The present invention relates to a filter for gas and to its use for the filtration of a stream of gas contaminated with liquid in the form of mist.

Air flow from oil-lubricated gas compressors or vacuum pumps is normally contaminated with oil mist. In order to remove this contamination the air is passed through a coalescing filter and an example of a known type of such filter is shown in Figure 1 of the accompanying drawings. The filter is generally tubular and comprises a coalescing layer 10 of microporous material which fits closely between inner and outer fora inous metal cylinders 12,14 and extends between upper and lower end caps 16,18. The upper end cap 16 is formed with a port 20 to permit the stream 22 of oil-contaminated air to enter the interior of ^■ the filter and the lo .er end cap 18 is closed as shown. The coalescing layer 10 is sealed gas-tightly to the end caps 16,18 by a curable sealant 20, e.g. an epo^y resin. The coalescing layer 10 is in this instance of one or more pleated sheets of resin-impregnated borosilicate glass microfibres as

disclosed in Patent No. GB-A-1557821. An alternative form of coalescing layer which is formed by moulding is disclosed in Patent No. GB-A-1603519. Also extending between the end caps 16,18 and covering the outer surface of the foraminous cylinder 14 is a drainage layer 24 which in this instance is of an open-celled foam plastics material, although coalescing filters whose drainage layers are of a felted fabric material e.g. , as disclosed in our Patent Specification No. PCT/GB 89/00182 are also known. To minimise oil carryover, the drainage layer is preferably treated with a fluorocarbon. The air entering the filter travels radially outward through the coalescing layer 10 and through the drainage layer 24 from which it is discharged radially as indicated at 26. Coalescing filters of this kind can be used in a number of attitudes, but this invention is concerned with coalescing filters which are intended to depend vertically downwards from a filter head and to fit with the removable bowl of such a filter head.

Droplets of oil carried with the air 22 are coalesced by the microporous coalescing layer 22 where the individual tiny droplets are combined, and the oil is carried outwards to the drainage layer 24 where it

- 3 - flows downwards to form an oil-saturated band or wet zone 28 at the lower end of the drainage layer 24, the oil gradually dropping from the wet zone 24 as individual large drops 30.

It will be noted from Figure 1 that the wet zone 28 which forms in normal use of the filter is of sufficient height that a significant portion thereof is in overlapping relationship with the coalescing layer 10. Air from the lower part of the coalescing filter 10 is forced to travel outward through the wet zone 28 where it disturbs the oil and can form bursting bubbles on or adjacent the outer surface of the drainage layer 24. These bursting bubbles of oil in the wet zone 28 re-form fine oil particles on the clean side of the filter which are carried into the clean air stream 26 and reduce the filtration efficiency.

The particular design of coalescing filter shown in Figure 1 is unusual in that the lower end of the drainage layer 24 is not confined within the lower end cap 18 but is free on th- outsi e thereof. More usually, both the coalescing and the drainage layer fit on or into the lower end cap as is shown in Patent

No. GB-A-1345155. The filter may be connected to the filter head with which it is intended to be used by a screw connection 32 in the top cap 16. Alternatively, a tie rod threaded at its lower end can depend from the filter head and can fit into a threaded socket in the end cap 18. The filter may also fit to the filter head by a bayonet connection. With all these arrangements, during the fitting operation the filter is gripped in the fingers of, the user's hand at its lower end and is offered up to the filter head where it is twisted or screwed into position.

This invention is based on the realisation that provided the drainage layer is undisturbed at its lower end and terminates at a quiet region axially below the coalescing layer, the oil saturated band or wet zone that forms in the drainage layer can be caused substantially not to overlap the coalescing layer or do so to a greatly reduced extent. The disturbance which can lead to an undesired increase in the height of the wet zone includes pressure, compression or indenting of the drainage layer, for example when it is pressed into a channel in an end cap and also includes compression of the lower end of the drainage layer 24 by handling with the user's

fingers when the filter is fitted to the filter head. Disturbance which raises the height of the wet band also includes obstructing the lower end of the drainage layer so that the initial escape of oil therefrom is solely through the side and not through the end of the drainage layer.

