A METHOD AND A DEVICE IN THE PRESS SECTION OF PAPERMAKING MACHINES

Abstract: A method and a device for controlling a paper web (W) which is leaving a press nip in the press section of a papermaking machine. The paper web (W) is intended to follow a felt (7, 11) after the press nip. Control of the paper web is achieved in accordance with the teachings of the invention by preventing air admission to the side of the felt (7, 11) facing away from the paper web during the expansion phase of said felt (7, 11) after the press nip, whereby a pressure is generated, sucking the paper web (W) against the felt (7, 11). Air admission to the felt (7, 11) is prevented by means of an impermeable belt (9, 12) which is arranged to travel in a separate loop at the same speed as the felt (7, 11). The belt (9, 12) extends closely adjacent the felt (7, 11) against the side thereof facing away from the web (W) through one or several press nips and over a distance following said press nip/nips.

Description: A Method and a Device in the Press Section of Paper aking Machines. The invention relates to a method of controlling the paper web when leaving the press nip, and to a device for performing the method. A paper web is dewatered when passing through a press nip together with one or several felts and/or belts. In one type of press nip the paper web is running through the press nip between two press felts, known as a double-felted press nip. After the press nip the paper web should follow one of the felts. It is sometimes a problem to control the paper web path, and the paper web may sometimes also have a tendency to follow the other felt, especially after a stop or break of the paper web. The higher speeds of modern papermaking machines have made the problem more pronounced. Another reason for the problem is that all the felts are not replaced at the same time but individually, depending on felt wear. The paper web tends to follow the felt having the smoothest surface in general the felt which has been in operation for the longest period of time. The problem is more obvious in the case of tail-threading through the press nip or after web breaks. In such cases the web may follow an undesirable path through the press section, causing time-consum ng operational standstills. Different ways to achieve control of the web have been tried. One way is to adjust the press roll interrelationship, i.e. the geometry of the rolls, in such a manner that the felt which the web has to follow expands, while its reverse side still abuts the press roll whereas the second felt expands by means of air supplied to its reverse side. The vacuum thus generated in the felt to which air is not supp¬ lied, exerts a certain pressure against the web for guiding the web in the desired web path. However, this method suffers from considerable drawbacks since the geometry of the press and the felt angles are fixed within narrow limits. The force acting on the web is not sufficient to press the paper web against the desired one of the felts in a satisfac¬ tory manner. Another prior-art way of solving the problem is to manu¬ facture one of the felts impermeable, at least over a certain part of its thickness, to such an extent that it becomes practically impossible for air to penetrate through the felt. A felt of this kind will then serve as a web transport felt on which the web is carried through the papermaking machine, and particularly through the machine press sec¬ tion. After a certain period of operation, the felt surface becomes very smooth and this creates difficulties when the web is about to leave the transport felt at the entrance to the dryer section. At this point the web adheres so firmly to the felt that it cannot be removed therefrom. It is an object of the invention to eliminate these draw¬ backs and to this end the invention has for its purpose to indicate a method of controlling the paper web leaving the press nip in the press section of a papermaking machine, which paper web, after the press nip, is intended to follow a permeable felt. The method is characterized therein that at . least during the expansion of the felt after the press nip air admission is prevented to the side of that felt which faces away from the paper web. The invention also concerns a device for performing the method in accordance with claim 1 in press nips of papermaking machines for the purpose of controlling the paper web leaving the press nip. The device is characterized by an impermeable belt which is arranged to run in contact with the felt which the web is intended to follow after the press nip so as to abut against that s de of the felt which faces away from the paper web, said abutment being effected through one or several press nips and over a certain distance thereafter, said belt also arranged to be advanced in a separate loop and at the same speed as the felt. Further characteristics of the invention will appear from the dependent claims. The invention will be described in closer detail in the following with reference to the accompanying single figure of the drawings, labelled FIGURE 1. The illustrated embodiment of a dewatering press section in accordance with the present invention consists of two press nips 1 and 2 each one formed by two solid press rolls 3, 4 and 5, 6, respectively. Through the first nip 1 a paper web runs between two air permeable press felts, an upper pick-up felt 7 and a bottom felt 8. Arranged in a loop inside the pick-up felt 7 is an air impermeable belt 9. In the same way the paper web runs through the second nip 2 between two air permeable felts, a top felt 10 and a bottom felt 12. Arranged in a loop inside the bottom felt 11 is an air impermeable belt 12. The felts and belts are guided by lead rolls 13, stretch rolls 14 and guide rolls 15. Each one of the permeable felts also runs over suction boxes 16 for dewatering of the felts. The web is formed on a forming fabric 17 and is picked up by vacuum in a suction pick-up roll 18. The pick-up felt 7 wraps the pick-up roll and the paper web is transferred from the forming fabric to the face side of the pick-up felt 7 and is carried by the felt to the first press nip 1. When only the two felts 7 and 8 together with the paper web run through the press nip, a problem could arise in guiding the web still in contact with the face of the pick-up felt 7 after leaving the press nip. Without suction boxes in the top roll 4 a smooth surface of felt 8 may transfer the web to the bottom press felt instead of letting it be carried on the bottom side of the pick-up felt 7. However, the impermeable belt 9 is running close together with the pick-up felt 7 through and after the press nip 1. Thus air supply to the pick-up felt 7 is prevented when the felt expands after the nip. A vacuum is created in the felt to press the web against the bottom face of the pick-up felt 7. When running from the first to the second press nip the web is transferred from the pick-up felt 7 onto the bottom felt 11. This transfer is facilitated, because ahead of the transfer point the belt 9 no longer has contact with the pick-up felt 7 thus allowing air to penetrate the felt from the reverse side thereof and remove the vacuum. The web is carried by the bottom press felt 11 into the second press nip 2. Also the second press nip is a double-felted press nip, where the web is pressed between the top felt 10 and the bottom felt 11. The impermeable belt 12 is running close together with the top part of the bottom felt . ll> The combination felt 11/belt 12 prevents the air from penetrating the felt. It makes it even easier to transfer the web from the pick-up felt 7 to the bottom felt 11, but above all, it keeps the web onto the bottom felt 11 after the press nip. The web is carried by the bottom felt 11 to the first dryer roll 19. Before the transfer point the belt 12 loses contact with the felt 11 making the felt accessible to admission of air from the inside of the felt. The vacuum is released and the web can easily be separated from the felt 11 and be transferred onto the dryer cylinder 19. It has been suggested to make the felt 11 impermeable. Such an arrangement has the same effect after the press nip as the combination of the felt 11 and the belt 9, but it causes a problem when transferring the web from the felt 11 to the dryer cylinder 19. This is the case particularly after some time of operation when the surface of the felt 12 becomes.smooth. The felts and belts may be made endless, but it is likewise possible to use seamed felts and belts. The method and the device in accordance with the invention provide several advantages, among them less risk for rewetting and in addition, the arrangement ensures that the web will always follow the desired one of the felts. The impermeable belts 9 and 12 for instance could be a stable plastics belt or a woven belt which is impregnated in any con¬ venient manner in order to become impermeable. The embodiments of the invention described in the aforegoing are to be regarded as examples only, and a variety of other modifica¬ tions and embodiments are possible within the scope of the appended cl aims. The bel ts 9 and 12 need not be in engagement with the fel ts 7 and 11 , respecti vely, a di stance ahead of the entrance i nto the press ni ps . Al ternati vely, the bel ts -9 and 12 may be wrapped around the press roll s 4 and 5, respectively. It is al so possible to arrange pick-up from the formi ng fabric 17 to the pick-up fel t 7 by wrappi ng the bel t 9 around a solid pick-up roll 18. Further, the felts and/or belts may travel through several press nips. In its longest extension a felt may serve as a pick-up felt for removing the paper web from the forming fabric and then carry the web through all presses up to the dryer section. Such a felt can be sealed by positioning impermeable belts in the felt parts that are of critical importance to web guidance. The material making up the belts 9 and 12 can be chosen freely. The purpose of the belts 9 and 12 is to block admission of air to the permeable felts from the side of the felt opposite that of the paper web over part of the felt loops. Such periodical blockage may be achieved by other means, e.g. by using fixed elements or surfces in the form of e.g. a table below the felt.

