METHOD AND DEVICE FOR DEWATERING OF A PAPER OR BOARD WEB AND FOR PASSING THE WEB INTO THE PRESS SECTION

Abstract: The invention concerns a method and a device for dewatering a paper or board web (W) and for passing the web as a closed draw from the lower wire (17) of the web forming section formed between a lower wire (17) and an upper wire (15) to the press section and through one or several dewatering press nips (NP1) in said press section. In the invention, the web (W) is separated from the upper wire (15) on the first suction block (a) of the suction roll (14), after which the web is affixed to the lower wire (17) on the second suction block (b) of said suction roll (14). After this the web (W0) is affixed, in a transfer and pre-press nip (NP0) formed by said suction roll (14) and by a smooth-faced (20a) roll (20), onto the outer face of a transfer belt (24) substantially not receiving water, the web (W0) being passed on support of said belt (24) into the first press nip (NP1). In the press nip (NP1), water is pressed out of the web (W1) into the press felt (25).

Description: Method and device for dewatering of a paper or board web and for passing the web into the press section The invention concerns a method in accordance with the preamble of claim 1 for dewatering of a paper or board web and for passing the web into the press section as well as a press section in accordance with the preamble of claim 5 for a paper or board machine. Constantly increasing running speeds of paper and board machines provide new problems to be solved, mostly related to runnability. With the present paper machine speeds of about 25 metres per second, closed press sections, which comprise a compact combination of press rolls fitted around a smooth-faced centre roll, still operate satisfactorily, but with the future speeds of 25...40 metres per second, new solutions are needed. From the point of view of energy economy, dewatering taking place by pressing is preferable to dewatering taking place by evaporation. The more water can be removed by pressing, the less water has to be removed by evaporation. When the machine speed is increased, the nip times become shorter, in which connection it would be necessary to increase the compression pressure accordingly in order that the press impulse could remain unchanged. The peak pressure of pressing can, however, be increased up to a certain limit only, after which the structure of the web is broken. By means of the present invention, attempts are made, among other things, to make it possible to increase the running speeds of paper and board machines and to reduce the consumption of energy without deterioration of the quality properties of the web. From the prior art, a number of different solutions are known for removal of water out of a paper or board web and for passing the web from the web forming section to the press section. In the following, some of them will be described. The applicant's FI Patent Application No. 954698 constitutes the prior art most closely related to the present invention. In said patent application, a solution is suggested for removal of water out of a paper or board web and for passing said web as a closed draw from the forming wire or transfer wire of the web former to the press section and through one or several dewatering press nips in said press section. The solution is characterized in that the web that runs on the forming wire or on the transfer wire is made to adhere, in a transfer and pre-press zone, to the outside face of a transfer belt substantially not receiving water, and that, after said pre-press zone, the web is separated substantially immediately from said wire and passed on support of said transfer-belt loop onto the next press fabric in the press section and/or into the press nip. In the embodiment shown in Fig. 8 in the applicant's FI Pat. Appl. 954698, there is a first pre-press zone between a first smooth-faced upper roll and an open-faced lower roll. Moreover, in the solution, there is a second pre-press nip proper between a second smooth-faced press roll and said open-faced lower roll. In the second prepress nip the web is separated from the open-faced lower roll and made to adhere to the outer face of the transfer felt that is impermeable to water, from which belt face the web is passed further into the press unit between a transfer belt and a lower felt. In the (German) E»E Patent Application 43 01 750 Al (Voith) a solution is suggested for dewatering of a web formed on an endless wire, wherein the web is passed into a first press nip, which is formed between the wire, which is in contact with the web from below, and a first smooth-faced press roll, after which the web is passed into a second press nip, which has been formed substantially in the horizontal plane and which is formed between the first smooth-faced press roll and a felt, after which the web is passed into a third press nip, which is formed between the first smooth-faced press roll, which is in contact with the upper face of the web, and a second smooth- faced roll. The solution is characterized in that the second press nip consists of a shoe press. In said application, in Fig. 4, a solution is illustrated in which the first press nip is formed directly between a suction roll of the wire unit and the first smooth-faced press