Device in a paper machine in guiding the leader of a web

Abstract: Device in a paper machine in guiding the web (W) and in particular in guiding the leader (R) of the web. The device operates in a paper machine in the area between the centre roll (12) of the press section and a separate press nip (N₄) in the press section. The separate press nip (N₄) is formed between a smooth-faced upper roll (25) and a hollow-faced lower roll (26). Around this lower roll (26), the lower felt (17) of the separate press is placed, said lower felt (17) running from a suction-transfer roll (16) placed at the proximity of the centre roll (12) as a straight run into the separate press nip (N₄). At the initial end of the straight run of the felt (17) of the separate press nip (N₄), in connection with the guide roll (16), air-transfer devices (30,40,50) are fitted, by whose means a reduced pressure is produced at least across the width of the leader (R) in connection with the closing inlet nip (N) between said felt (17) and the leader (R) and/or in connection with the opening outlet nip (N a ) between said guide roll (16) and the straight run of the felt (17).

Description: The invention concerns a paper machine according to the preamble of claim 1. Such a paper machine is known from EP-A-0 401 190. As is well known, when a paper machine is started or after a web break, the end of the web is passed through the dryer section by from the web, for example by means of a water jet, against the centre roll of the press, cutting a narrow leader or lateral band, typically of a width of about 200 mm, which leader was earlier usually guided manually by means of air jets. The constantly increasing running speeds of paper machines have resulted in increasing difficulties in the threading of the web. These difficulties are worst, when a separate press is employed, inside the press section, immediately after the press section, in the initial part of the dryer, and in the gaps between the groups of drying cylinders. In prior art, press sections are used that comprise a compact combination of press rolls consisting of two or three press nips placed closed to each other, the web having a closed draw between said nips. After said combination of rolls, a third or fourth, separate press follows, which is formed between a smooth-faced upper roll and a hollow-faced lower roll. Through this separate nip, a lower felt is passed, said hollow-faced lower press roll being placed inside the loop of said lower felt. The web of full width and the leader are guided from the smooth-faced centre roll of the compact combination of rolls as a short free draw, as a rule, over a guide roll onto the upper face of the felt in said separate nip. On the upper roll in a separate press, a broke conveyor and a doctor are commonly employed, by means of which doctor the lateral band (leader) that attempts to be wound around the upper roll is shifted to the side in the transverse direction. This broke conveyor is placed above the substantially horizontal run of the felt of the separate press, said run being guided by a suction-transfer roll into the separate nip. On said run, immediately before the separate nip, a suction box is also employed, by whose means it is ensured that the web and the leader remain on the top face of the felt. Below said broke conveyor, there is, as a rule, a wall, which is substantially parallel to the run of the felt that runs underneath said wall. Said wall and the straight felt run define a gap between them, which gap is a difficult problem point in the threading of the web because of the air flows induced in said gap and because of the dynamic pressure effects of said air flows. Having been separated from the centre roll and when blown further, the lateral band does not adhere to the felt of the separate press, but air remains between the lateral band and the felt. Separation of the web from the felt is also promoted by said strong air current that takes place in the gap defined by said felt and by the bottom wall of the broke conveyor placed above the felt, which air current attempts to absorb the web towards the bottom wall of the broke conveyor. These problems have become more serious with increasing running speeds of paper machines. These problems of threading are also increased by the fact that, in the threading of the leader, the suction zone on the suction roll of the lower felt of the separate press cannot be utilized efficiently in order to made the leader adhere to the lower felt, because the suction cannot be applied effectively to the area of the leader, for the suction zone "leaks" over the major part of its width, in which there is no sealing web. A further problem is constituted by the bag formations arising from the slackness of the leader after the separation from the centre roll, and by other, corresponding problems. In a way known from prior art, attempts have been made to reduce the slackness of the leader after the centre roll in the press by means of a difference in the running speed of the felt of the separate press, but, with increasing running speeds of paper machines, a sufficient reserve for an increase in the running speed in this respect is, as a rule, not available. The object of the present invention is to provide novel solutions for the problems discussed above. In respect of the prior art related to the present invention, reference is made to the applicant's FI Patent 69,145 (equivalent of US-A-4,692,215) and FI Patent 78,528 (equivalent of US-A-4,923,567) and to the FI Patent FI-B-86 901 (published on 15/07/92), in which attempts have been made to solve the problems discussed above. In said FI-B-86 