Patent Publication Number: US-2004055723-A1

Title: Press section of a board machine provided with a pre-press and method for removing water from a web in such a press section

Description:
[0001] The invention relates to a press section of a board machine provided with a pre-press according to the preamble of claim 1.  
       [0002] The invention also relates to a method for removing water from a web in a press section of a board machine provided with a pre-press according to the preamble of claim 8.  
       [0003] In this application, a separate press nip means a press nip in which opposing press members form only one press nip. The separate press nip may be a roll nip or an extended nip. As a contrast to a separate press nip there is a press which is provided with a centre roll and in which the centre roll forms press nips with at least two other rolls.  
       [0004] The function of the press section is to remove water from a web and make the web symmetric, so that the absorption and roughness properties of both sides of the web are symmetric. In addition, the press section should be simple and efficient enough in order that it might be applied to several board grades.  
       [0005] At high speeds in particular, the travel of the web in the press section should be as straight as possible. The water-containing and weak web arriving at the press section does not withstand the dynamic forces produced by high speeds and changes of direction, but there are web breaks and other operational malfunctions, which cause shutdowns.  
       [0006] The applicant&#39;s U.S. Pat. No. 5,792,320 discloses a method for removing water from a paper or board web by pressing. In FIG. 2 of the patent, a pre-press nip is formed between a hollow-faced lower roll placed inside the loop of a forming wire and an upper shoe roll placed outside the loop of the forming wire. In the pre-press nip, a substantially non-water-receiving, smooth-faced transfer belt runs on the side of the upper shoe roll, by means of which belt the web is transferred from the pre-press nip onto a lower press felt of the first press nip proper. Removal of water takes place in the pre-press nip, which forms an extended nip, only in one direction, i.e. through the forming wire into the shell of the hollow-faced roll.  
       [0007] The applicant&#39;s U.S. Pat. No. 5,522,959 discloses a press section intended in particular for printing paper qualities. In FIG. 6 of the patent, a pre-press nip is formed between a lower hollow-faced roll placed inside the loop of a forming wire and an upper shoe roll placed outside the loop of the forming wire. In the pre-press nip, a press felt runs on the side of the upper shoe roll. Water is removed in the pre-press nip, which forms an extended nip, in two directions, i.e. through the forming wire into the shell of the lower hollow-faced roll and into the upper press felt. The transfer of the web to the press section proper takes place on a pick-up roll from the forming wire after the pre-press nip.  
       [0008] The applicant&#39;s U.S. Pat. No. 5,820,731 discloses a method and an apparatus for removing water from a web in a paper or board machine. In FIG. 3 of the patent, a first pre-press nip is formed between a hollow-faced first lower roll placed inside the loop of a forming wire and a smooth-faced first upper roll placed outside the loop of the forming wire. A second pre-press nip is formed between a smooth-faced second lower roll placed inside the loop of the forming wire and a hollow-faced second upper roll placed outside the loop of the forming wire. The web runs in both pre-press nips between the lower forming wire and an upper wire running over the upper rolls. Removal of water takes place in the first pre-press nip mainly through the forming wire into the shell of the first lower hollow-faced roll and in the second pre-press nip mainly through the upper wire into the shell of the second upper hollow-faced roll. Both pre-press nips are roll nips. The transfer of the web to the press section proper takes place on a pick-up roll from the forming wire after the second pre-press nip.  
       [0009] It is an object of the present invention to provide a press section of a board machine provided with a pre-press, which press section is particularly suitable for the manufacture of board grades having a high basis weight. By a high basis weight is meant here a basis weight which is over 100 g/m 2 , advantageously over 200 g/m 2 .  
       [0010] The press section according to the invention is mainly characterised by the characterising part of claim 1.  
       [0011] The method for removing water in a press section according to the invention is mainly characterised by the characterising part of claim 8.  
