PRESS SECTION OF A PAPER MACHINE

Abstract: Press section of a paper machine, in which the paper web (W) coming from the forming wire (10) of the paper machine is passed through the first two-felt nip (N₁). The press section includes a smooth-faced centre roll (40), against which two single-felt nips (N₂,N₃) are formed. The nip (N₂) is placed at a certain distance from the first two-felt nip (N₁), whereat the web (W) runs the passage determined by said distance as a straight run supported by the first upper fabric (20). The web (W) departs from the first upper fabric (20) when carried by the smooth (41) face of the centre roll (40) and is shifted on it further into the third nip (N₃). The web (W) is detached from the face (41) of the centre roll (40) and passed further. The first nip (N₁) is formed between a pick-up suction roll (21 ) placed inside the first upper fabric (20) and a hollow-faced (32) press roll (31). The web (W) is transferred by means of the suction zone (21a) of the pick-up suction roll (21) from the forming wire (10) onto the first fabric (20) and on that fabric further into the first nip (N₁). A first lower press fabric (30) is provided around said press-suction roll (31). The web (W) is carried on the lower face of the first upper fabric (20) into the second nip (N₂).

Description: This invention concerns a press section of a paper machine of the type defined in the preamble of appended claim 1. A press section of this general structure is disclosed in US-A-4 440 598. The starting point of the present invention has been a further development of the applicant's 'Sym-Press II' press section ('Sym-Press' = Finnish trade mark) on the basis of the experience of operation that has been obtained with the 'Sym-Press II' press over a number of years. In respect of the details of the 'Sym-Press II' press section, reference is made to the applicant's Finnish Announcement Publication No. 50,651 (equivalent of US Patent No. 4,209,361). In respect of the prior art related to the present invention, reference is made further to the US Patents Nos. 4,257,844 and 4,192,711, to the Finnish Patent 72,159 corresponding to US-A-4 440 598, as well as to the papers in the journals Das Papier Heft 1, pages 33 to 34, 1981, and Norsk Skogindustri No. 3, 1974, page 80. In the latter papers, such a modification of the above 'Sym-Press II' press section is described, in which the 'Sym-Press II' suction roll does not form a nip with the smooth-faced centre roll and in which, in connection with or ahead of this suction roll, the first two-felt press nip in the press section is provided, wherein the dewatering takes place in two directions. In connection with the smooth-faced centre roll, substantially in the place of the 'Sym-Press' suction roll, a hollow-faced press roll is placed, which forms the second press nip in the press section, together with the smooth-faced centre roll. The third press nip is formed substantially at the opposite side of the smooth-faced centre roll, in relation to the second nip. In the press sections known from the above cited publications, it has been necessary to use a suction roll either as a press roll or as a roll within whose area the web supported by the pick-up felt alters its direction upwards to the second nip. The use of a suction roll or of any other, corresponding suction device causes considerable drawbacks, which will be dealt with in more detail in the following. The perforation in a suction roll may provide the web with a marking, which is detrimental to the appearance of the paper and may affect the surface properties of the paper. Press-suction rolls are expensive, they require a drive motor of their own together with its regulation system, and they cause noise. In particular in the case of press-suction rolls, it would often be necessary to arrange crown variation, but, as a rule, this is impossible, because the mantle of the suction roll is perforated and/or because the interior space in the roll is to such an extent occupied by the suction box that, consequently, the crown variation devices in themselves known cannot be fitted therein. One of the objects of the present invention is also to eliminate this drawback. In connection with the prior-art frame parts of said press sections, difficulties have occurred in relation to the replacement both of the press fabrics and of the press rolls. These problems have increased with the increase in the widths of the paper machines, in particular owing to the fact that the press rolls have become ever longer and heavier. Said problems have, for its part, also been increased by the fact that press fabrics which are made of plastic materials and which are rigid have started being used ever increasingly, which said press fabrics