901, a device in a paper machine similar to that defined above is described for the threading of the leader of the web, in which device it has been considered novel that, before the separate press nip, on the straight run of the felt of the separate press nip, a transverse blow pipe or pipes is/are fitted, from which blowings are applied in a direction opposite to the running direction of the felt and of the web, by means of which blowings induction of air into the gap space between said straight run of the felt and the wall placed above said run is reduced and, at the same time, the staying of the leader on the face of the felt on said straight run of the felt before the separate press nip is promoted. An object of the present invention is to provide novel solutions for the problems that have been dealt with in said FI-B-86 901. It is a particular object of the invention to provide a device which guides the lateral band to the separate press and by whose means the lateral band can be made to adhere to the upper face of the felt of the separate press so that any slackness of the lateral band can be eliminated as it arrives from the centre roll of the press. After this, the paper guide roll placed at the proximity of the centre roll can, if necessary, be shifted to a lower position and the web be spread by means of a diagonal cutter to full width. In view of achieving the objectives stated above and those that will come out later, the paper machine according to the invention is characterized by the features of the characterizing clause of claim 1. 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 drawings, the invention being by no means strictly confined to the details of said embodiments. Figure 1 is a schematic side view of a press section provided with a separate press. Figure 1A is an axonometric view, seen in the direction of the arrow indicated in Fig. 1, of the suction box of the suction-transfer roll with its different suction zones. Figure 2 shows a threading device in accordance with the invention as placed at the inlet side of the transfer roll. Figure 3 shows a threading device similar to that shown in Fig. 2 and accomplished by means of a blow pipe. Figure 4 shows a threading device in accordance with the invention in which a suction-transfer roll and a blow device in accordance with the invention fitted at the outlet side of said suction-transfer roll are employed. Figure 5 shows a threading device in accordance with the invention in which blow boxes are employed both at the outlet side and at the inlet side of the suction-transfer roll, together with the environment of application. The paper web W is brought from the pick-up point P on the lower face of the upper felt 11 into the first press nip N 1 , which is a two-felt nip. After the first press nip N 1 between the rolls 10a and 10b, the web W is transferred on said upper felt 11 into the second nip N 2 . After the second nip N 2 , the web W follows the centre roll 12 of the press, which is provided with a smooth mantle face 12' and in connection with which there is a third press nip N 3 provided with a felt 13. After the third nip N 3 , the web W continues to follow the smooth face of the centre roll 12, from which it is separated by means of the paper guide roll 15 as a short free draw W o . Hereupon the web W is transferred onto the lower felt 17 of the separate press with the aid of the suction zone 16a of the suction-transfer roll 16. After the suction-transfer roll 16, there is a straight run of the felt 17, which is slightly inclined downwards and on which the web W is passed into the separate press nip N 4 , which is formed between a smooth-faced upper roll 25 and a hollow-faced lower roll 26. After the separate nip N 4 , being guided by a guide roll 29, the web W is transferred onto the drying wire 28 of the dryer section. The press section described above is just an example of an environment of application of the lateral-band guide device in accordance with the invention. In the following, with reference to Fig. 2, a preliminary description will be given of the environment of application of the invention. A leader R of a width of, e.g., about 200 mm is cut by means of a diagonal cutter (not shown) from one edge of a web W of full width against the face of the centre roll 12 in a situation in which the web W is separated from the face of the centre roll 12 by means of the blade of a doctor 14 and is passed into the pulper placed underneath. To the lateral band R placed on the cylinder 12 face 12', detaching blowings are applied from the nozzle pipes of the blow device 19 in the way described in the above FI Patent 78,528. At this stage, the paper guide roll 15 may have been shifted to the upper position 15'. The guide plate of the guide device 19 and the blowings provided in connection with same guide and direct the leader R along the path R 1 towards the transfer roll 16. According to Figs. 1 and 1A, a suction box 30 has been fitted in the interior of the suction-transfer roll 16. The suction zone 16a of the suction roll 16 extends across the entire width of the web W in the transverse direction. The suction zone 16 is defined by the longitudinal sealing ribs 32 and by the end seals 33a of the suction box 30. The suction zone 16a has been divided into two zones 31a and 31b in the transverse direction by means of an intermediate sealing rib 33b, of which zones the latter one extends over the width of the leader R or slightly beyond said width. The rib 33b may be arranged so that its position can be adjusted in the direction of the arrow T, so that the width of the suction zone 31b can be adjusted. According to the schematic