       [0012] In the press section in accordance with the invention there are at least two separate press nips. A pre-press nip serving as a first press nip is formed between an inner roll placed inside the loop of a forming wire and an outer roll placed outside the loop of the forming wire. The pre-press nip is an extended nip which is formed between a shoe roll and a counter roll. An extended nip provides a higher dry solids content than a roll nip because of a longer nip and a higher linear load.  
       [0013] Two separate press fabrics are used as an upper press fabric in the pre-press nip. A press fabric that receives water very well is placed against the outer roll and a press fabric that is highly permeable to water is placed against the outer surface of this water-receiving press fabric. The press fabric which receives water very well is advantageously a press felt and the press fabric which is highly permeable to water is advantageously a wire. In that connection, the web runs between two wires in the pre-press nip.  
       [0014] By using a separate wire loop on the outside of the upper press felt loop in the pre-press nip, contamination of the upper press felt can be substantially reduced. This is particularly important when board grades having a high basis weight are manufactured at high speed. The dry solids content of a board web is, depending on running speed, basis weight and the quality of pulp, generally below  20 % when it enters the pre-press nip. In the pre-press nip, the dry solids content of the web rises by about 10-15%.  
       [0015] The press section in accordance with the invention is also not sensitive to variations in the quality of pulp nor to the abrasive effects of pulp. Because of the upper wire, the pulp cannot come into direct contact with the upper press felt in the pre-press nip.  
       [0016] The wire loop placed on the outside of the upper press felt also reduces the rewetting of the web after the pre-press nip. The water bound into the upper press felt in the pre-press nip cannot be absorbed through the upper wire back into the web after the pre-press nip.  
       [0017] On the press section in accordance with the invention, a board web can be run whose basis weight varies in a range of about 100 to 500 g/m 2 . A board web having a basis weight of about 400 g/m 2  and a dry solids content of about 20% before the pre-press nip can be run at a speed of about 600 m/min, in which case the production capacity of the press will be about 4 kg/sm.  
       [0018] In the method in accordance with the invention, water is removed from the web in a press section of a board machine provided with a pre-press. The web to be pressed is passed as a closed draw through at least two separate press nips. The first press nip of the press section is a pre-press nip formed between a roll placed inside the loop of a forming wire and a roll placed outside the loop of the forming wire, which pre-press nip is an extended nip. The second press nip of the press section is a press nip proper which is provided with two water-receiving press felts and through which the web runs between said press felts. The web runs in the pre-press nip between the lower forming wire and an upper water-permeable press fabric, which water-permeable press fabric is in turn positioned against the outer surface of a water-receiving press fabric lying against the outer roll. 
     
    
    
     [0019] In the following, the invention will be described in detail with reference to the embodiments of the invention shown in the appended figures, to the details of which embodiments the invention is, however, not by any means narrowly confined.  
     [0020]FIG. 1 is a schematic view of a press section of a board machine provided with one pre-press nip and with one press nip proper in accordance with the invention.  
     [0021]FIG. 2 is a schematic view of a variant of the press section shown in FIG. 1 in accordance with the invention.  
     [0022]FIG. 3 is a schematic view of a second variant of the press section shown in FIG. 1 in accordance with the invention.  
     [0023]FIG. 4 is a schematic view of a third variant of the press section shown in FIG. 1 in accordance with the invention. 
    
    
     [0024]FIG. 1 shows a press section of a board machine provided with two separate press nips N 1  and N 2  in accordance with the invention. The first press nip is a pre-press nip N 1  and the second press nip is a press nip proper N 2 .  
     [0025] The pre-press nip N 1  is formed of an upper first shoe roll  22  and of a lower first counter roll  11  provided with a hollow-faced  11   a  shell. The first counter roll  11  is located inside a forming wire loop  12  and the shoe roll  22  is located outside the forming wire loop  12 . An upper first press felt  24  runs around the shoe roll  21  as an inner loop and an upper wire  26  runs around the shoe roll  21  as an outer loop. On the inner press felt loop  24  there are conditioning devices  25  for the press felt  24 .  
