GB2065852A - Method and apparatus for de- watering a water pervious conveyor belt and/or a moisture containing material on said belt - Google Patents

Abstract

In order to increase the efficiency of de-watering a layer of water- containing material and/or a water- pervious conveyor belt 6 upon which the layer is being conveyed towards an accumulator roll, the viscosity of the water to be removed is decreased by heating it, preferably by use of saturated steam supplied from a steam header 12 whose vertical walls 14, 15, 16 and 17 are vertically aligned with the vertical walls of a suction box 9, the steam being distributed by a plurality of pipes 25 connected by pipes 23 with an inlet steam pipe 19, the exhaust steam being passed to an exhaust space 30 and exhausted through an outlet dome 31. <IMAGE>

Description

SPECIFICATION Method and apparatus for de-watering a water pervious conveyor belt and/or a moisture containing material on said belt This invention relates to a method and apparatus for de-watering a water pervious conveyor belt and/or a moisture containing material on said belt.

There are a number of industries in which a moisture-laden layer of material on a continuously moving conveyor belt has to be deprived of its water content, or a major portion of that content, prior to delivery to apparatus for at least partial formation of the material into a required product.

Paper manufacture is an example of such an industry; and the production of fibre-reinforced cementitious articles is another.

A problem in the de-watering process, as briefly referred to above, to the solution of which the present invention is contributive, arises in connection with the production of asbestoscement pipes, sheets and other articles; and, although not limited thereto, the present invention is described herein largely in terms of that particular production.

The usual method of making asbestos-cement articles is to mix the solids ingredients with water to provide a slurry in which the solids content is about 10% by weight. This slurry is then partly dewatered by external application of the slurry to a rotary seive; the water fraction thus separated is drawn off from the sieve interior and the thickened slurry accumulates as a film-like layer on the outside of the sieve. This layer is then transferred to the trailing end of the top flight of a waterpervious, moving endless belt conveyor (usually referred to as a "felt"). The felt carries the layer with it and presents it to a rotary, cylindrical, accumulator roll upon which the layer is built up, as a closely compacted scroll until the required product thickness is achieved.

If the required product is a pipe, the accumulator roll may be a mandrel from which the scroll is removed axially. If the final product is to be of sheet form, the accumulator roll is one known as a "size" roll whereof the circumference is a little longer than the length of the finished sheets. To remove the scroll from the size roll, it is cut (in the axial direction) and peeled from the size roll. It is then flattened, edge-trimmed or otherwise further formed, by corrugating for example. Whatever its form, the fully shaped "green" article is then cured by autoclaving or otherwise.

On its way to the accumulator roll, the layer is further de-watered. While the layer is on the top or working flight of the conveyor belt, water is drawn off by use of suction boxes (applied to the underside of the working flight) or otherwise. This brings the water content of the layer, as it leaves the working flight, down to about 50%.

As the layer arrives at the accumulator roll, it is still further de-watered by use of a "tread" roll.

This roll is parallel to the accumulator roll, being in "wringer" relationship therewith. The tread roll compacts the layer relative to the being-formed scroll and, at the same time removes water so that the moisture content of the green product is brought down to about from 15% to 20%.

This green-product, water-content proportion is something of a compromise. Obviously, no more water is necessary in the green product than is sufficient to satisfy the chemical and physical requirements of curing. The ideal proportion could be more nearly approached by increasing the dewatering means operative on the layer prior to and during its incorporation into the scroll, but this is economically prohibitive (by use of the conventional apparatus outlined above) mainly due to the fact that the "drier" the layer becomes the harder it is to get out the water still remaining in it.

As already indicated, a green-product watercontent of about 15% and 20%, under presently prevailing practice, is generally regarded in the art as the least economically disadvantageous compromise between the mutually conflicting aspects discussed above.

According to the present invention in one aspect there is provided a method of de-watering a water-pervious conveyor belt and/or a layer of water-containing material when being fed by said belt to an accumulator roll; including the step or reducing the viscosity of the water to be removed by heating it at least during its approach towards said accumulator roll.

