Paper making and fiber liberation – Processes and products – Lead strip forming and/or guiding
Reexamination Certificate
2002-12-05
2004-09-21
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
Lead strip forming and/or guiding
C162S255000, C162S289000, C226S091000, C034S117000, C034S114000
Reexamination Certificate
active
06793773
ABSTRACT:
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
The present invention relates to a method and a system in a paper machine or the like for passing a web from the press section to the drying section. Thus the invention relates particularly to such paper machine solutions, where the web is passed from the press section to the drying section in a closed draw, i.e. directly from the press fabric, felt or other belt, such as a transport belt, to the drying wire of the first drying group of the drying section. The web is passed with the aid of a transport suction roll.
In a drying section provided with a single wire draw, the web is directed in the first drying group, supported by the wire, over the first drying cylinder and from there further in a way known as such, supported by a suction roll or the like, to the second drying cylinder of the drying group, and so on, alternately over a hitch roll, typically a suction roll, and alternately over a drying cylinder. The passage of the web from a drying cylinder to a hitch roll is stabilised in a manner known as such, with the aid of a vacuum box arranged in a pocket formed between the drying cylinders and the hitch roll, preferably at the opening gap between the wire and the first drying cylinder in the travelling direction of the web. Corresponding vacuum boxes are typically arranged also in the pockets formed between the next drying cylinders and the hitch rolls.
FIG. 1
shows a typical travel of the web w in the opening gap between the drying cylinder A and the drying wire F. As shown in
FIG. 1
, the web tends to follow the surface of the drying cylinder and momentarily to become detached from the drying wire. As the web is detached it creates a pouch, which is difficult to return into contact with the wire in a neat manner. Further problems are caused by the fact that the wire does not detach from the surface of the drying cylinder in a completely symmetrical manner in the cross direction of the web, whereby the travel of the web is not completely uniform even in the cross direction of the web, and pouches, folds and other damages, even breaks, are caused in the web. Pouches in the machine direction are visible all the way to the next cylinder.
The above mentioned problems occur particularly at high running speeds, and when the effect of the vacuum box is not sufficient to control the travel of the web in the opening gap. Within certain limits it is possible to treat the problems with the aid of vacuum boxes, i.e. it is possible to aid the web to detach from the drying cylinder and to be reattached to the drying wire.
However, it may be problematic to adjust the negative pressure in the vacuum box, so that full control over the travel of the web is obtained. If the negative pressure of the vacuum box is too low, e.g. −100 Pa, there will be difficulties in detaching the wire from the cylinder, which results in the above mentioned problems. However, a low negative pressure is advantageous in that the negative pressure returns a detached web to the wire at a small angle, i.e. in a relatively well controlled manner. If, on the contrary, the negative pressure is relatively high, e.g. −500 Pa . . . −1000 Pa, depending on the box type, then the web will detach more easily from the cylinder and it will not form a pouch which is as large as when running at a low negative pressure. However, then there may rise a problem in that the web is returned too violently to the wire. A high negative pressure will violently suck the web with a sharp twitch against the wire, which may cause damages in the web or further weaken already weak spots in the web, which easily results in a web break in the respective pocket or later on.
A too high negative pressure also causes difficulties by bending the wire towards the vacuum box. Thus a high negative pressure will hinder the wire from passing straight towards the hitch roll, but it will form an arch on the section between the drying cylinder and the hitch roll. Then the wire may come into contact with structures of the vacuum box, if the box is not shaped to be arched in a corresponding manner. Even the shaping of the box will not always help, as the wire is arched in a different way in different phases of a run.
The detachment of the web can be secured and bending of the wire can be avoided by using a vacuum box divided into different negative pressure levels, such as the HiRun box presented in WO 00/50693 of Metso Paper, Inc., where the negative pressure in the travel direction of the web in the first part of the box, i.e. at the opening gap, is higher than in the next part or parts of the box.
However, it would be advantageous if the detachment of the web from the drying wire could be totally or almost totally avoided, and if the web could be made to pass from the drying cylinder to the hitch roll in a controlled manner, so that it follows the wire.
A poor detachment of the web from the cylinder, and the creation of pouches, folds or other damages makes it more difficult to run the web in the next stages. A web break due to a poor detachment can occur both immediately after the gap and even after several drying cylinders. The number of breaks in the drying section depends also on other interfering factors impeding the travel of the web at the beginning of the drying section. There will be less breaks if the web passes faultlessly in the gaps opening from the cylinders. The number of breaks can on their part be regarded to represent the runability.
Many factors have an effect on the above mentioned travel of the web in the opening gap of a drying cylinder, and thus on the runability in the drying section, such factors being e.g.
the quality of the pulp, e.g. the length of the fibres,
the dry solid contents of the web,
the tension difference between the press section and the drying section,
the mechanical shaping of the web in the press section and in the passing of the web before the drying cylinder,
the relaxation velocity of the web
the strength of the web, and
the temperature of the cylinders. Attempts have been made to act on some of these factors in order to improve the runability.
In order to remove the problems caused by the detachment of the web there has been traditionally attempts e.g. to increase the tension difference between the press section and the drying section. This causes an internal tension in the web, which aids in detaching the web from the cylinder and thus it improves the runability. However, there is a limit to the increase of the tension difference. An increased tension difference increases the number of breaks. Further, a too much increased tension difference has an disadvantageous effect for instance on the quality characteristics of the web. As the speeds of the paper machine increase there has also emerged a growing need to increase the tension differences in order to retain the runability.
There have also been attempts to keep the temperature of the surface of the drying cylinder at the beginning of the drying section on a level at which the web does not bum to the surface of the cylinder. This solution is less good regarding the drying efficiency of the drying section.
The above mentioned runability problems at the beginning of the drying section have among other things often resulted in that it has been necessary to shorten the length of the cylinder group at the beginning of the drying section, which increases costs.
SUMMARY OF THE INVENTION
The object of the invention is to achieve a method and a system for passing a paper web or the like from the press section to the cylinder drying group, where the above mentioned problems are minimised.
Thus an object of the invention is to provide a reliable method and system for passing a paper web or the like from a press section to a cylinder drying group.
Then the object of the solution according to the invention is among other things to pass the web from the press section to the drying section with a minimum tension differ
Juppi Kari
Komulainen Antti
Chin Peter
Halpern Mark
Metso Paper Inc.
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