cannot be jammed into a bundle easily. Beloit Corporation has attempted to solve the problems described above by means of a so-called "Flip-Top" (trade mark of Beloit Corporation) frame construction, in which such a top frame is used as is provided with a pivot shaft parallel to the transverse direction of the paper machine and placed above the press rolls, whereby either the top part of the front frame or the top part of the rear frame can be opened around said pivot shaft. Said two top parts of the frame cannot be opened at the same time. By opening said top part of the front frame, it is possible to facilitate the replacement of the pick-up fabric of the press section and the replacement of the fabric that usually acts as the press fabric in the first nip and in the second nip. Thereat, the top part of the rear frame is locked as a frame part on whose support the opening-dumping of the top part of the front frame takes place. Correspondingly, when the top part of the front frame is in the closed-locked position, the top part of the rear frame can be dumped so that the press fabric of the third press nip can be replaced. The lower fabric of the first nip, which runs mainly in the basement space, is replaced by means of arrangements in themselves known. The above "Flip-Top" solution can be characterized as a sort of a "drawbridge" which is opened around an articulated joint placed at the middle of the bridge, and only one half of the bridge can be opened at a time. From the applicant's Finnish Patent Applications Nos. 844693 (corresponding to US-A-4 608 125 and US-A-4 699 692) and 854959 (corresponding to US-A-4 657 634), such frame constructions for press sections are known in which the front frame and the rear frame are connected to each other and/or to the intermediate frame of the central roll in the press by means of various openable and closable intermediate frames placed above. When press roll arrangements with several nips are used, problems of space are also encountered, because several different press rolls with their auxiliary devices must be accommodated in a relatively limited space. This is why it has been necessary to place the frame parts that connect the front frame and the rear frame to each other or to the intermediate frame, with their opening and closing means, in highly congested spaces, which results in problems both for the construction and for the operation. The object of the present invention is also to provide improvements for these problems. The object of the present invention is to provide a synergically novel press section, wherein the various press rolls are fitted in such a way that a more advantageous press construction unit is obtained both in view of the pressing process and in view of the maintenance of the machine, which includes the replacements both of the press rolls and of the various fabrics. A further object of the invention is to provide such a press section of a paper machine wherein the web can be passed to and through the press section reliably as a closed draw without risk of breaks. A further object of the present invention is to attempt to retain the most important advantageous properties of the 'Sym-Press' press section, which have made the 'Sym-Press' press section a popular, even a predominant construction. In view of achieving the objectives stated above and those that will come out later, the press section in accordance with the invention is mainly characterized by the features stated in the characterising clause of appended claim 1. Further features of the invention are stated in the appended sub-claims. 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, whereat the invention is not confined to the details of said exemplifying embodiments. Figure 1 is a schematical illustration of a three-nip embodiment of the press section in accordance with the invention. Figure 2 shows such an embodiment of the invention wherein there is a separate nip placed after a compact press-roll arrangement. Figure 3 is a more detailed side view of the press section shown in Fig. 2 with its frame construction. The web W entering into the press section shown in Figures 1,2 and 3 is formed on the wire 10, from which the web W is detached at the point P on the downwardly inclined run in the wire part between the rolls 11 and 12 and transferred by the effect of the negative pressure in the suction sector 21a of the pick-up roll 21 onto the first upper fabric 20. Besides acting as the pick-up fabric, the first upper fabric 20 also acts as a press fabric in the first press nip N₁ and in the second press nip N₂ in the press section. The first press nip N₁ is formed between the pick-up suction roll 21 and the hollow-faced 32 press roll 31, and the nip N₁ is a two-felt