roll. In this solution, for the transfer of the web, no transfer belt is employed, but the transfer takes place on the face of a smooth-faced roll. In solution described in the (German) DE Patent Application 43 01 751 Al (Voith), the web is formed between a lower wire and an upper wire, in which connection the web and the upper wire are separated momentarily, by means of a difference in pressure, from the lower wire before the web is transferred into the press unit. In said application, the transfer of the web into the press unit, in the embodiment shown in Fig. 2, has been accomplished so that the first press nip is formed directly between a suction roll of the wire unit and a first smooth-faced press roll, in which connection the web is separated from the upper wire on the suction zone of said suction roll of the wire unit. In this solution, no transfer belt is employed, but the transfer takes place on the face of a smooth-faced roll. In the (German) Z E Patent Application 43 21 399 Al (Voith), a solution is described in which there is a transfer of the web taking place from a wire or felt, at least one shoe press following directly after the transfer of the web, and one press felt running through the first press nip. The solution is characterized in that an endless transfer belt impermeable to water is used for the transfer of the web. In this solution, in the transfer of the web, pre-pressing taking place between two rolls is not employed. In the (German) E»E Patent Application 43 21 406 Al (Voith) , a solution of a press section is described in which there are several press nips formed between press rolls and backup rolls, a transfer belt that is impermeable to water and that has a closed, smooth upper face and that forms an endless loop, which loop always runs around one press roll in each of two successive press nips. The solution is characterized in that the first two press rolls that remain inside a loop, together with their backup rolls or shoe units, form the first two press nips in the press section, and of the first and the second press roll, the transfer felt runs around at least one of said rolls over a distance of at least 90°. In this solution, in the transfer of the web, pre-pressing taking place between two rolls is not employed. By means of the solution in accordance with the present invention, the above prior- art solutions can be improved further. In the solution in accordance with the invention, it is also possible to reduce the number of flat suction boxes in the web former. As a result of this, the friction produced by the suction boxes and abrading the wire becomes lower, and the service life of the wire becomes longer. The principal characteristics of the method in accordance with the present invention are given in the characterizing part of claim 1 , and the principal characteristics of the press section in accordance with the present invention are given in the characterizing part of claim 5. The invention will be described in the following with reference to the figures in the accompanying drawings, wherein a preferred embodiment of the invention is illustrated, the invention being, however, not supposed to be confined to the details of said embodiment alone. Figure 1 is a schematic illustration of the wet end of a paper machine and of the initial end of the press section in accordance with the invention. Figure 2 is a schematic enlarged illustration of the transfer of the web in accordance with the invention from the wet end of the paper machine to the initial end of the press section. Fig. 1 is a schematic illustration of a twin-wire gap former in a paper machine, comprising a lower wire 17 and an upper wire 15, the headbox 11 of the paper machine feeding the stock suspension into the forming gap G defined by said wires. The forming gap G is defined between the runs of the wires 17,15 guided by the lower wire 17, by the breast roll 12 placed inside the loop of the upper wire 15 and by the forming-suction roll 13 placed inside the loop of the lower wire 17. After the curved twin-wire forming zone, there follows a zone provided with suction boxes 18, 19. At the end of the twin-wire zone, there is the reversing roll 16 of the upper- wire loop 15 and the suction roll 14 of the lower-wire loop 17. The suction roll 14 includes at least one suction zone, preferably two separate suction zones a, b, whose vacuum levels can be regulated independently from one another. On the first suction zone a of the suction roll 14 the web W Q is separated from the upper wire 15, and on the second suction zone b the web W 0 is made to adhere to the outer face of the lower wire 17 placed on the face of the suction roll 14. The suction roll 14 may also include three separate suction zones. In the other respects, the wet end involves gap former technology in itself known, which will not be described in more detail in this connection. From the point of view of the invention, the type/construction of the wet end in itself has no decisive significance, and, in principle, the invention can be applied both in a paper machine and in a board machine. In the end of the wet end, there must, however, be a suction roll 14 provided with at least one suction zone, at which roll a pre-press and transfer nip NP 0 can be formed by means of a backup roll 20. After the twin-wire