WO2008125885A2

WO2008125885A2 - Improvements in coalescing filters

Info

Publication number: WO2008125885A2
Application number: PCT/GB2008/050264
Authority: WO
Inventors: Hans Gunter Alexander Waltl
Original assignee: Psi Global Ltd
Priority date: 2007-04-16
Filing date: 2008-04-16
Publication date: 2008-10-23
Also published as: GB2448865B; WO2008125885A3; EP2152382A2; GB2448865A; GB0707283D0

Abstract

A filter is provided for coalescing droplets of oil in a stream of gas, comprising an oil coalescing layer of a microfibrous material and a second layer of an oil drainage material located downstream of and in face to face contact with the first layer. The drainage layer is for receiving oil from the coalescing layer and for providing a path for oil to flow by gravity from the filter. Oil carry-over is reduced by providing a drainage layer which comprises at least one inner layer of polyester fabric in an embodiment of spunbonded construction and in an embodiment of polyester filament of trilobal cross- section and an outer layer of different construction.

Description

IMPROVEMENTS IN COALESCING FILTERS

FIELD OF THE INVENTION

The present invention relates to an improved coalescing filter and to its use for the removal of oil droplets from an air or gas stream. Such a stream may, for example, be an oil or gas stream from an oil- lubricated compressor which may be of the rotary vane or the screw type, may be from a vacuum pump or may be traveling along an air or gas line having provision for line filtration.

BACKGROUND TO THE INVENTION

Many manufacturers/suppliers in the compressed air and vacuum industries make filters but few make authentic separators. There is a basic functional difference between the two types. A filter removes solid particles from an air stream via a matrix whilst a separator causes sub-microscopic liquid aerosols to coalesce into larger droplets that can be collected and drained. An efficient separator allows droplets to be retrained without being re-entrained into the air stream whilst allowing oil to drain away fast enough to prevent an undesirable increase in pressure.

In air purification systems, primary separation filters (coalescing filters) are commonly provided downstream of an oil lubricated compressor (see US-A-4231768, Pall Corporation). Coalescing filters are also commonly fitted to vacuum pumps for purifying the air stream from the exhaust side of the pump. In either case, the filter is likely to be challenged by a stream of air containing oil in the form of an aerosol of particle size 0.01-50μm. The air stream may be passed in an in-to out direction through a tubular filter having two working components, a layer within which the oil droplets coalesce and a drainage layer which collects the oil leaving the coalescing layer and retains it until it drips by gravity from the filter, and an in-to-out flow direction is also possible. The coalescing layer may be of borosilicate glass microfibres (see GB-A- 1603519, the disclosure of which is incorporated herein by reference). The drainage layer may be provided by a porous sleeve of plastics foam or by a non-woven fabric. Coalescing filters may be used with their axes vertical, horizontal or less commonly inclined.

Coalescing filters commonly spend their service lives wetted out with oil, and the problem of production of secondary aerosols from such filters by "spitting" is disclosed, for example, in GB-A-2177019 (Pall Corporation). One avenue of research has been to try to reduce spitting and oil-carry-over from a coalescing filter by improving the drainage layer. It should be mentioned that oil carry-over is not in itself necessarily a sign of overall ineffectiveness of the filter. The more efficient the coalescing layer is the more oily liquid is supplied to the drainage layer and the greater the risk that some of the oily liquid may become re-entrained in the air stream.

Open-celled polyurethane foam having about 60 pores/inch and an acrylic coating to provide resistance to chemical attack is a known drainage layer material with low carry-over and suitable in some applications for line filters but is used only rarely if ever as a drainage layer in separators.. The material may also be colored with a dye or pigment to indicate the grade of filter. It has the advantage that its pore structure can be made very uniform which assists drainage and reduces the tendency for oil blisters to form in the exterior of the drainage layer which can be expanded and burst by the stream of compressed air and give rise to spitting. However, in other respects the properties of the material are poor. Its maximum working temperature is 6O⁰C whereas for compressor and vacuum pump applications an ability to withstand 12O⁰C is desirable since the working temperatures of such compressors and vacuum pumps are commonly above 100⁰C. It has poor resistance to contaminants in the oil and is attacked by some of the newer diester synthetic oils. It is easily damaged through handling and becomes brittle on exposure to UV light.

