Paper making and fiber liberation – Processes and products – Running or indefinite length work forming and/or treating...
Patent
1988-05-16
1989-08-29
Hastings, Karen
Paper making and fiber liberation
Processes and products
Running or indefinite length work forming and/or treating...
162305, 162358, 1623601, D21F 302
Patent
active
048614300
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
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-consuming 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 against 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 supplied, 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 satisfactory manner.
Another prior-art way of solving the problem is to manufacture 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 section. 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.
SUMMARY OF THE INVENTION
It is an object of the invention to eliminate these drawbacks 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 the 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 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.
BRIEF DESCRIPTION OF THE DRAWING
The invention will be described in closer detail in the following with reference to th
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Hastings Karen
Nordiskafilt AB
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