zone, the dry solids content kg of the web W Q is, depending on the paper grade, as a rule, about 10...20 %. Fig. 2 illustrates the pre-pressing of the web (W 0 ) and the transfer of the web into the press section. In the initial end of the press section, there is a pre-press roll 20, which forms a pre-press and transfer nip NP 0 with the suction roll 14 of the web former. The pre-press roll 20 is preferably a solid-mantle smooth-faced 20a roll, whose radius R 2 is considerably smaller than the radius R j of the suction roll 14 of the web former. The ratio of the radii R1 R2 is preferably about 2...5. The transfer belt 24 imposes a requirement of minimum diameter on the pre-press roll 20, which minimum diameter is about 500 mm. If necessary, the outer mantle 20a of the pre- press roll 20 can also be made of a resilient material. After the pre-press and transfer nip NP 0 , there follows the first press zone NP j proper, which is an extended-nip zone and is formed between the shoe press roll 21 and the backup roll 23. In the extended nip NP- , above the web W, there is a transfer belt 24 substantially not receiving water, and below the web W j there is a press felt 25 which receives water. In the pre-press and transfer nip NP 0 , considerable amounts of water are transferred from the paper web W 0 , with the aid of the vacuum in the latter suction zone b in the suction roll 14, through the lower wire 17 in one direction into the latter suction zone b in the suction roll 14 of the web former. In the pre-press and transfer nip NP 0 , the web W 0 is also made to adhere to the smooth outer face of the transfer belt 24 and passed on support of the transfer belt 24 into the first press zone NP j proper. After the web W 0 has been made to adhere to the outer face of the transfer belt 24 in the pre-press nip NP 0 , the web W j is first turned substantially to the horizontal plane while following the curve radius R 2 of the outer face 20a of the pre-press roll 20, after which the web W j is transferred substantially in the horizontal plane, on support of the outer face of the transfer belt 24, into the first press zone NP j proper. The first suction zone a on the suction roll 14 of the web former is in the sector of the suction roll 14 starting from the point at which the inner wire 17 meets the outer circumference of the suction roll 14 for the first time, and ends at a short distance, preferably a few centimetres, from the point at which the outer wire 15 is separated from the outer circumference of the suction roll 14. The second suction zone b on the suction roll 14 is in a sector that starts from the point at which the first suction zone a ends, and ends right before the beginning of said pre-press and transfer nip NP 0 . In the suction zones a, b of the suction roll 14 of the web former and in the prepress and transfer nip NP 0 , water is drained from the web W 0 to such an extent that the dry solids content Δk=k 1 -k 0 of the web W j becomes about 7...10 percentage units higher. After the pre-press and transfer nip NP 0 , the dry solids content of the web W j is about 20...30 % . In the pre-press and transfer nip NP 0 , a linear load of maximally about 100 kN per metre is used. The first press zone NP j proper is preferably an extended-nip zone. In the extended nip NP j , the lower roll is a hose roll 21 provided with a pressure-loaded press shoe 22, and the upper roll is a hollow-faced 23 a press roll 23. The outer face of the belt mantle 21a on the roll 21 can be hollow or smooth. In some cases, it is possible to substitute for the extended-nip zone NP j by a corresponding roll nip. In the first extended nip NP j , water continues to be removed from the web W j in one direction only, in the extended nip NP j into the press felt 25 placed below the web W j . In the extended nip NP j , the dry solids content of the web W j becomes about 20...30 percentage units higher. In the extended nip NP j , maximally a linear load of about 1500 kN per metre is used. After the first press zone NP j proper, there follows at least a second press zone, after which the web W is transferred as a closed draw into the dryer section. The second press zone is not shown in the figures. In respect of the second and/ or more press zones following after the first press zone and of the other parts included in the press section, reference is made to the applicant's said FI Patent Application No. 954698. In the invention, an essential component is the transfer belt 24 substantially not receiving water and arranged in the way described above. It is characteristic of this transfer belt 24 that it is substantially impermeable, i.e. either does not receive water at all or receives water just to a little extent. An important feature is also the capability of adhesion of the transfer belt 24 so that it can separate the web W Q directly after the pre-press zone or equivalent without risk of rewetting. This capability of adhesion is partly based on the smooth or almost smooth outer face of the transfer belt 24 and on the choice of its material. The transfer belt 24 is substantially non-extensible. As the material of the transfer belt 24, it is possible to use various synthetic materials, and it may be reinforced by means of a metal-composite or a fabric. The thickness of the transfer belt 24 is, as a rule, dimensioned in the range 1...5 mm so that it endures bending, compression pressures in the various nips, doctoring, and cleaning by means of a high-pressure water jet. For example, the transfer belt sold with the Transbelt™ product name is suitable for this purpose. 