illustration in Fig. 1A, each of the suction zones 31a and 31b communicates with a suction pump 34 through a pipe duct 36a,36b. The pipe ducts 36a and 36b are provided with valves 35a and 35b, by whose means, when necessary, the suction can be shut off from the wider zone 31a, in which case the entire suction effect of the pump 34 can be concentrated efficiently on the narrow threading zone 31b in connection with the threading of the web. By means of the narrow suction zone 31b, air can be removed efficiently in the critical area after the guide roll 15, so that the leader R can be made to adhere to the upper face of the felt 17 and transferred on said felt reliably into the separate nip N 4 . When a web W of full width is being run, the suction of the suction box 30 may be preferably applied to the entire width of the web W, whereby it is ensured that the web W adheres efficiently to the upper face of the felt 17 and remains on it up to the separate nip N 4 . According to Fig. 2, after and underneath the centre roll 12 of the press, a blow box 40 is fitted, which is placed directly underneath the guide roll 15. In Fig. 2, the guide roll 15 is shown in the upper position 15', to which it has been shifted for the time of the threading. The blowings applied from the blow pipe 19 separate the leader R 1 from the smooth face 12' of the centre roll 12 and guide it over the blow box 40. The blow box 40 is provided with an upper guide face 41, which is substantially plane and which terminates in an edge fold 41a. An extension of said edge fold 41a is constituted by a curved wall 43 of the blow box, which is placed at the distance of a small gap G from the transfer roll 16 and from the felt 17 that runs over said roll. The outer wall 43 and the wall of the blow box 40 define a nozzle duct 44 between them, through which duct air is blown in the direction of the arrow B in the direction opposite to the direction of movement of the transfer roll 16 and of the felt 17 and substantially tangentially in relation to the roll 16. By means of these blowings B, air is ejected in the direction of the arrow E out of the intermediate space G, and flowing of the boundary layer of air, which arrives on the face of the felt 17, into the closing nip N between the felt 17 and the leader R 1 is prevented, which nip is placed at the level of the front edge 41a of the wall 41. In this way, a slight negative pressure is produced in the nip N, whereby it is ensured that the leader R 1 adheres to the upper face of the felt 17. Together with the blow box 40, it is possible to make use of the transfer-suction roll 16 described above and of its suction zones 31a and/or 31b. Fig. 3 shows a version of the invention in which a blow-pipe device 45 has been fitted in the closing nip N between the leader R 2 and the transfer roll 16, a guide plate 46 being placed at the front side of said device 45. The device 45 is provided with a blow pipe 47, through whose nozzle slot 48 or an equivalent series of nozzle holes strong air blowings B are blown tangentially in relation to the mantle of the transfer roll 16 and to the direction of the felt 17 that runs over said mantle. By means of these blowings, an ejection effect E similar to that described above is produced so that the inlet nip N of the band R 2 is subjected to negative pressure. In the other respects, the construction and the operation of the device are similar to that described above in relation to Fig. 2. The blow box 40 and the blow-pipe device 45 are fitted in the transverse direction either over the width of the leader R only, or the devices are extended across the entire width of the web W, in which case it is provided with a separate edge compartment over the width of the leader R in a constructionally and functionally equivalent way as compared with the end zone 31b of the suction box 30 shown in Fig. 1A. Fig. 4 shows an embodiment of the invention and its environment. In the position shown in Fig. 4, the web W is of full width, and the guide roll 15 is in the lower position, in which it forms a transfer nip N s with slight load at the initial end of the suction zone 16a of the roll 16. The web W is separated from the smooth face of the centre roll 12 and passed as a short free draw W o over the guide roll 15 into the transfer nip N s , in which it adheres to the upper face of the felt 17 by the effect of the negative pressure prevailing in the suction zone 16a. According to Fig. 4, a blow box 50 is placed at the rear side of the roll 16, which blow box 50 produces a reduced pressure, in a way corresponding to the blow box 40 in Fig. 2, in the opening nip N a , which is formed between the roll 16 mantle and the straight run of the felt 17 that is separated from said roll 16. The scope of the invention also includes the combination of devices shown in Fig. 5, which comprises a blow box 40 or a blow-pipe device 45, which is placed at the inlet side of the suction-transfer roll 16 and which is similar to that described in Fig. 2 or equivalent, and a second blow box 50, which is fitted in the opening nip N a at the rear side of the transfer-suction roll 16 and which is similar to that described above in relation to Fig. 4. The construction as shown in Fig. 5 is usable when quite high web W speeds are used and when particular difficulties occur in the threading of the web and/or in the staying of the web on the face of the felt 17. In the following, the patent claims will be given, and the different details of the invention may show variation within the scope of the inventive idea defined in said claims and differ from the details described above for the sake of example only.