The viscosity of the water to be extracted is reduced by heating it, thus making the water more fluent, and hence more amenable to suctional removal, easier to express, and more readily able to flow out of and from the felt and any matters carried thereby.

In this last connection, it will be appreciated that although the invention is primarily directed to the de-watering of a product layer proceeding to an accumulator roll, it is also concerned with dewatering of the felt irrespective of whether the felt is or is not in use as a conveyor for the layer. It will be obvious that any de-watering treatment applied to a layer on the working flight must equally, and just as importantly, be applied to the working flight itself. Even so, the invention is applicable to any non-working flight of the felt; for example, merely to de-water it in readiness to resume working situation, or for removal of water used in washing it, while it is non-working.

According to the present invention in another aspect there is provided apparatus for use in carrying out the present invention, comprising a steam header of inverted box-like form; means for moving a flight of a conveyor belt across the open bottom of said header; and means for feeding saturated steam into said header for direct application to said belt and/or a layer of watercontaining material when being fed by said belt to an accumulator roll.

In the course of the research which led up to the present invention, consideration was given to the worthwhileness of reducing the viscosity of water in this particular de-watering art having regard to all the other economic factors involved therein. It was however, decided to put the point to experiment, and, the experimental work showed that the objects set forth earlier herein were not merely realisable, but realisable to a degree such that saturated steam could be used as the heating medium notwithstanding the fact that steam heating will inevitably add water (in the form of condensate) to a situation the whole point of which is the removal of water.

Indeed, while radiant or some other form of dry heating could be employed, saturated steam heating is preferred since it readily enables, without need for elaborate control devices, the temperature of the water to be extracted to be brought to, or close to, the maximum degree of fluidity as a liquid; that is, without substantial vaporisation; the main requirement being to get rid of the unwanted water in its dense liquid form without having to handle relatively large volumes of water vapour which would arise if the process was a true drying procedure as distinct from de watering.

The reduction of water viscosity as referred to above is best expressed in terms of the curve (5) shown in the graph of Figure 4 of the accompanying drawings. In the graph, the absolute viscosity of water (in the liquid phase) is plotted as a function of temperature; the temperature range running from the freezing point to the boiling point of water, those points being the limits of the range relevant hereto.

Where the invention is applied to the production of fibre-reinforced cementitious articles, the process water will contain dissolved calcium hydroxide and calcium sulphate. For practicel purposes the viscosity of this process water can be taken as equal to that of ordinary water.

An embodiment of apparatus suitable for carrying out the method according to the present invention will now be described, subject hereof is illustrated, more-or-less by way of an example, with reference to Figures 1 to 3 of the accompanying drawings, in which: Figure 1 is a longitudinal sectional view of the apparatus, Figure 2 is a cross-sectional view taken along the line 2-2 indicated in Figure 1, and Figure 3 is a detail of an item already shown in Figures 1 and 2, on a somewhat enlarged scale.

Referring to Figures 1 to 3, a conventional water-pervious conveyor belt or "felt" is indicated at 6. This felt, as illustrated, may be regarded as part of a working flight carrying a film-like product layer (not shown) to be de-watered at the same time as the working flight, or it may be regarded as a non-working flight being de-watered by itself.

In either case, the flight is to be regarded as moving in the direction indicated by arrows 7 in Figure 1. Felt 6 rides the rim 8 of an open-top suction box 9 on all sides thereof. Box 9 is of conventional kind. It is furnished with a liquid drain 10 through which vacuum suction is applied, and it is preferably provided with support bars 11 to prevent over-sagging of the felt 6.

Located above the suction box 9 is a steam header 12. This header 12 is in the form of an inverted box consisting of a roof 13, end walls 14 and 1 5 and side walls 16 and 17. These walls are substantially in vertical register with those of the suction box.