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: Claims 1. A method for dewatering of a paper or board web (W) and for passing the web as a closed draw from the lower wire (17) of the web forming section formed between a lower wire (17) and an upper wire (15) to the press section and through one or several dewatering press nips (NP j ) in said press section, characterized in that the web (W) is separated from the upper wire (15) at the end of the web former and affixed to the lower wire (17) on a suction block of the suction roll (14) placed inside the lower-wire loop (17), after which the web (W 0 ) is affixed, in a transfer and pre-press nip (NP 0 ) formed by said suction roll (14) and by a smooth-faced (20a) roll (20), onto the outer face of a transfer belt (24) substantially not receiving water, the web (W 0 ) being passed on support of said belt (24) into the first press nip (NP j ) proper, in which nip water is pressed out of the web (W j ) into the press felt (25), which is placed, in the press nip (NP j ), at the opposite side of the web (W ) in relation to the transfer belt (24). 2. A method as claimed in claim 1, characterized in that the web (W) is separated from the upper wire (15) in the final end of the web former on the first suction block (a) provided inside the suction roll (14) placed inside the lower-wire loop (17), and the web is affixed to the lower wire (17) on the second suction block (b), which follows after the first suction block (a) of said suction roll (14). 3. A method as claimed in claim 1 or 2, characterized in that, in the pre-press and transfer nip (NP Q ) consisting of a suction roll (14) and of a smooth-faced (20a) press roll (20), a substantial amount of water is drained from the web (W 0 ) primarily in one direction only through the lower wire (17) into the suction roll (14), and that, at the same time, the web (W 0 ) is affixed reliably to the outer face of the transfer-belt loop (24). 4. A method as claimed in claim 3, characterized in that, in the suction zone or suction zones (a, b) of the suction roll (14) and in the pre-press and transfer nip (NP 0 ), water is drained from the web (W 0 ) totally to such an extent that the dry solids content of the web (W j ) is about 20...30 %. 5. A press section of a paper or board machine, comprising a number of successive press zones, the web (W) having been transferred into the first press zone (NP j ) of said press zones as a closed draw from the lower wire (17) of the web former section formed between the lower wire (17) and the upper wire (15) in the paper or board machine, and the web (W) to be pressed being transferred between the different zones in said press section as a supported and closed draw, and from which press zones, after the last press zone, the web (W) has been transferred into the dryer section of the paper or board machine as a closed draw, characterized in that the press section includes a pre-press and transfer nip (NP 0 ) formed between the suction roll (14) of the web former and the smooth-faced (20a) press roll (20), that the press section includes a transfer belt loop (24), which does substantially not receive water and whose outer face is capable of adhering to the paper web, that said transfer belt loop (24) has been passed through said pre-press and transfer nip (NP 0 ), that in said pre-press and transfer nip (NP 0 ) the web (W 0 ) is made to adhere to the outer face of the transfer belt loop (24) and is, after said pre-press and transfer nip (NP 0 ), substantially immediately separated from the lower wire (17) without substantial rewetting of the web (W 0 ), and that on said transfer belt (24) the web (W ) has been passed as a closed and supported draw through the first press zone (NP j ) in the press section. 6. A press section as claimed in claim 5, characterized in that the suction roll (14) of the web former includes two suction zones (a, b) one after the other, the web (W) being separated from the upper wire (15) by means of the first suction zone (a) and being kept on the suction roll (14) on the outer face of the lower wire (17) by means of the second suction zone (b) of the suction roll (14). 7. A press section as claimed in claim 6, characterized in that the first suction zone (a) of the suction roll (14) of the web former is placed in a sector which starts from the point at which, in the running direction (S) of the web (W), the inner wire (17) meets the outer circumference of the suction roll (14) for the first time, and ends at a short distance from the point at which the outer wire (15) is again separated from the outer circumference of the suction roll (14), and the second suction zone (b) is placed in a sector which starts from the point at which the first suction zone (a) ends, and ends directly before the pre-press and transfer nip (NP 0 ).

Inventors: HAAVANLAMMI ARTO, PUURTINEN ARI, VIERTOLA MIKA, HAAVANLAMMI, ARTO, PUURTINEN, ARI, VIERTOLA, MIKA
Applicants: VALMET CORP, VALMET CORPORATION
Application year: 1998
Main classification: D21F3
IPC codes: D21F2, D21F3