GB2274406A - Coalescing filter - Google Patents

Abstract

A layer 1 of fine mesh filter medium laid around a rotary mandrel is strengthened by bundles or tows 2 of continuous fibres impregnated with suitable resin and wound or laid in controlled helical patterns under controlled tension. This creates a three-dimensional reinforcing structure which is bonded by the resin to layer 1, the layer being exposed over an array of diamond shape areas between the windings. A layer 4 of coalescent filter medium is applied to the outside of the reinforcing structure. The fine mesh filter medium slows down the rate of flow of the liquid so that it tends to be spread more evenly over the whole of the filter, and the mesh size may vary laterally of the medium to assist such uniform spreading. The filter is used to coalesce oil or petrol in water, drained from a garage forecourt, to assist gravity separation. An alternative filter (Fig. 2) is flat with a peripheral frame and a filter medium whose mesh size is smaller in the central area. <IMAGE>

Description

'Improvements relating to Filters A common method of removing hydrocarbon contamination from rainwater such as oil and petrol spillages on carparks and garage forecourts, is to use a separator fitted with a coalescent filter. The coalescent filter medium is usually a thick, random, fibrous material such as reticulated plastic foam, glassfibre or metallic wool, etc. This medium traps the particles of hydrocarbon, coalesces them into larger droplets which then float upwards due to their lower specific gravity. Two main disadvantages are that it does not stop smaller entrained or emulsified hydrocarbon and it allows uneven and irregular flow paths to be created.This is because the open mesh structure allows the liquid to take the shortest route to the outlet thus creating high flow velocities nearest the flow path and low flows furthest away. These high velocities carry hydrocarbon particles straight through.

In accordance with the invention there is provided a dual filter device comprising a layered structure incorporating a support frame, a fine mesh filter membrane, and a coalescent filter membrane, such that the support frame provides support for the filter membranes.

The provision of the fine mesh filter membrane as the membrane which is carried on the support frame alleviates problems previously experienced when using the coalescent filter on its own. Thus the fine mesh filter membrane removes the small entrained and emulsified particles of hydrocarbon which then float to the surface. Furthermore, the fine mesh filter membrane provides a resistance to flow which is proportional to pore size and this creates a back pressure which tends to even out the flow over the entire surface and reduces the tendency of high velocity in some areas. This enables the coalescent filter membrane to operate efficiently so as to take out the larger hydrocarbon droplets so that they coalesce and float to the surface.

These large droplets are then, of course, prevented from reaching the fine mesh filter membrane which would tend to clog with such particles if the fine mesh filter membrane was used on its own. A further advantage is that a combined fine mesh filter membrane and support frame provides a support for the coalescent filter membrane which is inherently weak and incapable of supporting itself.

The coalescent filter membrane is ideally formed from a reticulated foam or other body of fibres laid in a random array but with a generally consistent average density. The filter material may be plastics material, glassfibre or metallic wool.

If the fine mesh filter membrane is designed to have pore sizes which vary over the surface areas this will provide varying flow resistance to even out expected flow rates across the filter when in use. By this means, the problem of high flow velocity is reduced still further.

In the preferred arrangement the fine mesh filter membrane comprises a layer of foraminous material integrated with an open-grid support structure formed from fibre bundles with resin impregnated between the fibres and through adjacent portions of the fine mesh filter membrane and set to bond the support structure to the fine mesh filter membrane. Ideally, the support frame will be in the form of a cage, perforated tube or frame for supporting a filter membrane of cylindrical or flat form or a filter membrane having a non-symmetrical shape or topography.

For added strength or for particular purposes the support structure and filter material could be built up in multiple layers. Thus, for example, a layer of filter material can be carried between two support structure layers. Alternatively, layers of filter material can be supported on either side of a single support structure.

Many more layers may be built up if desired. For increased strength, layers of fibre bundles can be built up one on top of the other. In order to combat the possibility that several such layers of fibre bundles might tend to flatten out the arrangement could be such that one fibre bundle is pre-wound around another fibre bundle.

More specifically, the fine mesh filter membrane could be of a construction as is defined in our co-pending European Patent Application No. 93310453.1 which claims priority from British Patent Application No. 9226769.9.