nip. The first fabric 20 acts as the upper felt, and the lower felt consists of the first lower fabric 30, which is guided by the guide rolls 33. Some of the guide rolls 33 are placed below the floor plane L-L. The lower roll 31 in the nip N₁ may be a grooved roll, a blind-drilled roll, or a suction roll and, in some special cases, also a smooth-faced roll. The first lower fabric 30 may also have a guide roll before the first nip N₁. After the nip N₁, on the run R, the first upper fabric 20 is upwardly inclined, and on its support the web W moves into the second nip N₂, wherein the first upper fabric 20 acts as a press fabric. The second nip N₂ is formed between the smooth-faced centre roll 40 and the hollow-faced press roll 41. In the press roll geometry in accordance with the invention, it is an essential feature that the second nip N₂ is formed on the upper quarter of the centre roll 40 that is placed next to the first nip N₁. Correspondingly, the third nip N₃ is formed on the latter upper quarter of the centre roll, in relation to the first nip N₁, or at the proximity of the horizontal plane placed around the axis of rotation of the centre roll 40. The smooth-faced 41 centre roll 40, whose radius is preferably larger than the radii of the other press rolls, is preferably a metal roll, for whose smooth face 41 the material has been chosen so that the adhesion of the web W to be pressed to the roll 40 face 41 is stronger than its adhesion to the fabric 20 and that the web W can, however, be detached from the roll 40 face 41 sufficiently readily by the effect of a difference in speed as the web W is transferred as an open draw W o from the press section to the drying section (Fig. 1) or into a fourth separate nip N₄ (Figs. 2 and 3). Against the centre roll 40, a third press nip N₃ is also provided, which is formed by a hollow-faced 52 press roll 51, which is provided with a press fabric loop 50 of its own, said loop acting as the second upper fabric. The guide roll of the fabric 50 is denoted with the reference numeral 53, and its tensioning roll with the reference numeral 53b. The guide rolls of the first fabric 20 are denoted with the reference numerals 23,23a, and the tensioning roll with the reference numeral 23b, as well as the felt reconditioning devices with the reference numeral 24 (Fig. 3). The smooth-faced 41 centre roll 40, which is journalled permanently on the frame part 110 of the paper machine, has, substantially at the side opposite to the nips N₂ and N₃, a downwardly open space U and a sector that is provided with a doctor 42. In the event of a break, the doctor 42 guides the web W through the open space U into the broke pulper (not shown), which is placed underneath the press section. In accordance with Fig. 1, the web W is detached as an open draw W o from the smooth face 41 of the centre roll 40 by making use of difference in speed, and is transferred onto the drying fabric 70 in the drying section, which said fabric 70 runs as guided by the guide rolls 71 and 73. Thereupon, having been made to adhere to the lower face of the drying fabric by means of suction devices 72, the web is transferred onto the first drying cylinder 81 or onto a corresponding lead-in cylinder. Fig. 3 shows two upper cylinders 81 and a lower cylinder 82 of the drying section. According to Fig. 2, the web W is detached as an open draw W o from the roll 40 face 41 and transferred by means of difference in speed over the guide roll 43 onto the second lower fabric 60, to which the web W is made to adhere by means of the suction zone 66a of the guide roll 66. The fabric 60 carries the web W into the fourth separate nip N₄, which is formed between a smooth-faced upper roll 61 and a hollow-faced 64 lower roll 63. The upper roll 61 is provided with a doctor 62. The web W is detached from the smooth face of the upper roll 61 and passed via the guide roll 68 onto the drying fabric 70 in the way described above. According to Figs. 1 and 2,between the nips N₁ and N₂, when the web W runs on support of the first upper fabric 20, a steam treatment is applied to the web by means of the steam box 25, said steam treatment intensifying the dewatering in the second nip N₂ and possibly in the third and fourth nip N₃, N₄. The operation of the steam box may be intensified by a suction box 25a placed at the opposite side of the press felt 20. According to Fig. 2, a steam box 65 and a suction box 65a are also placed between the third nip N₃ and the fourth nip N₄ at the proximity of the second lower fabric 60, which said boxes 65 and 65a intensify the dewatering in the last, i.e. fourth nip N₄. The geometry of the press section in accordance with the invention is preferably such that there is the difference in height H mentioned above