Our WO 89/07484, the contents of which are incorporated herein by reference, discloses another solution of the spitting problem based on the impregnation of the drainage layer with a fluorocarbon or other low surface energy material. As a result, the region of the drainage layer that is wetted out with oil becomes smaller. Treatment of both foams and felts is disclosed. A practical embodiment of that invention employs a drainage layer of a non- woven fabric which is a 50:50 blend of nylon (3.3 d.tex) and polyester (5.3 d.tex) with an acrylic binder and fiuorochemical finish. The weight of the drainage layer is 252 g/m² and thickness 3.2-3.5mm. However, we have found that this material has limitations arising from the way in which it is made. During the manufacturing process, the fibers are formed into a web which is subsequently heavily needled, after which it is dipped into an acrylic binder and finally passed through a fiuorochemical dip in order to reduce the surface energy of the resulting structure. The heavy needling leaves visible holes in the fabric. In use of the filter, oil emerges through the holes and forms droplets at the surface of the fabric which become exploded by the following stream of air, causing oil re-entrainment and reduced separation performance.

A coalescing filter whose drainage layer is simple to make, in use gives air with low oil carryover, can be operated a temperatures above 6O⁰C, and is resistant to light and to chemical attack is disclosed in our EP-B- 1077756, the contents of which are also incorporated herein by reference. That specification discloses a filter for coalescing droplets of oil in a stream of gas, comprising an oil coalescing layer of a micro fibrous material and a second layer of an oil drainage material located downstream of the first layer, said drainage layer being for receiving oil from the coalescing layer and providing a path for oil to flow by gravity from the filter, characterized in that the drainage layer is a non-woven felt or wadding thermally bonded by fusible bi-component fibers.

SUMMARY OF THE INVENTION

A problem with which the invention is concerned is to produce a further filter having a further advantageous combination of properties including e.g. low oil carryover and/or low spitting.

That problem is solved, according to the invention, by providing a filter for coalescing droplets of oil in a stream of gas, comprising an oil coalescing layer of a micro fibrous material and a second layer of an oil drainage material located downstream of the first layer, said drainage layer being for receiving oil from the coalescing layer and providing a path for oil to flow by gravity from the filter, characterized in that the drainage layer comprises at least one inner layer and e.g. of polyester and an outer layer of different construction also e.g. in polyester. The aforesaid filter may be tubular and have a coalescing layer which fits within a drainage sleeve, coalesced oil draining from the sleeve which is of a coarser porosity than the coalescing layer.

Reduced spitting and/or reduced oil carryover may be associated with the different construction and/or properties of the inner and outer drainage layers including in particular different depths, different densities and different fiber diameters. In embodiments the inner drainage layer is of spunbonded material or other material of relatively small depth and relatively high density whereas the outer drainage layer is of greater depth and more open structure. In embodiments the depth of the outer drainage layer is 7-35 times the depth of the inner drainage layer, e.g. 8-15 times the depth of the inner drainage layer, commonly 9-12 times the depth of the inner drainage layer. Compared to the outer drainage layer the inner drainage layer may be relatively thin and relatively high density, though both inner and outer drainage layers may have high gas permeability so as to offer only a minor contribution to pressure drop. The uncompressed density of the inner drainage layer may be 1.5-3 times the uncompressed density of the outer drainage layer e.g. about twice the uncompressed density of the outer drainage layer. The diameter of the fibers of the outer drainage layer may be 2-5 times the diameter of the fibers of the inner drainage layer, e.g. 3-4 times the diameter of the fibers of the inner drainage layer and in an embodiment 4 times the diameter of the fibers of the inner drainage layer. The inner drainage layer or layers is or are may be of spunbonded polyester and the outer drainage layer may be of partly needled and partly thermally bonded polyester.

