Paper making and fiber liberation – Apparatus – Running or indefinite length product forming and/or treating...
Patent
1994-02-02
1996-04-16
Hastings, Karen M.
Paper making and fiber liberation
Apparatus
Running or indefinite length product forming and/or treating...
162300, 162352, D21F 100
Patent
active
055079180
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a twin-wire former for the production of a fiber web, in particular a web of paper or board, from a fiber suspension, and particularly to a twin-wire former having ledges above the forming wires of the twin-wire former for aiding in directing the water away from the wires. In such a twin-wire former, there are two paper machine wire belts or wires which together form a twin-wire zone, and the fiber suspension travels between the belts. Each of the wire belts travels over a respective plurality of ledges which are arranged on the outsides of the belts. The ledges in each plurality thereof are arranged at a distance apart from each other. One plurality of ledges above one wire are rigid ledges. The other plurality of ledges above the other wire are resiliently supported to be pressed against the other wire belt. The following publications are indicated as prior art:
Documents 1 and 2 disclose different twin-wire formers in each of which the top wire travels along the (substantially flat) lower side of a dewatering box. According to EP '786 this dewatering box has rigid ledges on its bottom. Directly below said rigid ledges the bottom wire travels over ledges which can be applied against it resiliently. In several embodiments of GB '120 the ledges are close together so that water cannot discharge downward through the bottom wire in this region. The same is true of other embodiments of GB '120 in which a flexible plate is provided instead of ledges. In further embodiments of GB '120, smaller or larger spaces are present between resiliently appliable ledges, which spaces can receive smaller or larger amounts of water and discharge them laterally to the outside. This is true also of the twin-wire former in accordance with EP '786. In all of these cases, the entire flat lower surface of the dewatering box which is arranged in the top wire is covered by the resiliently appliable ledges present in the bottom wire or by the said flexible plate, with the exception of at most the narrow zones of the intermediate spaces. This has the result that the discharge of the water downward is prevented to a greater or lesser extent in the entire region of the flat bottom side of the dewatering box.
Therefore, one of the disadvantages of all of these known arrangements is that the dewatering takes place exclusively (or practically exclusively) in the upward direction in the region of the resiliently appliable ledges (or of the flexible plate). Therefore, the quality of the fiber webs produced leaves something to be desired, in particular with regard to the "formation" or "cloudiness". There is also the problem that the said intermediate spaces become clogged with the passage of time, so that the formation is not uniform over the width of the web.
Therefore, a construction was adopted in the prior art which only relatively few ledges which could be pressed resiliently against the one wire are provided. Here, large spaces which can receive large amounts of water are present between the ledges. Furthermore, openings are provided so that these amounts of water can discharge downward over the shortest possible path. Twin-wire formers of this type are described in Publications 3 and 4. In general, the following is true of the twin-wire formers in accordance with Publications 1-4: Due to the resiliently supported ledges which are arranged opposite the rigid ledges, the following can be achieved: For instance, upon an increase in the amount of suspension flowing in between the two wire belts, the resiliently supported ledges can move away somewhat. In this way, the danger (which exists when rigidly supported ledges alone are used) of a damming up occurring in the fiber suspension in front of the ledges is eliminated. Such a damming up could destroy the fiber layers formed up to that time on the two wire belts. In other words: In the known twin-wire formers in accordance with Publications 1-4, a dewatering pressure which has once been set remains constant due to the resiliently supported ledges even upon
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Egelhof Dieter
Grossmann Udo
Pisinger Franz
Polifke Hubert
Sahlin Sven-Ake
Hastings Karen M.
J.M. Voith & GmbH
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