Drying and gas or vapor contact with solids – Apparatus – Rotary drums or receptacles
Patent
1997-01-28
1998-01-27
Bennett, Henry A.
Drying and gas or vapor contact with solids
Apparatus
Rotary drums or receptacles
34117, 34122, 34130, D21G 500
Patent
active
057110883
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a device in the drying section of a papermaking machine, which comprises a plurality of drying cylinders and guide rolls and through which a paper web, supported by a drying fabric, is conducted alternatingly over the drying cylinders and the guide rolls in such a manner that the drying fabric presses the paper web against the drying cylinders and is positioned between the paper web and the guide rolls, said device being adapted to reduce the effects of the tendency of the paper web to adhere to a drying cylinder as it passes from the drying cylinder to a subsequent guide roll and comprising a blow box, which has a wall arranged in the portion where the paper web and the drying fabric pass from the drying cylinder to the guide roll and which faces the drying fabric and extends substantially in parallel therewith to form a narrow air gap between the wall and the drying fabric, and a nozzle arranged at the wall end positioned upstream with respect to the direction of travel of the paper web and the drying fabric, said nozzle being adapted to eject air at high velocity towards the drying fabric away from the air gap.
In a papermaking machine with a drying section of the above-mentioned type, the paper web tends to adhere to the drying cylinders when passing to the following guide roll. If such adhesion occurs, the aerodynamic forces are allowed to act on the paper web via the drying fabric in a negative fashion. The negative effect of these forces is particularly obvious when the papermaking machine is operated at high speed, i.e. when the paper web and the drying fabric are passed through the drying section at high speed (above 1200 m/min). The adhesion tendency is at a maximum at the beginning of the drying section and decreases as the paper web dries, and is at a maximum when the paper web is thin and intended for manufacturing of low-grammage paper.
If the paper web adheres to a drying cylinder and then leaves the web and follows the drying cylinder a distance, before releasing it and being returned to the drying fabric, a blister arises between the paper web and the drying fabric. Between this blister and the space at the opposite side of the drying fabric, a communication of air occurs via the drying fabric. This communication of air forms a complex and unstable flow field which can increase the blister and cause the paper web to flap, which may induce folds or breaks in the paper web.
The object of the present invention is to provide a device which guides the aerodynamic forces in such a manner that their negative effect on the paper web is eliminated or at least reduced and, consequently, makes it possible to operate the papermaking machine at higher speed, and which besides is designed such that the action of the air, which is necessary for achieving the intended effect, can be reduced.
According to the present invention, this object is achieved by a device which is of the type mentioned by way of introduction and characterised in that the nozzle has a slot-shaped opening and is designed to eject a well-defined air jet directed along the centre plane of the opening, that the nozzle is directed such that its centre plane intersects the circumferential surface of the drying cylinder along a first straight line which is positioned upstream of a second straight line, along which the paper web leaves the drying cylinder, and that the centre plane of the opening of the nozzle and the tangent plane of the drying cylinder, said tangent plane extending through said first straight line, make an angle between themselves, which is 10.degree.-20.degree., preferably 15.degree..
The distance between these two straight lines along the circumferential surface of the drying cylinder preferably is 30-50 mm.
The invention will now be described in more detail with reference to the accompanying drawings.
FIG. 1 is a schematic view of part of the drying section of a papermaking machine.
FIG. 2 illustrates a device according to the invention, mounted in the drying section of the papermaking
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Halldin Claes
Lindqvist Niclas
ABB Flakt AB
Bennett Henry A.
Gravini Steve
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