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

Abstract

The press section comprises at least two separate press nips (N 1, N 2 ) through which a web (W) to be pressed is passed as a closed draw. The first press nip of the press section is a pre-press nip (N 1 ) formed between a roll ( 11 ) placed inside a forming wire loop ( 12 ) and a roll ( 21 ) placed outside the forming wire loop ( 12 ), which pre-press nip is an extended nip. The second press nip of the press section is a press nip proper (N 2 ) provided with two water-receiving press felts ( 34, 42 ), through which nip the web (W) runs between said press felts ( 34, 42 ). An upper press fabric in the pre-press nip (N 1 ) is formed of a water-receiving press fabric ( 24 ) lying against the outer surface of the water-receiving press fabric ( 24 ), in which connection the web (W) runs in the pre-press nip (N 1 ) between the lower forming wire ( 12 ) and the upper water-permeable press fabric ( 26 ). The invention also relates to a method for removing water from a web in a press section provided with a pre-press nip.

1 . A press section of a board machine provided with a pre-press, which press section comprises at least two separate press nips (N 1 , N 2 ) through which a web (W) to be pressed is passed as a closed draw, in which connection the first press nip of the press section is a pre-press nip (N 1 ) formed between a roll ( 11 ) placed inside a forming wire loop ( 12 ) and a roll ( 21 ) placed outside the forming wire loop ( 12 ), which pre-press nip is an extended nip, and the second press nip of the press section is a press nip proper (N 2 ) provided with two water-receiving press felts ( 34 ,  42 ), through which nip the web (W) runs between said press felts ( 34 ,  42 ), characterised in that an upper press fabric in the pre-press nip (N 1 ) is formed of a water-receiving press felt ( 24 ) lying against the outer roll ( 21 ) and of a water-permeable wire ( 26 ) lying against the outer surface of the water-receiving press felt ( 24 ), in which connection the web (W) runs in the pre-press nip (N 1 ) between the lower forming wire ( 12 ) and the upper water-permeable wire ( 26 ).  
     
     
         2 . A press section according to  claim 1 , characterised in that the outer roll in the pre-press nip (N 1 ) is a shoe roll ( 21 ) provided with a press shoe arrangement ( 23 ) and with a belt shell ( 22 ) and the inner counter roll in the pre-press nip (N 1 ) is a hollow-faced ( 11   a ) roll ( 11 ), in which connection removal of water in the pre-press nip (Nl) takes place through the forming wire ( 12 ) into the hollow-faced ( 11   a ) shell of the counter roll ( 11 ) and through the upper wire ( 26 ) into the press felt ( 24 ).  
     
     
         3 . A press section according to  claim 2 , characterised in that the belt shell of the outer roll in the pre-press nip (N 1 ) is hollow-faced ( 22   a ), in which connection water is also removed through the press felt ( 24 ) into the hollow-faced ( 22   a ) belt shell ( 22 ) of the shoe roll ( 21 ).  
     
     
         4 . A press section according to  claim 1 , characterised in that the inner roll in the pre-press nip (N 1 ) is a shoe roll ( 21 ) provided with a press shoe arrangement ( 23 ) and with a hollow-faced ( 22   a ) belt shell ( 22 ) and the outer counter roll in the pre-press nip (N 1 ) is a smooth-faced roll ( 11 ), in which connection removal of water in the pre-press nip (N 1 ) takes place through the forming wire ( 12 ) into the hollow-faced ( 22   a ) belt shell ( 22 ) of the shoe roll ( 21 ) and through the upper wire ( 26 ) into the press felt ( 24 ).  
     
     
         5 . A press section according to any one of  claims 1  to  4 , characterised in that the second press nip after the pre-press nip (N 1 ) is an extended nip (N 2 ).  
     
     
         6 . A press section according to any one of  claims 1  to  5 , characterised in that the second press nip (N 2 ) is formed between a shoe roll ( 21 ) provided with a hollow-faced ( 22   a ) belt shell ( 22 ) and a hollow-faced ( 41   a ) counter roll ( 41 ).  
     
     
         7 . A press section according to any one of  claims 1  to  5 , characterised in that the second press nip (N 2 ) is formed between a shoe roll ( 21 ) provided with a smooth-faced belt shell ( 22 ) and a smooth-faced counter roll ( 41 ).  
     
     
         8 . Method for removing water from a web (W) in a press section of a board machine provided with a pre-press, in which press section the web (W) to be pressed is passed as a closed draw through at least two separate press nips (N 1 , N 2 ), in which connection the first press nip of the press section is a pre-press nip (N 1 ) formed between a roll ( 11 ) placed inside a forming wire loop ( 12 ) and a roll ( 21 ) placed outside the forming wire loop ( 12 ), which pre-press nip is an extended nip, and the second press nip of the press section is a press nip proper (N 2 ) provided with two water-receiving press felts ( 34 ,  42 ), through which nip the web (W) runs between said press felts ( 34 ,  42 ), characterised in that the web (W) runs in the pre-press nip (N 1 ) between the lower forming wire ( 12 ) and an upper water-permeable wire ( 26 ), which water-permeable wire ( 26 ) in turn lies against the outer surface of a water-receiving press felt ( 24 ) lying against the outer roll ( 21 ).