Pressing arrangement

Details Pressing arrangement Pressing arrangement

2001-03-08

2003-12-23

Chin, Peter (Department: 1731)

Paper making and fiber liberation

Processes and products

Running or indefinite length work forming and/or treating...

C162S358100, C162S358300, C162S360200

Reexamination Certificate

active

06666954

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims priority under 35 U.S.C. § 119 of German Patent Application No. 100 12 148.9, filed on Mar. 13, 2000 and German Patent Application No. 200 19 256.6, filed on Nov. 13, 2000, the disclosures of which are expressly incorporated by reference herein in their entireties.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a pressing arrangement with at least one pressing nip formed by two pressing rolls for dewatering a paper, cardboard, tissue, or other fibrous material web, through which one dewatering belt is guided next to the fibrous material web on each side for the purpose of absorbing the pressed-out water, in which the fibrous material web is guided by both dewatering belts in a common guidance path after the pressing nip, the lower dewatering belt running below the fibrous material web wraps a preferably suctioned separating roll along with the fibrous material web at the end of the guidance path, the upper dewatering belt arranged above the fibrous material web is guided away from the fibrous material web in a separation path in the region of the separating roll, and the fibrous material web is subsequently accepted by a removal belt.
2. Discussion of Background Information
Such arrangements have been known for a long time in which the fibrous material web is held onto the lower dewatering belt after the removal of the upper dewatering belt with the aid of suction or blowing devices. The effect, especially of the suction devices, on the fibrous material web decreases with increased web speed, higher water contents in the dewatering belt, and increasing wear on the dewatering belt. As a result, the adhesion of the fibrous material web to the lower dewatering belt decreases, which can lead to fluttering of the web edges and thus to increased stretching of the web edges.
Furthermore, after the separation path, air flows into the wedge-shaped opening between the fibrous material web and the upper dewatering belt. This stream of air can also lead to a lifting of the web edges and to the formation of flutters.
SUMMARY OF THE INVENTION
The present invention provides for the guidance of the fibrous material web, in particular after the separation path, i.e., the removal of the upper dewatering belt.
According to the invention, the wrapping of the separating roll by the lower dewatering belt and the fibrous material web subsequent to the separation path occurs at a wrapping angle of at least about 30°. This relatively large wrapping angle leads to a rapidly widening wedge-shaped opening between the fibrous material web and the upper dewatering belt, which allows air to flow in without the danger of web fluttering. Advantageously, the wrapping angle is at least about 45°, preferably even at least about 60°.
By the vacuum acting on the fibrous material web, the suctioned separating roll increases the adhesion of the fibrous material web onto the lower, air-permeable dewatering belt over the entire region of the wrapping. So that the adhesion of the fibrous material web to the lower dewatering belt can increase sufficiently and its adhesion to the upper dewatering belt can decrease sufficiently, the time the fibrous material web remains in the separation path should be greater than about 1 ms, preferably greater than about 15 ms. This allows the subsequent removal of the upper dewatering belt without the danger of snagging with respect to the fibrous material web.
In order to greatly limit the length of the wedge-shaped opening between the upper and lower dewatering belts, the upper dewatering belt should be guided after the separation path to a guidance roll, which deflects this dewatering belt; it is advantageous for the unsupported progression of the dewatering belt to be as short as possible and between about 20 and about 200 mm, preferably between about 20 and about 80 mm, long. This results in a decrease in the air flow into the wedge-shaped opening that undermines the guidance of the fibrous material web on the lower dewatering belt. Furthermore, the position of the guidance roll of the upper dewatering belt should be changeable and a wrapping angle of the upper dewatering belt with respect to the separating roll of the lower dewatering belt should be adjustable thereby between about 0° and about 5°.
Furthermore, the angle between the common guidance path (sandwich guidance) subsequent to the pressing nip and the pressing plane formed by the pressing rolls should preferably be variable in the range of ±about 5°. If the sandwich guidance runs at an incline, this is connected with a more intensive downwards dewatering and thus a greater compacting of the lower side of the fibrous material web. On the other hand, a declining sandwich guidance leads to a more intensive upwards dewatering and thus to a greater compacting of the upper side. This offers an effective way for influencing the two-sidedness of the fibrous material web, in particular with regard to surface strength, oil absorption, and printability.
The goal of the invention is to avoid long, level sandwich guidances of the dewatering belt and the fibrous material web. The lower belt can also be embodied as a plastic belt with a dense surface that is closed to the greatest extent possible. In this case, a normal guidance roll is used instead of the suctioned separating roll. Here, a relatively smooth surface guarantees the adhesion of the fibrous material web to the lower belt.
It is advantageous with respect to the further secure travel of the fibrous material web for the fibrous material web to travel from the separating roll to a suctioned removal roll arranged at a distance from it and wrapped by the air-permeable removal belt, where the fibrous material web is accepted by the removal belt. Here, the clear distance between the separating roll and the removal roll should be as small as possible, i.e., smaller than about 500 mm, and preferably between about 20 and about 200 mm, in particular between about 20 and about 80 mm. Because of the short distance between the separating and removal rolls, web stabilizers and additional blowing or suction devices can be eliminated in this region. Furthermore, this leads to a shortening of the wedge-shaped opening and thus to a decrease in the working surface of the fibrous material web.
It is also advantageous for the lower dewatering belt to wrap an adjustable guiding roll after the transfer of the fibrous material web and for a gap, a contact, or a wrapping of the lower dewatering belt to be adjustable in relation to the removal roll by this guide roll. This allows the formation of a relatively large wrapping angle, for example, for the secure transfer of the fibrous material web.
In order to be able to compensate for stretching of the moist fibrous material web for the purpose of preventing the formation of folds, the accepting belt should have a higher speed than the lower dewatering belt.
The removal belt can be formed as a pressing felt of a subsequent pressing nip, as a transfer belt for subsequent transport, or as a drying wire of a subsequent drying group for drying the fibrous material web. Here, the distance between the guiding roll of the upper dewatering belt and the removal belt should be adjustable so as to control thereby the air introduced into the wedge-shaped opening.
In order to achieve an intensive but gentle dewatering of the fibrous material web, it is advantageous for the pressing nip of the pressing arrangement to be embodied in an elongated fashion. Because of the considerable amounts of water that result in this case, at least the lower dewatering belt, preferably both dewatering belts, should be embodied as a pressing felt.
Particular advantages result with regard to an improved guidance of the fibrous material web in pressing arrangements with two pressing nips, both of which should preferably be embodied in an elongated fashion. However, at least the second pressing nip should be embodied in an elongated fashion. Here, the arrangement according to

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