The invention also includes an oil-lubricated compressor or vacuum pump provided with an oil-coalescing filter as aforesaid. The invention further provides a process for purifying air from an oil lubricated compressor or vacuum pump which comprises passing the air through a coalescing filter as aforesaid. The process may be applied to oil lubricated and oil-free rotary screw compressors e.g. for refrigeration systems or for compressed air supply and e.g. as supplied by manufacturers such as CompAir, Atlas Copco and Durr Technik, oil lubricated and oil free rotary vane compressors (e.g. Hydrovane compressors) and oil lubricated and oil free rotary vane vacuum pumps. The coalescing filter may have an out-to-in or an in-to-out flow direction and may be used as primary and/or secondary oil separator where there is two- stage separation of oil.

BRIEF DESCRIPTION OF THE DRAWINGS

How the invention may be put into effect will now be further described, by way of example only, with reference to the accompanying drawings in which: Fig. 1 is a view in isometric projection and obliquely from above of a filter according to the invention;

Fig. 2 is a view of the filter of Fig. 1 in vertical section; and Fig. 3 is a graph of oil carryover mg/cm³ against gauge pressure (bar) for a filter according to the invention and a standard PSI filter of otherwise generally similar structure.

DESCRIPTION OF PREFERRED EMBODIMENTS

The filter 10 of the invention is of generally conventional structure and comprises first and second end caps 12, 16 with an inlet 14 in the first end cap. A single foraminous tubular member 18 e.g. of steel having an open area of 45-70% has in contact with in its inner surface a coalescing layer 20 and on its outer surface a drainage layer 22, which in the present embodiment comprises inner and outer drainage layers.

The filters preferably have end caps 12, 16 which have low affinity for contaminants, and glass-filled nylon which has an undesirably high affinity for water is advantageously not used, PBT or other polyester or metal e.g. steel being preferable end cap material. As previously explained, a coalescing filter or separator for compressed air causes sub-microscopic liquid aerosols to coalesce into larger droplets that drain away from the air stream. An efficient separator allows droplets to be retrained without being re-entrained whilst allowing oily liquid to drain away fast enough to prevent an increase in pressure. In embodiments, the coalescing filter may be of external diameter 50-150 mm, internal diameter 25-100 mm, height 50-500 mm and rated flow of 0.3-5 M³/min. In embodiments it may be required to operate at temperatures up to 12O⁰C give rise to a remaining oil content 1-3 mg/m³ (more preferably < 1 mg/m³ and more preferably <0.5 mg/m³) with a pressure loss that can be as low as 200mb (3psi).

Coalescing layer

The coalescing layer 20 may be of glass micro fibres or other inorganic material, e.g. borosilicate glass micro fibres and may be molded, wrapped or pleated. It may also be of organic micro fibres e.g. polyester fibers. Coalescing layers of all three of the above mentioned constructions are used in filters made and sold by the present applicants and by other filter manufacturers.

In embodiments of the invention the coalescing layer is a molding in borosilicate glass microfibres as described in our GB-A- 1603519 and US-A-4303472, the disclosures of which are incorporated herein by reference. These specifications disclose a method for forming a tubular filter element which includes the steps of: (a) forming a slurry of fibers in a liquid; (b) introducing the slurry under pressure into the top of an annular molding space defined between a central core, a vertical cylindrical screen spaced from and outward of said core and a support defining a lower boundary for the molding space so that a mass of fibers becomes compacted on the screen and liquid is discharged from the molding space through the screen; (c) progressively increasing the height of the effective open area of the cylindrical screen by moving upwardly a sleeve in sliding contact with the cylindrical screen at a rate substantially equal to the rate at which the height of the mass of fibers increases above the support; and (d) removing the resulting tubular mass of fibers from the molding space.