     [0026] The press nip proper N 2  is formed of a lower second shoe roll  31  and of an upper second counter roll  41  provided with a hollow-faced  41   a  shell. The second shoe roll  31  is located inside a lower second press felt loop  34  and the second counter roll  41  is located inside an upper third press felt loop  42 . On the lower second press felt loop  34  there are conditioning devices  35  for the press felt  34  and, in a corresponding manner, on the upper third press felt loop  42  there are conditioning devices  43  for the press felt  42 .  
     [0027] The structure of the shoe rolls, i.e. hose rolls  21 ,  31  used in the press nips N 1 , N 2  is known in itself, comprising a flexible hose shell  22 ,  32 , inside which there is a press shoe  23 ,  33  which can be loaded with a hydraulic pressure medium and which provides the necessary compression pressure on a web W running through the nips N 1 , N 2 . In this kind of structure, the hose shell is advantageously hollow-faced  22   a ,  32   a , but it can also be smooth-faced.  
     [0028] The web W is brought into the pre-press nip N 1  on the forming wire  12  over suction boxes  13  placed at the end of a forming section. The web W runs in the pre-press nip N 1  forming an extended nip between the lower forming wire  12  and the upper wire  26 . The upper first press felt  24  additionally runs between the upper wire  26  and the hollow-faced belt shell  22  of the first shoe roll  21 . Removal of water takes place in the pre-press nip N 1  in two directions, i.e. through the lower forming wire  12  into the hollow-faced  11   a  shell of the lower first counter roll  11  and through the upper wire  26  into the upper first press felt  25  as well as into the hollow face of the belt shell  22  of the upper first shoe roll  21 .  
     [0029] After the pre-press nip N 1 , the wires  12 ,  26  and the press felt  24  follow some distance the outlet edge of the shoe  23 , after which the lower forming wire  12  is separated from the upper wire  26 . The angle α between the nip plane T of the pre-press nip N 1  and the running direction S of the web W after the nip N 1  is an acute angle, in which connection the web W follows the upper wire  26  after the pre-press nip N 1 . The adherence and sticking of the web W to the bottom surface of the upper wire  26  after the pre-press nip N 1  is enhanced by means of a suction box  27  placed inside the upper first press felt loop  24 . After that, the web W is passed on support of the bottom surface of the upper wire  26  onwards as a straight draw and transferred at a first transfer point S 1  in a suction zone  36   a  of a first transfer suction roll  36  onto the top surface of the lower second press felt  34  running around said transfer suction roll  36 , the web W being passed on support of said top surface into the press nip proper N 2 .  
     [0030] The figure also shows dropping jets  28 , by which the web W can be detached from the upper wire  26  and run as broke into a pulper located beneath the press section.  
     [0031] In the press nip proper N 2  forming an extended nip, the web W runs between the lower second press felt  34  and the upper third press felt  42 . Thus, removal of water in the press nip proper N 2  takes place into the lower second press felt  34  and into the hollow face of the belt shell  32  of the lower second shoe roll  31  as well as into the upper third press felt  42  and-into the hollow face  41   a  of the upper second counter roll  41 .  
     [0032] After the press nip proper N 2 , the upper third press felt  42  is detached from the lower second press felt  34 . In this press nip proper N 2 , the angle between the running direction of the web W after the press nip N 2  and the nip plane is an obtuse angle, in which connection the web W follows the lower second press felt  34  as a straight draw onwards. From the lower second press felt  34  the web W is transferred at a second transfer point S 2  in a suction zone  51  a of a second transfer suction roll  51  onto the bottom surface of an upper drying wire  52  running around said transfer suction roll  51 , on support of which bottom surface the web W is transferred further to a dryer section.  
