Improvements in or relating to apparatus for dewatering fibrous suspensions on a paper forming machine

The present invention relates to apparatus for dewatering fibrous suspensions on a paper forming machine.

In a known paper forming machine the fibrous suspension passes between top and bottom forming wires and water is extracted from the suspension un an upward direction by use of an auto-slice and a curved inverted suction box which contacts the top wire. The two wires trap the stock over a lead-in-foil and water flows in an upward direction into the auto-slice and afterwards, due to the continuing pressure of the two wires, the dewatering action is continued in an upward direction into the apertures of the inverted suction box (FR-A-2350422).

A transfer suction box is disposed downstream of the inverted suction box on the side of the bottom wire and vacuum applied to the suction box reverses the direction of flow and a downward dewatering action takes place together with the transfer of stock to the bottom wire. The cover has a typical radius of five metres and its contacting surface with the top wire is open to vacuum. The openings in the cover are in the form of slots or drilled holes. The arrangement is such as to produce upward dewatering throughout its whole length.

Stock dewatering using the existing arrangement requires vacuum to be applied to the cover or active surface throughout its whole length and this in turn increases the power required to drive the top wire. It is an aim of the present invention to provide apparatus for dewatering which is more economical in terms of capital and running costs than the present arrangement and which gives improved retention of the stock on the bottom wire, and which gives a longer suction transfer box cover life.

According to one aspect of the present invention there is provided apparatus for dewatering a fibrous suspension on a paper forming machine comprising a suction housing having an active surface or cover adapted to co-operate with a top wire to urge that wire towards a forming wire, the fibrous suspension being trapped between the top wire and the forming wire, and the cover having a first open area by means of which water is removed upwardly and a second part downstream of the first open area which urges water downwardly.

The first open area comprises a plurality of slots or foils defining slots or a plurality of drilled holes whilst the second part is preferably solid. The open area in the first part of the cover may be reduced in the downstream direction.

The solid part of the cover will not permit any dewatering to take place through the cover and, consequently, the pressure of the two wires will force the dewatering action to reverse its direction and go downwards.

According to another aspect of the present invention there is provided a method of dewatering a fibrous suspension on a paper forming machine in which a fibrous suspension trapped between a top wire and a bottom forming wire is moved past an active surface or cover of a suction housing with the top wire contacting the cover for the purpose of removing water from the fibrous suspension, wherein the water is extracted in an upward direction as the fibrous suspension moves past a first open area of the cover, and wherein the water is directed downwardly as the fibrous suspension moves past a second part of the cover disposed downstream of the first area.

The present invention will now be described further, by way of example only, with reference to the accompanying drawing which is a diagrammatic end view of dewatering apparatus on a paper forming machine.

The machine comprises an endless top-wire 1 which is entrained around rolls 3. Dewatering apparatus in the form of a curved inverted vacuum box having a suction housing 7 is disposed within the top-wire 1 and has an active surface or cover 9 which co-operates with the top-wire 1. The cover 9 urges the top-wire 1 towards a forming wire 5. The fibrous suspension is trapped between the top-wire 1 and the forming wire 5.

The wires 1, 5 move in the direction indicated by arrows A. The forming wire 5 passes over a lead-in-foil 11 which urges water upwardly. An auto-slice 13 immediately downstream of the lead-in-foil removes a proportion of the water in the upward direction. A further portion of the water is removed upwardly by a first part of the cover 9 which has an open area and to which suction is applied from the suction housing. The open area extends over the region shown at 15 and is formed by a pluraltiy of foils defining slots or by drilled holes. The open area may have a compound curvature with a radius which decreases in the downstream direction in order to generate a progressively higher dewatering pressure.

A second part of the cover, generally indicated at 17, is solid. Thus, no dewatering can take place through the cover in this part and, consequently, the pressure of the two wires will force the dewatering action to reverse its direction and go downwards.

Thus, downward dewatering will start before a transverse suction box 19 disposed downstream of the inverted vacuum box 7. The water, dewatered due to the downward action, will be doctored away from the bottom wire 5 by the leading edge of the transfer suction box and through the open area of the transfer suction box cover 21.

The dewatering apparatus of the present invention has the following advantages:-

• The downward dewatering of the second part of the curved inverted vacuum box will drain downwardly due to gravity, and will not require any vacuum assistance (as is presently required). Thus, a saving is achieved on Capital and Running costs in respect of:-

  • the vacuum pump and its installation,
  • an upward stainless steel compartment with its inlet and discharge equipment is no longer necessary,
  • and the running cost of energy required to maintain VACUUM.

Due to not having VACUUM on the second part of the shoe, the power required to drive the top wire is reduced and while presently the last part of the vacuum box can have a tendency to plug due to the low flow of water in it, the new solid cover will not plug.

The retention of the Bel Bond will improve. The water, which goes downward, passes through the formed sheet and the bottom wire, and will be of lower consistency than the water presently drained upwards.

The water coming down from the second part of the vacuum box will lubricate the transverse box cover and contribute to a longer suction box cover life.