In a practical embodiment of the above mentioned moulding process, the filter element comprises a mass of borosilicate glass microfibres bounded by a foraminous outer support sheet e.g. of steel mesh with an open area of 45-70%. The borosilicate fibers are dispersed in water in a blending tank under mechanical agitation, and an acid, e.g. hydrochloric or sulfuric acid is added to give a pH of 2.9-3.1 at which the dispersion is stable, the fiber to water ratio being 0.01 - 0.5 wt%, typically 0.05 wt%. The resulting slurry is introduced into the molding space under a pressure of typically 414-689 millibar (6-10 p.s.i). and molded as described above. The sleeve is raised progressively at substantially the same rate as that at which the height of the fiber mass increases in order to maintain a flow of the dispersion to the point where the mass of fibers is building up, after which air may be passed through the molded element to reduce the content of residual water. The formed filter element is removed from the molding space, oven dried, resin impregnated and heated to harden the resin. The resin may be e.g. a silicone or an epoxy resin and may be impregnated in a solvent such as acetone, but it is now in some embodiments preferred that the resin should be a phenolic resin which may be impregnated as an aqueous solution. The fibers in a finished filter element produced by the above method are predominantly layered in planes perpendicular to the direction in which the dispersion flows into the molding space, and the same packing pattern arises throughout the range of forming pressures that can be used in practice. This non-random packing pattern results in a filter element that provides efficient depth filtration and has an advantageous combination of properties including high burst strength and low pressure drop. The molded tubular elements may be bonded to end caps to complete the formation of the filter.

In further embodiments the resin used may be an acrylate, see WO

2007/088398, the disclosure of which is incorporated herein by reference. That reference describes and claims inter alia a method of moulding a filter, which comprises the steps of: forming a tubular article from an aqueous dispersion comprising glass micro-fibers and a water-soluble acid-based resin binder comprising a carboxylated acrylic polymer and a polyfunctional alcohol; and heating the article to successively drive off water and cure the resin.

The above process has been used e.g. to manufacture air/oil separators designed to remove oil mist generated in screw or sliding vane compressors or in vacuum pumps where the size of the particles generated lies in the range 0.3-1.5 microns (μm) and also to manufacture in-line filters for removing oil, water and contaminants from a stream of compressed air. Inner drainage layer

The inner drainage layer may comprise one or more e.g. two wrapped layers of non-woven fabric. Each of the layers may be formed wholly or predominantly of polyester fibers. The polyester fibers may have a softening temperature of at least 18O⁰C and a melting temperature of at least 25O⁰C. Such polyester fibers can be formed by melt extrusion and are commonly obtained from an aromatic dicarboxylic acid (e.g., terephthalic acid, isophthalic acid, naphthalene dicarboxylic acid, etc.) and an alkylene glycol (e.g., ethylene glycol, propylene glycol, etc.) as the diol component. In a preferred embodiment the polyester comprises at least 85 mole percent of polyethylene terephthalate fibers. As the polyester fibers are melt extruded during fiber formation they optionally may be blended with approximately 5 to 20 percent by weight of similarly melt-extruded binder fibers. In some embodiments the predominant polyethylene terephthalate fibers are blended with binder fibers of polyethylene isophthalate which melt about 4O⁰C to 5O⁰C below the melting temperature of the polyethylene terephthalate fibers. One advantage of using binder fibers for bonding the fabric is that there need be no added chemical binder in the resulting spunbonded fabric. In embodiments, the polyester fibers may be of non-circular cross-section and may be lobed or striated being e.g. bilobal, trilobal, tetralobal, heptalobal or of more complex non-cylindrical shapes.

