nip (N 2 ;NP 2 ;N 3 ) from the lower fabric (17,28) of the preceding nip (NP 1 ;NP 2 ), the angle of change in direction is a 0 < 45°, preferably a 0 < 30°. A press section as claimed in any of the claims 1 to 5, characterized in that the magnitude a 1 of the turning sector of the upper fabric (38;38B) preceding the last nip (N 2 ;NP 2 ;N 3 ) has been chosen in the range of a 1 ≈ 45...130°, preferably in the range of a 1 ≈ 60...100°. A press section as claimed in any of the claims 1 to 6, characterized in that the press section includes two press nips, of which the first nip is an extended nip (NP 1 ) and the last nip (N 2 ) is a sharp roll nip. A press section as claimed in any of the claims 1 to 6, characterized in that the press section includes two press nips, both of which are extended nips (NP 1 ,NP 2 ), preferably arranged so that the lower press member of the first extended nip (NP 1 ) is a hose roll (20) provided with a flexible hose mantle (20'), and the upper press member of the last nip (NP 2 ) is a similar hose roll (30). A press section as claimed in any of the claims 1 to 6, characterized in that the press section includes three successive nips, preferably arranged so that the first two nips are extended nips (NP 1 ,NP 2 ) provided with two press fabrics, and the last nip is a sharp roll nip (N 3 ). A press section as claimed in any of the claims 1 to 6, in particular for the manufacture of thin printing and writing papers, characterized in that the first nip in the press section is a roll nip (NP 10 ), preferably an extended roll nip (NP 10 ), through which two press fabrics (16,17) that receive water are passed. A press section as claimed in claim 10, characterized in that the extending of the first roll nip (NP 10 ) has been accomplished by using roll diameters larger than normal diameters of press rolls, said diameters being preferably chosen in the range of D 0 ≈ 1000...2000 mm, and/or that the extending of the roll nip has been accomplished by using press fabrics (16,17) that receive water and that are thicker than normal. A press section as claimed in any of the claims 1 to 11, characterized in that the web (W) is transferred from the smooth-faced lower roll (40;31) of the last nip (N 2 ;NP 2 ;N 3 ) as a short free draw (WD) onto the drying wire (45) of the first cylinder group (R 1 ) in the dryer section. A press section as claimed in any of the claims 1 to 11, characterized in that the upper fabric of the last press nip (N 2 ) is a substantially impervious transfer band (38B) that does not receive water and on whose lower face the web (W) is passed, after the last nip (N 2 ;NP 2 ;N 3 ), to the transfer point (TR), at which the web (W) is transferred, preferably on the suction zone (51a) of a transfer-suction roll (51), onto the drying wire (45) of the first cylinder group (R 1 ) in the dryer section. A press section as claimed in any of the claims 1 to 11, characterized in that a smooth-faced transfer band (38A) which does substantially not receive water has been arranged to run around the lower roll (40) of the last press nip (N 2 ;NP 2 ;N 3 ), by means of which transfer band (38A) the web (W) is transferred, preferably from the suction zone (44a) of a transfer-suction roll (44B), onto the drying wire (45) in the first cylinder group (R 1 ) in the dryer section. A press section as claimed in any of the claims 1 to 14, characterized in that the first extended nip (NP 1 ) and the point of transfer (TR) of the web (W) onto the drying wire (45) after the press section are placed substantially on the same horizontal level, compared with one another, the last nip (N 2 ;NP 2 ;N 3 ) in the press section being placed above said level with the difference in height H 0 , which is dimensioned in the range of H 0 ≈ 500...2000 mm. A press section as claimed in any of the claims 1 to 15, characterized in that the length L 0 of the gently upwards inclined straight run of the upper fabric (38;38B) of the last nip (N 2 ;NP 2 ;N 3 ) after the web (W) transfer point is L 0 ≈ 400...2000 mm, preferably L 0 ≈ 700...1400 mm, and/or that the overall height of the press section above the machine plane has been dimensioned in the range of H ≈ 5...12 m, preferably in the range of H ≈ 6...9 m.

Inventors: KIVIMAA JUHA, PAJULA JUHANI, KIVIMAA, JUHA, PAJULA, JUHANI
Applicants: METSO PAPER INC, METSO PAPER, INC.
Application year: 1995
Main classification: D21F3
IPC codes: D21F5, D21F3