Header 12 has a saturated steam inlet 1 8 which extends into a multi-perforate main distributor pipe 1 9 encased in a trunk 20 lined with insulatory material 21. Pipe 19 supplies steam, by way of ports 22, to a lattice reticulation consisting of longitudinally extending intermediate distributor pipes 23 which communicate by means of ports 24, with transversely extending final distributor pipes 25. These pipes 25 open to the space above the throughgoing felt 6 by way of ports 26.

The header end wall 14 is a single-skin. The other three walls of the header 12 may be of the same construction. In that case, all four walls may extend downwardly into close proximity with the felt 6 thus to provide an all-around opening through which used steam may escape. If desired, each of these walls may have its lower edge provided with a flexible apron such as that shown at 27 in relation to the ieading end wall 14. These aprons 27 may be made of rubber and their lower edges are brought as close to the felt 6, or a product layer thereon, as is compatible with providing mechanical clearance between them and the product layer passing thereunder.

For preference, however, the header 12 is provided with an internal steam exhaust system. It consists in making each of the walls 1 5, 1 6 and 1 7 of double-skin construction defining cavities 28. These cavities 28 open, by way of holes 29 to an exhaust main space 30 which, in turn, leads the used steam to an exhaust steam dome 31. Where this cavity wall arrangement is employed, each of the wall skins is furnished with an apron similar to that indicated at 27 in Figure 1. These twin aprons, indicated at 32 and 33 in Figure 3, are held to the skins of the related wall by bolts 34 having spacer collars 35 mounted thereon. The twin aprons 32 and 33 are provided for the following reason. In operation of the apparatus it is found that the felt 6, or the product layer thereon, approaching the header tends to carry with it a surface film of cold air. When this air arrives between the twin aprons 32 and 33 it is confronted with departing steam and by it is swept to exhaust thus to avoid any cooling effect which the air might otherwise exercise on the steam still working within the header.

The pressure of the steam employed is not critical. It may be as low as will suffice to drive the steam through the system.

Claims (11)

1. A method of de-watering a water-pervious conveyor belt and/or a layer of water-containing material when being fed by said belt to an accumulator roll; including the step of reducing the viscosity of the water to be removed by heating it at least during its approach towards said accumulator roll.

2. A method as claimed in claim 1, in which the step of reducing the viscosity of the water by heating it is performed by heating the water with saturated steam.

3. A method as claimed in claim 2, in which the said step is performed by saturated steam so conditioned that the temperature of the water to be removed is brought nea? to but not in excess of the vaporising temperature of the water.

4. A method as claimed in claim 2 or claim 3, in which said conveyor belt is moved across the open top of a suction box and the saturated steam is applied to the belt by way of an open-bottom steam header which overlies said suction box.

5. A method of de-watering a water-pervious conveyor belt and/or a layer of water-containing material when being fed by said belt to an accumulator roll, substantially as hereinbefore described with reference to the accompanying drawings.

6. Apparatus for use in performing a method as claimed in any preceding claim, comprising a steam header of inverted box-like form; means for moving a flight of a conveyor belt across the open bottom of said header; and means for feeding saturated steam into said header for direct application to said belt and/or layer of watercontaining material when being fed by said belt to an accumulator roll.

7. Apparatus as claimed in claim 6, in which the means to feed steam to said header comprises a main distributor pipe and a reticulation lattice of orificed pipes in communication with said main pipe.

8. Apparatus as claimed in claim 6 or 7, in which said header includes a steam exhaust system in which a plurality of the header walls are of twin-skin construction to act as exhaust steam ducts leading to an exhaust dome.

9. Apparatus as claimed in any of claims 6 to 8, including a suction box over which said belt passes and wherein said header has its side walls in register with those of said suction box.

10. Apparatus as claimed in claim 8, in which each of the skins of said header side walls has its lower edge provided with a flexible apron and the aprons of the two skins associated with each of said walls are so spaced apart that air entering the space between them beneath the outermost skin is swept to exhaust by steam entering that space beneath of the innermost skin.

11. Apparatus for de-watering a water-pervious conveyor belt and/or a layer of water-containing material when carried by said belt, substantially as hereinbefore described with reference to and as illustrated in Figures 1 to 3 of the accompanying drawings.