The inner drainage layer may be of substantially random fabric which is non- directional in nature. The continuous polyester filaments thereof are bonded to each other at points of contact (i.e., at cross-over points) with the retention of permeability to gases. The layer or web may be substantially fully bonded in the sense that the filaments are bonded together at a multitude of cross-over points as can be confirmed through microscopic examination. The bonding at substantially all cross-over points among the continuous filaments within the web can be accomplished by the melting of binder fibers e.g. by calendering or hot air bonding. The continuous filaments of such web may have a denier per filament of approximately 2 to 6 (e.g. about 4), see e.g. US-A- 5750151 (Brignola et al, Reemay Inc the disclosure of which is incorporated herein by reference). Preferably the inner drainage layer is a spunbonded favric i.e. it has been spunlaid and then bonded by a combination of heat and pressure by heated calender rollers which may be patterned but are preferably smooth faced for an overall bond. Apparatus for spunbonding polyester and other fibers is available e.g. from Reifenhauser Gruppe under the trade name Reicofil. In the above process melt is fed via a coathanger die to a spinneret which forms a curtain of filaments which are then cooled in a blowing zone by means of a stream of air, drawn by aerodynamic forces and then conveyed to a discharge channel where the fibers are swirled around and deposited randomly on a wire mesh belt. The deposited material is heat bonded by calender rolls, after which the calendered material is cooled to give material of grammage typically 10- 100 g.s.m.

A suitable non-woven spunbonded polyester fabric is of thickness 0.2-0.4 mm e.g. about 0.3 mm and of basis weight 10-50 g.s.m, more typically 25-35 g.s.m. e.g. about 30 g.s.m, corresponding to uncompressed densities in the range about 5-25 mg/ml, in some embodiments about 100-200 mg/ml. A preferred inner drainage layer is one or two layer Reemay 2014 spunbonded polyester fabric of randomly arranged continuous filament of trilobal section, of thickness about 0.228 mm (9 mils) and of basis weight about 34 g.s.m. available from Reemay, Inc., Old Hickory, Tenn., USA and corresponding to an uncompressed density of about 100 mg/ml and an air permeability of 3500 l/m2/s measured according to DIN 53887 B.

Outer drainage layer

The outer drainage layer may have a weight of 100-300 g.s.m., typically about 200 g.s.m., and a thickness of about 2-7 mm, typically about 5mm. The fibres of the outer drainage layer advantageously have minimal intra-fibre and inter-fibre affinity for oil or other contaminants, and can be formed into a dimensionally stable felt or wadding of reproducible pore size with little or no needling. For reduced affinity for contaminants, nylon fibres (which absorb water) are not used and the outer drainage layer comprises inert e.g. polyester fibres only. For satisfactory dimensional stability it has been found in an embodiment of the invention that typically about 10-15 wt% of the fibres of the outer drainage layer should be fibres which are wholly or partly fusible, e.g. bi-component thermally bondable fibres. If the proportion of bi-component or other fusible fibres is less than 5%, there is little bonding, whereas if there are more than 25% the bonded fabric becomes very stiff. We have found that with minimal needling and thermal bonding the resulting fabric has a generally uniform pore size which reduces or prevents preferential local oil through- flow.

The outer drainage layer material may on its intended outer face be subjected to a conventional treatment intended to reduce outwardly projecting fibres which provide return paths for oil to the air stream. Such treatments include application of resin and surface heating or singeing, but obstruction of the exit pores of the drainage layer should be avoided. The material may also incorporate a dye or pigment for identification purposes. Embodiments of the drainage layer are resistant to the stress of pulses of air and are resistant to contact e.g. from the user's fingers, whereas a foam drainage layer exhibits poor resistance to such contact.

The majority fibres of the outer drainage layer may comprise polyester fibres of more than about 6 d.tex, and suitable fibres currently available are of sizes 7, 17 and 24 d.tex, of which the 17 d.tex size has been found in some embodiments to give the best results, the 7 d.tex fibre size giving a smaller pore structure in which oil may be retained by capillary action. Polyester fibres have been found to combine the properties of quick absorption of oil droplets into the material, ability to absorb a large mass of oil, quick oil drainage, and low final retention mass leading to a low final wet-band height when the resulting filter is in use.