Claims: Paper machine comprising a press section with a device for guiding a web (W) and in particular for guiding the leader (R) of the web, said press section comprising press rolls, forming press nips with a centre roll (12), from which the web is transferred to the lower felt (17) of a separate press nip (N 4 ) formed between an upper roll and a lower roll, preferably a smooth-faced upper roll and a hollow-faced lower roll (26), which device is fitted in the paper machine in the area between the centre roll (12) and the separate press nip (N4), around which lower roll (26) the lower felt (17) of the separate press is arranged, said lower felt (17) being fitted to run from a guide roll (16), preferably a suction-transfer roll (16), placed at the proximity of said centre roll (12), as a straight run into said separate press nip (N 4 ), characterized in that, within the closing inlet nip (N) between said felt (17) and the leader (R) and/or within the opening outlet nip (Na) between said guide roll (16) and the straight run of the felt (17), as said device such air-transfer means (30, 40, 50) have been arranged, by which a reduced pressure is produced at least across the width of the leader (R) within the closing inlet nip (N) between said felt (17) and the leader (R) and/or within the opening outlet nip (N a ) between said guide roll (16) and the straight run of the felt (17). Paper machine as claimed in claim 1, wherein said guide roll (16) is a suction-transfer roll, characterized in that the suction zone (16a) of the suction box (30) placed inside said suction-transfer roll (16) has been divided into two compartments (31a and 31b) by means of a partition wall (33b), of which compartments the lateral compartment (31b) extends substantially over the width of the leader (R) and the other compartment (31a) extends substantially over the rest of the width of the web (W) (Fig. 1A). Paper machine as claimed in claim 2, characterized in that, in connection with the threading of the web, the entire suction effect of the suction zone (16a) can be concentrated on said lateral compartment (31b). Paper machine as claimed in any of the claims 1 to 3, characterized in that the guide device comprises a blow box (40), which is fitted at the proximity of said guide roll (16) at the inlet side thereof, below the path of running (R 1 ) of the leader (R), which blow box includes a blow nozzle arrangement (44), through which air jets (B) of a direction opposite to the direction of movement of the guide roll (16) and of the felt (17) running over said roll can be applied, by means of which air jets (B) air can be ejected (E) out of the closing inlet nip (N) between the leader (R) and the felt (17), whereby it is possible to promote the adhesion of the leader (R) to, and the staying of the leader (R) on, the upper face of the felt (17). Paper machine as claimed in claim 4, characterized in that the blow box (40) has a substantially plane guide wall (41), over which the path of the leader (R 1 ) runs, that the front edge (41a) of said guide wall (41) is connected with a curved wall (43), which is placed at the proximity of said felt (17) and separated therefrom by a small gap (G), and that the latter wall (43) defines a nozzle opening (44), out of which said air jets (B) are blown. Paper machine as claimed in any of the claims 1 to 3, characterized in that, at the proximity of said guide roll (16), underneath the path (R 2 ) of the leader, within the closing inlet nip (N) between the felt (17) and the leader a blow-pipe device (45) is fitted, whose blow pipe (47) is provided with a nozzle slot or with a series of nozzle holes (48), through which air jets (B) of a direction opposite to the running direction of the felt (17) can be applied, by means of which air jets it is possible to eject air out of the closing inlet nip (N) between the felt (17) and the leader (R 2 ). Paper machine as claimed in claim 6, characterized in that, to said blow pipe (47), a guide plate (46) has been attached, which extends from said blow pipe towards the mantle of the centre roll (12) in the press section. Paper machine as claimed in any of the claims 1 to 7, characterized in that, in the opening nip (N a ) at the outlet side of said guide roll (16) and said press felt (17) running over said roll, a blow device (50) is fitted, by whose means it is possible to lower the pressure level in said opening nip (N a ) and to promote the staying of the leader (R) and possibly also of the web (W) of full width on the top face of said press felt (17) as it runs into said separate nip (N 4 ).

Inventors: HIRSIMAEKI MARTTI, KIVIMAA JUHA, HIRSIMAEKI, MARTTI, KIVIMAA, JUHA, Hirsimäki, Martti
Applicants: VALMET CORP, VALMET CORPORATION
Application year: 1993
Main classification: D21F3
IPC codes: D21F3, D21G9