The invention may be performed in various ways and preferred embodiments will now be described, by way of example, with reference to the accompanying drawings, in which:- Figures 1 is a perspective view of a supported filter structure forming part of a dual filter arrangement of this invention; and Figure 2 is a view of an alternative form of supported filter structure for a dual filter arrangement of this invention.

In the arrangement shown in Figure 1 a sheet of filter medium 2 is laid down or wound round onto a rotating mandrel so as to create a complete sleeve. Bundles or tows 1 of continuous fibres (for example, glassfibre, carbon aramide, thermoplastic or other fibres) are impregnated with a suitable resin and are wound or laid in controlled helical patterns at a pre-determined pitch and under controlled tension. This creates openings of a generally diamond shape between the impregnated fibres. The tension of the resin impregnated fibres is controlled in such a way that the resin is forced through the filter medium onto the supporting mandrel so as to impregnate those parts of the filter medium. As the resin sets, the fibres 2 become bonded to parts of the filter sheet 1 whilst leaving portions of filter material within the diamond openings.As an alternative, the fibres can be wound first onto the mandrel and then the layer of filter material can be laid over the top under tension so that the resin will soak up from underneath.

It is also possible to build up several layers of filter fabric each sandwiched between impregnated fibres to form multi layer filters. In order to provide sufficient strength for the larger filters of say 6m diameter, multiple 'tows' of fibres can be introduced and a progressive buildup of the cage structure can be achieved by local reinforcement at strategic points. When these multiple 'tows' become large in cross section they tend to flatten out due to the controlled tension. This can be prevented by using a secondary winding operation which lays or winds another 'tow' of fibres circumferentially around the primary tow in advance of it being laid onto the mandrel. This effectively prevents the primary tow from spreading out due to the tension applied onto the surface of the mandrel.

Local reinforcement may be particularly necessary at the ends and in the drawings this is illustrated at 3 where progressive build-up of fibres has been created.

The dual filter is created by wrapping a layer of a coalescent filter 4 around the supporting filter medium 1.

This coalescent filter comprises a random array of fibres having a generally consistent averagely density, the filter body being constructed from plastics material. The coalescent filter is of lower inherent strength but is supported by the inner cage-like filter member. The coalescent filter provides for filtration of large hydrocarbon particles and any smaller particles falling through will be stopped by the fine mesh filter 1. The fine mesh filter also slows down the rate of flow of the liquid so that it tends to be spread more evenly over the whole of the dual filter structure.

In the alternative design shown in Figure 2 the support filter is set into a flat square frame and the coalescent filter will be laid on top. This drawing shows how the mesh size of the filter may be varied between a small mesh size 5 and a larger mesh size 6. This helps even further to ensure that the flow rate is more evenly distributed across the filter rather than being concentrated through the central region where the fine mesh region 5 creates added resistance to flow. The mesh sizes could, of course, be made to vary gradually from the centre to the sides of the filter.

The provision of a coalescent filter membrane directly onto a filter cage structure, with the fine mesh filter membrane on the outside, prevents the ingress of grit or other particles into the coalescent filter which could otherwise cause clogging. Again the outer fine mesh filter membrane could be specially formed so as to even out the flow.

Claims (7)

1. A dual filter device comprising layered structure incorporating a support frame, a fine mesh filter membrane, and a coalescent filter membrane, such that the support frame provides support for the filter membranes.

2. A device according to Claim 1, wherein the coalescent filter is formed from a reticulated foam or other body of fibres laid in a random array but with a generally consistent average density.

3. A device according to Claim 1 or Claim 2, wherein the coalescent filter is formed from plastics material, glassfibre or metallic wool.

4. A device according to any one of Claims 1 to 3, wherein the fine mesh filter has pore sizes which vary over the surface area to provide varying flow resistance to even out expected flow rates across the filter when in use.

5. A device according to any one of Claims 1 to 4, wherein the fine mesh filter comprises a layer of foraminous material integrated with an open-grid support structure formed from fibre bundles with resin impregnated between the fibres and through adjacent portions of the fine mesh filter and set to bond the support structure to the fine mesh filter.

6. A device according to any one of Claims 1 to 5, wherein the support frame is in the form of a cage, perforated tube or frame for supporting a filter membrane of cylindrical or flat form or a filter membrane having a nonsymmetrical shape or topography.

7. A dual filter device substantially as herein described with reference to the accompanying drawings.