The bicomponent fibers which are preferred for use in the outer drainage layer have a relatively high-melting core and a lower-melting sheath e.g. a core which melts at above about 200⁰C and a sheath which melts at about 110-175⁰C. They may comprise about 10 wt % of the fibers of the drainage layer. The felt or wadding may be obtained by forming a loose web of the fibers, and passing the loose web between heated rollers so as to form a structure of an intended thickness and pore size, and it need not contain fluorocarbon. The minority bi-component fibers may be of the same chemical composition as the majority fibers of the drainage layer, or they may be of different composition. They may be of the same diameter as the majority fibers, or they may be larger or smaller, the effect of the relatively low proportion of thermally bondable fibres on the overall pore structure of the drainage layer being significantly less than that of the majority fibres. For example the bi-component thermally bondable fibres may be polyester fibres of the same diameter as the remaining fibers. A suitable drainage layer may be made from a 200 g/m² thermally bonded 17 d.tex polyester needle felt (available from Lantor (UK) Limited) sprayed with 30 g/m² water-based nitrile binder containing a white pigment, crushed to a thickness of 5mm and formed into a sleeve (density including nitrile binder about 46 mg/ml) which fits over the coalescing layer. The following other heat-fusible fibres which are smaller than the majority fibres may be suitable: (a) PES/PROP 2.2d.tex x 40 mm fibres fusing at temperatures of 130-140⁰C and sold under the trade name Damaklon ESC fusible bi-component by Daqmaklon Europe Ltd.

(b) PES 5.5 d.tex x 60 mm bi-component fibres fusing at 165-175⁰C available from EMS Griltex. (c) PES 4.4 d.tex x 50mm bi-component T91 Terital fibres fusing at 110-

12O⁰C and available from TBM.

Thus a fabric for an outer drainage layer may be made from 85 wt % of 17 d.tex polyester fibres and 15 wt% of any of the fibres (a) to (c) above, the fibre mixture being carded, crossfolded, needled, sprayed by means of a spray line successively with nitrile rubber (Synthomer 5046), resin (BT 336, Beatle) and colourant (Artilene Red PBL, Clariant or a white pigment), and passed through an oven to cure the resin. The resulting material may have an air permeability of 62 cm³/cm²/s at 1 mm water according to BS 9237. In more recent embodiments the outer drainage layer may be of thermally bonded material rather than needled material. An embodiment of the invention will now be described in the following Example.

Example

A tubular micro fibre coalescing element is made based on glass micro fibres of diameter 0.5-10μm and aspect ratio 500:1 - 4000:1 using the moulding procedure of our GB-A- 1603519 and using a single foraminous steel support cylinder. The element has an inside diameter of 75mm, an outside diameter of 95mm and a length of 250mm. It is impregnated with a phenolic resin and cured in an oven, after which the ends are sanded flat.

An inner drainage layer is formed by wrapping two layers of Reemay 2014 spun-bonded polyester fabric around the moulded filter element and support cylinder then pulling over an outer drainage layer constructed from Lantor XS 11 (polyester bi- component non- woven fleece material). The outer drainage layer, when inspected by eye, has a uniform appearance. The resulting tubular structure is fitted with steel end caps to form a filter element for in-to-out air or gas through-flow. The filter element is placed with its axis horizontal in a filter housing and challenged with air from an oil- lubricated rotary vane vacuum pump (Busch R5 0302D operating at 415V and 50 Hz). Aerosol carryover is a measurement of how much contaminated air leaves the pump, hence an important factor when considering filter efficiency.

From tests, we found that the prototype described herein exhibits a "cold start" pressure drop of <350 mbar in the first few tens of seconds after start up with the vacuum pump being allowed to cool to ambient temperature and then restarted. A high peak pressure drop in this period can arise because oil in some filter constructions the pores of filter become clogged with oil from previous use of the pump, the oil being more viscous when cold and therefore offering greater resistance to gas flow through the filter. A high cold start pressure drop is potentially damaging the pump when recurring over time. The relatively low cold start pressure drop of the present filter may be compared to a pressure drop approaching 600 mbar from other commercially available filters. The wet flow pressure drop of the filter is similar to that of a conventionally constructed filter from the present applicants.

