EP0122701B1

EP0122701B1 - Improvements in or relating to dewatering apparatus

Info

Publication number: EP0122701B1
Application number: EP84301198A
Authority: EP
Inventors: Ronnie Abraham Arav
Original assignee: Beloit Walmsley Ltd
Current assignee: Beloit Walmsley Ltd
Priority date: 1983-03-17
Filing date: 1984-02-24
Publication date: 1987-06-03
Also published as: PL143764B1; FI79729B; FI840901A; EP0122701A1; ZA841486B; PL246711A1; DE3464073D1; FI79729C; FI840901A0; GB8307435D0

Description

The present invention relates to apparatus for dewatering a fibrous stock and in particular for dewatering a fibrous web formed between top and bottom wires of a paper forming machine.
In a known dewatering apparatus disclosed in GB-A 1,582,342 a web is formed between the top and bottom forming wires and water is extracted in an upward direction by suction applied through a so-called inverted vacuum box. The vacuum box has an active surface which is curved convexly, the curvature being of a constant radius or nonuniform, varying from a larger to smaller radius in a downstream direction. In another apparatus disclosed in FR-A-2,289,671 top and bottom wires having a fibrous stock sandwiched therebetween are led over a convexly-curved surface of a solidd shoe of constant radius, and are subsequently trained around a pressurised roller of smaller radius. Both these prior arrangements are satisfactory, but produce limited dewatering or drainage.
An aim of the present invention is to increase the drainage capabilities of the curved inverted vacuum box without detracting from its paper- forming qualities.
According to the present invention then, there is provided apparatus for dewatering a fibrous suspension on a paper forming machine comprising a suction housing having a convexly non-uniformly curved active surface adapted to contact a pair of travelling forming wires of a paper machine trained therearound with the fibrous suspenson sandwiched therebetween for dewatering and formation, said active surface having a radius of curvature upstream greater than that downstream, which apparatus is characterised in that said active surface has at least three discrete curved sections, the raddi of curvature of each pair of adjacent sections being coincident at the junction between said pair of adjacent sections to define a compound curvature composed of decreasing radius in a downstream direction.
The suction housing is preferably an inverted suction or vacuum box in which water is extracted upwardly from the suspension which is constrained between top and bottom forming wires. When the stock consistency is low, on entry between the top and bottom forming wires, the large radius contributes to low pressure between the two wires, and the stock is dewatered gently. As the stock between the wires increases in consistency due to the dewatering action, higher pressures resulting from the decreasing radius can be applied to the stock without having a crushing effect, and more water can be dewatered from it.
The pressure applied at any point on the suction housing is inversely proportional to the radius of the active surface thereof at that point and many increase incrementally with each reduction in radius over the length of the active surface.
The initial large radius gives the same high quality formation as present arrangements, but the improvement of the invention has the additional advantage that the higher pressures automatically applied later in the dewatering process increase the drainage capabilities of the apparatus and hence the formation of a web from the firbous suspension.
The present invention will now be described further, by way of example only, with reference to the accompanying drawing, which illustrates a diagrammatic side view of dewatering apparatus on a paper forming machine.
Referring to the drawing, an endless top wire 1 is entrained around rollers 3 and is co-operable with a bottom forming wire 5 of a paper forming machine. The wires 5 and 1 move in the direction indicated by the respective arrows A and B. The fibrous suspension or stock is deposited on the forming wire 5. Suction apparatus in the form of a curved inverted vacuum box 7 is disposed within the endless top wire 1 and has an active cover surface 9 which contacts the top wire 1. The active surface 9 is open to vacuum and may comprise a plurality of bars defining slots, or holes drilled in a plate. Thhus, the suspension is dewatered upwardly in the illustrated embodiment. The active surface is curved convexly and according to the present invention the radius of curvature of the active surface or cover is not constant throughout but decreases in the direction of wire movement. The cover has three or more curvatures which are themselves of constant radius but which are different from one another. Thus, in the ittustrated embodiment the cover has three discrete curvatures, R_i, R₂ and R₃, with R, > R₂ > R₃. For example, R, = 5 metres, R₂ = 3 metres and R₃ = 2 metres. A lead in foil 11 is provided, and a transfer box 13.
The radii of curvature of each pair of adjacent sections are coincident at the junction between the pair of adjacent sections to define a compound curvature composed of decreasing radii.
Theoretically, the pressure between the two wires 1, 5 on the curved inverted suction box is inversely proportional to the radius of curvature. The progressively decreasing radius of curvature will have the effect of progressively increasing the pressure exerted by the wires on the stock betwen them. For example, if the starting radius is 5 metres, the pressure P₅ between the wires will be P₅ = k x Ys where k is a constant. If the following part of the curvature has a radius of 2.5m, the pressure that the stock will be subjected to will be 2.5 times the initial pressure, as can be seen from the relationship:-

P₂=kx
=2.51kx'/s1=2.5P5.

In addition to the above mentioned advantages with the cover of the present invention the bone dry moisture of the stock, after the compound cover, will be higher than from a cover with uniform radius. Additionally, due to the increased drainage capacity, more water will flow through the cover apertures and this increased flowflushes the internal passageways and clears up all stock build-ups. Thus, the tendency for the final part of the cover to plug up is reduced.

Claims

1. Apparatus for dewatering a fibrous suspension on a paper forming machine comprising a suction housing having convexly, non-uniformly curved active surface (9) adapted to contact a pair of travelling forming wires (1, 5) of the paper machine, trained therearound with the fibrous suspension sandwiched therebetween for dewatering and formation, said active surface (9) having a radius of curvature upstream greater than downstream, characterised in that said active surface (9) has at least three discrete curved sections, the radii (R_l, R₂, R₃) of curvature of each pair of adjacent sections being coincident at the junction between said pair of adjacent sections to define a compound curvature composed of sections of decreasing radius in a downstream direction.

2. Apparatus as claimed in claim 1 characterized in that the active surface (9) is the cover of an inverted suction box (7) by which water is extracted upwardly from the suspension which is constrained between the top and bottom forming wires (1, 5).