     [0033]FIG. 2 shows a variant of the embodiment shown in FIG. 1. An upper wire  26  and an upper first press felt  24  are separated from a lower forming wire  12  immediately after a pre-press nip N 1 . The angle α between the running direction S of the web W after the pre-press nip N 1  and the nip plane T is an obtuse angle, in which connection the web W follows the lower forming wire  12  after the pre-press nip N 1 . The web W is detached from the obliquely downward run of the forming wire  12  at a pick-up point P in a suction zone  44   a  of a pick-up suction roll  44  and caused to adhere to the outer surface of an upper third press felt  42  running around the pick-up suction roll  44 . When the running direction of the web W has been turned on the pick-up suction roll  44 , the web W is transferred as a straight draw first some distance merely on support of the bottom surface of the upper third press felt  42  and after that between a lower second press felt  34  and the upper third press felt  42  into a press nip N 2  proper. In other respects, the embodiment shown in FIG. 2 corresponds to the embodiment shown in FIG. 1.  
     [0034]FIG. 3 shows a second variant of the embodiment shown in FIG. 1. This embodiment differs from the embodiment of FIG. 2 in respect of the structure of the pre-press nip N 1  and the press nip proper N 2 . The pre-press nip N 1  is formed of a lower first shoe roll  21  provided with a hollow-faced  22   a  belt shell  22  and of an upper first counter roll  11  provided with a smooth-faced shell. The first shoe roll  21  is now located inside a forming wire loop  12  and the first counter roll  11  is located outside the forming wire loop  12 . An upper first press felt  24  runs around the first counter roll  11  as an inner loop and an upper wire  26  runs around it as an outer loop. The press nip proper N 2  is formed of a lower second shoe roll  31  provided with a smooth-faced belt shell  32  and of an upper second counter roll  41  provided with a smooth-faced shell. In other respects, this embodiment corresponds to the embodiment shown in FIG. 2.  
     [0035]FIG. 4 shows a third variant of the embodiment shown in FIG. 1. This embodiment differs from the embodiment of FIG. 3 in respect of the pre-press nip. The pre-press nip N 1  is formed of a lower hollow-faced  11   a  first counter roll  11  and of an upper first shoe roll  21  provided with a smooth-faced belt shell  22 . The nip plane of the pre-press nip N 1  forms here an angle of about 90° to the running direction of the web W after the pre-press nip N 1 .  
     [0036] The travel of the web through the press section is very linear in particular in the embodiment shown in FIG. 1, in which connection the web is not subjected to high dynamic forces caused by changes in the ruing direction. The web thus adheres very well to the surface which supports it, such as a wire or a press felt.  
     [0037] In the press sections shown in the figures there are only two press nips, i.e. a pre-press nip N 1  and a press nip proper N 2 . However, in addition to these two press nips N 1 , N 2 , the press section in accordance with the invention may include one or more press nips after the second press nip N 2 . If, for example, the basis weight of the web W to be formed is high, a third press nip can be used to raise the dry solids content of the web W before the web W is transferred to the dryer section.  
     [0038] In the press sections shown in the figures, the shoe roll  31  is always the lower roll of the press nip proper N 2 , but the shoe roll  31  may also function as the upper roll of the press nip proper N 2 , in which case the lower roll of the press nip N 2  is a counter roll.  
     [0039] The maximal linear load used in the pre-press nip N 1  in accordance with the invention depends on the structure of the counter roll  21  and of the wires  12 ,  26 . Linear loads of at least the order of about 600 kN/m are possible. In the press nip proper N 2  it is possible to use linear loads of at least the order of 1500 kN/m. The degree of openness of the hollow face  22   a ,  32   a  of the belt shell  22 ,  32  of the shoe rolls  21 ,  31  is in a range of 15 to 35%, advantageously 25%, and the degree of openness of the hollow face  11   a ,  41   a  of the counter rolls  11 ,  41  is in a range of  40  to 70%, advantageously 60%.  
     [0040] The wire  26  or an equivalent press fabric serving as the outer upper fabric in the pre-press nip N 1  shall be a thin fabric which is highly permeable to water but rigid enough to function. The press felt  24  or an equivalent press fabric serving as the inner upper fabric in the pre-press nip N 1  shall in turn be a fabric which receives water very well, which has enough space for water and whose weight is over 2000 g/m 2 .  
     [0041] The claims are presented in the following and the various details of the invention may vary within the inventive idea defined by said claims and differ from the disclosure given above by way of example only.