between the first nip N₁ and the centre plane of the centre roll 40, and the second nip N₂ is placed at the distance of an angle a from the vertical centre plane of the centre roll, the magnitude of said angle being a = 5...80°, preferably a = 30...60°. Correspondingly, the third nip is placed at the distance of an angle b from said centre plane. The magnitude of the angle b is, as a rule, b = 25...150°, preferably b = 45...100°. The diameter of the centre roll 40 is preferably about twice the diameter of the press rolls 22,31,51. The diameter of the suction roll 21 is preferably smaller than the diameter of the centre roll 40 but larger than the diameter of the press rolls 22,31 and 51. The diameters of the centre roll 40 and of the upper roll 61 in the separate nip N₄ are preferably substantially equally large as compared with each other. The press section described above is a further developed form of the applicant's 'Sym-Press II' press section. Of course, the 'Sym-Press II' press section, wherein a continuous series of nips is formed between the rolls and which includes one press-suction roll, is a compact press section. The press section in accordance with the present invention carries out the principle of closed draw, but it is not quite so compact in the same sense as the 'Sym-Press' press section. None of the rolls included in the first two-felt nip N₁ is in contact with the rolls that form the nip N₂. The press section in accordance with the invention can, however, be considered compact, because the space required by it, in particular in the vertical direction, is not substantially larger than the space required by a 'Sym-Press' press section. The linear loads prevailing in the nips N₁,N₂, N₃ are preferably as follows: N₁ 80 ± 10 kN/m, roll pair suction/hollow-faced, N₂ 110 ± 10 kN/m, roll pair hollow-faced/smooth, N₃ 140 ± 10 kN/m, roll pair hollow-faced/smooth.

Claims: Press section of a paper machine, in which a paper web (W) coming from a forming wire (10) of the paper machine is passed, being supported by a first upper fabric (20), between two fabrics (20,30) through a first two-felt nip (N₁) formed by rolls (21,31), wherein the dewatering of the paper web (W) takes place through both of its faces, the press section including a smooth-faced centre roll (40), against which at least one, preferably two single-felt nips (N₂,N₃) are formed, in the first one (N₂) of said nips the press fabric consisting of the said first upper fabric (20) and said single-felt nip (N₂) being placed at a certain distance from the first two-felt nip (N₁), whereat the web (W) runs a passage determined by said distance as a substantially straight run supported by the first upper fabric (20), the web (W) departing from said first upper fabric (20) when carried by the smooth (41) face of said centre roll (40) and being shifted on it further, preferably into a second single-felt nip (N₃), which is provided with a press fabric (50) of its own, and the web (W) being detached from the smooth face (41) of said centre roll (40), preferably as an open draw (W o ), and passed further, characterized in that said first nip (N₁) is formed between a pick-up suction roll (21) placed inside said first upper fabric (20) and a lower, hollow-faced (32) press roll (31), the web (W) being transferred by means of a suction zone (21a) of said pick-up suction roll (21) from the forming wire (10) onto the first fabric (20) and on that fabric further into the first nip (N₁), and a first lower press fabric (30) being provided around said lower press-suction roll (31), and that the web (W) is carried on the lower face of the first upper fabric (20) into the second nip (N₂). Press section as claimed in claim 1, wherein the centre roll (40) is mounted on fixed bearing supports (111). Press section as claimed in claim 1 or 2, wherein at the opposite side of the centre roll (40), in relation to the second nip (N₂), there is a third nip (N₃), through which a second upper press fabric (50) of its own runs. Press section as claimed in any of the claims 1 to 3, wherein the press section includes a separate third or fourth press nip (N₄) preferably comprising a smooth-faced upper roll (61) and a hollow-faced (64) lower roll (63), around which a second lower press-fabric loop (60) is provided. Press section as claimed in claims 3 or 4, wherein a rearmost guide roll (23a) of the first upper fabric (20) and/or a foremost guide roll (53a) of the second upper fabric (50) is attached by means of intermediate parts (112,113) in connection with a upper part of bearing supports (111) of the centre roll (40).

Inventors: LAAPOTTI, JORMA, TAPIO, OLLI
Applicants: VALMET PAPER MACHINERY INC.
Application year: 1988
Main classification: D21F3
IPC codes: D21F7, D21F3