Claims: AMENDED CLAIMS [received by the International Bureau on 31 March 1987 (31.03.87) original claims 1-5 replaced by amended claims 1-5(1 page)] 1. A method of controlling the paper web leaving the press nip in the press section of papermaking machines, which web (5) is intended to follow a felt (4) after the press nip, said felt (4) being permeable throughout its entire thickness, c h a r a c t e r¬ z e d therein that at least during the expansion of the felt (4) and after its separation from the press roller surface after the press nip admission of air is prevented to the felt side facing away from the paper web (5), and in that before or in connection with the separation of the paper web (5) from the felt (4), the obstacle preventing air admission to the felt side facing away from the paper web (5) is eliminated. 2. A method as claimed in claim 1,. c h a r a c t e r i z e d therein that admission of air to the side of the felt (4) facing away from the paper web is prevented also ahead of the ' press nip. 3. A device for performing the method in accordance with claim 1 in press nips of papermaking machines for the purpose of controlling the paper web (5) leaving the press nip, c h a r a c t e r i z e d by an impermeable belt (6) which is arranged to run in contact with the felt which the web (5) is intended to follow after the press nip so as to abut against that side of the felt which faces away from the paper web, said abutment being effected through one or several press nips and over a certain distance thereafter, said belt (6) also arranged to be advanced in a separate loop and at the same speed as the felt (4). 4. A device as claimed in claim 3, c h a r a c t e i z e d therein that the impermeable belt (6) is arranged to follow said felt (4) while abutting against it, from one of the rollers (2) forming the press nip and up to a deflector roller (7) positioned at a distance away from said roller (2) in the direction of travel of said web (5). 5. A device as claimed in claim 4, c h a r a c t e r i z e d therein that the impermeable belt (6) is arranged, while abutting against said felt (4), to follow said felt also over a distance ahead of the press nip.

Inventors: ANDERSSON NILS TORBJOERN, ANDERSSON, NILS, TORBJOERN
Applicants: NORDISKAFILT AB
Application year: 1986
Main classification: D21F3
IPC codes: D21F3