In one aspect, the invention provides a tubular filter for fitting into a housing with, its axis vertical and for filtration of a stream of gas contaminated with liquid in the form of mist, the filter comprising a microporous filter layer for coalescing the mist as the gas flows through the filter, a macroporous drainage layer into which coalesced liquid flows from the filter layer and then runs to a lower end o. the filter layer from which it drains as droplets, and an end cap which fits gas-tightly to the filter layer to close the lower end of the filter, characterised in that it has one or more of the following features:

(a) the drainage layer extends below the filter layer;

- 6 - (b) the drainage layer extends axially into the end cap, there being drainage channels through the base of the end cap;

(c) the end cap is formed to provide a dual purpose guard which surrounds the lower extremity of the drainage layer, and acts as a finger grip to permit mounting of the filter cartridge to the filter head without cqntact with the drainage layer.

The invention also provides the use for the filtration of a stream of gas contaminated with liquid in the form of mist of a tubular filter through which the gas passes, the filter being supported from a head of a filter housing so as to depend from the head with its axis vertical, and the filter comprising a microporous filter layer for coalescing the mist as the gas flows through the filter, a macroporous drainage layer into which coalesced liquid from the filter layer flows from the filter layer and then runs to the lower end of the filter from which it drains as drops, and an end cap which fits gas-tightly to the filter layer to close the lower end of the filter with the drainage layer having an undisturbed lower end that terminates

at a quiet zone axially below the filter layer to achieve the effect that the height of an oil saturated band at the lowest region of the drainage layer is reduced.

Various forms of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of a prior art coalescing filter;

Figure 2 is a half-sectioned elevation of a filter according to the invention;

Figure 3 is a perspective view of a lower end cap forming part of the filter of Figure 2; and

Figure 4 is a perspective view of an alternative form of the end cap.

In Figure 2, there is shown a coalescing filter of similar overall construction to tnat of Figure 1, except that the top end cap 16a no longer has a threaded connection and the lower end cap 18a which, is

moulded in glass-filled nylon or other suitable plastic material has on its inner end an internally threaded boss 40 which fits onto a threaded lower end 42 of a tie rod 44 depending from a filter head (not shown) . An upwardly facing flange 46 defines with the boss 44 and the intervening portions of the end cap 18a a channel into which the inner and outer foraminous cylinders 12,14, and the coalescing layer 10 are secured by resin 48. A plurality of relatively thin widely-spaced radial ribs 50 support an annular guard 52 in spaced relationship surrounding the lower end of the drainage layer 24 which rests on the ribs 50 but is otherwise unobstructed. The relatively large slots 51 which are defined between adjacent pairs of ribs 50 allow generally unobstructed drainage of droplets from the drainage layer 24. The guard 52 defines a relatively quiet zone for formation of the oil droplets i.e. air flow around the wet band is minimised. Finger grips 54 on the outside of the guard 52 enable the filter element to be tightened onto the end cap without risk of damage to the drainage layer.

- 9 - The end cap construction of Figure 4 is generally similar to that of Figure 3 except that the continuous guard 52 is replaced by a multiplicity of angularly spaced fingers 60, such that the drainage layer 24 fits between the fingers 60 and the flange 46.

In experimental tests it has been found that a filter according to the invention can provide a significant reduction in pressure drop at igh flow rates of air, that the amount of oil mist appearing on the clean si e of the filter is more than twice the amount of oil mist present using a coalescing filter of otherwise similar construction, except that the drainage sleeve is tightly fitted into a channel in the end cap. The oil drainage layer used is typically a PVC impregnated reticulated polyether or polyurethane foam having typically 60 pores per inch and optionally impregnated with a luorocarbon. The drainage layer can also be a felt of polyester or other material which may be impregnated with a fluorocarbon.