A spit test is performed by allowing the vacuum pump to run at -0.75 gauge pressure. A Gelman paper is placed 100 mm from the exhaust from the pump which has passed through the coalescing filter is caught on the Gelman paper over a period of four hours. The present filter exhibits oil spitting decreased by more than half (0.63 grams over a period of 4 hours) compared to the applicants' conventional filter of otherwise similar construction.

Aerosol carryover is a measurement of how much contaminated air leaves the pump, hence the most important factor when considering the filter efficiency. Fig. 3 shows oil carryover (mg/m³) as a function of pressure (bar g) for the applicant's conventional filter (upper line) and the filter according to the invention (lower line) together with an upper horizontal dotted line showing the current legal limit for oil- contaminated exhaust gases of 5 ppm and a lower horizontal dotted line showing the proposed new legal limit of 0.5 ppm. The filter of the invention exhibits surprisingly low aerosol carryover and is below the 0.5 ppm limit at all measured pressures. It will be noted that the change to the present construction produces an improvement in spitting combined with an improvement in carryover.

The filter may be used with its axis vertical instead of horizontal, e.g. as described in GB-B-2261830 or with its axis inclined.

Claims

1. A filter for coalescing droplets of oil in a stream of gas, comprising an oil coalescing layer of a microfibrous material and a second layer of an oil drainage material located downstream of the first layer, said drainage layer being for receiving oil from the coalescing layer and providing a path for oil to flow by gravity from the filter, characterized in that the drainage layer comprises inner and outer layers of fabrics of different construction.

2. The filter of claim 1, wherein the depth of the outer drainage layer is 7-35 times the depth of the inner drainage layer.

3. The filter of claim 1, wherein the depth of the outer drainage layer is 8-15 times the depth of the inner drainage layer.

4. The filter of claim 1, wherein the depth of the outer drainage layer is 9-12 times the depth of the inner drainage layer.

5. The filter of any preceding claim, wherein the uncompressed density of the inner drainage layer is 1.5-3 times the uncompressed density of the outer drainage layer.

6. The filter of any preceding claim, wherein the uncompressed density of the inner drainage layer is twice the uncompressed density of the outer drainage layer.

7. The filter of any preceding claim, wherein the diameter of the fibers of the outer drainage layer is 2-5 times the diameter of the fibers of the inner drainage layer.

8. The filter of any of claims 1-6, wherein the diameter of the fibers of the outer drainage layer is 3-4 times the diameter of the fibers of the inner drainage layer.

9. The filter of any of claims 1-6, wherein the diameter of the fibers of the outer drainage layer is about 4 times the diameter of the fibers of the inner drainage layer.

10. The filter of any preceding claim, wherein the inner drainage layer is of weight 10-50 g.s.m. and the outer drainage layer is of weight 100-300 g.s.m.

11. The filter of any preceding claim, wherein the inner drainage layer is of weight 25-35 g.s.m. and the outer drainage layer is of weight 150-250 g.s.m.

12. The filter of any preceding claim, wherein the inner drainage layer or layers is or are is of spunbonded polyester.

13. The filter of any preceding claim, wherein the inner drainage layer or layers are of polyester filament of trilobal cross-section.

14. The filter of any preceding claim, wherein the outer drainage layer is a non- woven felt or wadding thermally bonded by fusible bi-component fibres.

15. The filter of any preceding claim, which is tubular and has a coalescing layer within a drainage sleeve, coalesced oil draining from the sleeve which is of a coarser porosity than the coalescing layer.

16. The filter of claim 15, wherein the filter has a single foraminous tubular steel support located between end caps of the filter and between the coalescing layer and the inner drainage layer.

17. The filter of any preceding claim, wherein the coalescing layer is of glass microfibres or other inorganic material.

18. A compressor or vacuum pump provided with an oil-coalescing filter as defined in any preceding claim.

19. The compressor of claim 18 which is a rotary screw compressor.

20. The compressor of claim 8 which is a rotary vane compressor.

21. A rotary vane vacuum pump according to claim 18.

22. A process for purifying air or gas from a rotary screw compressor ,rotary vane compressor or vacuum pump which comprises passing the air through the coalescing filter of any o f claims 1-17.