GB2448865A

GB2448865A - A coalescing filter

Info

Publication number: GB2448865A
Application number: GB0707283A
Authority: GB
Inventors: Hans Gunter Alexander Watl
Original assignee: PSI Global Ltd
Current assignee: PSI Global Ltd
Priority date: 2007-04-16
Filing date: 2007-04-16
Publication date: 2008-11-05
Anticipated expiration: 2027-04-16
Also published as: EP2152382A2; GB2448865B; WO2008125885A3; GB0707283D0; WO2008125885A2

Abstract

A filter for coalescing droplets of oil in a stream of gas, comprises an oil coalescing layer 20, of a microfibrous material and a second layer 22, of an oil drainage material located downstream of the first layer. The second layer is for receiving oil from the coalescing layer and provides a path for oil to flow by gravity from the filter. The drainage layer comprises at least one inner layer of polyester fabric and an outer layer of different construction. Preferably the inner drainage layer is a spunbonded polyester filament of trilobal cross-section and the outer drainage layer is a non-woven felt or wadding thermally bonded by bi-fusable bi-component fibres. Preferably air enters the filter through an inlet 14 provided in one of two metal or plastic end caps 12, 16. Advantageously the filter may be included in a compressor or vacuum pump.

Description

IMPROVEMENTS IN COALESCING FILTERS

FIELD OF THE INVENTION

The present invention relates toan improved coalescing filter and to its use for the removal of oil droplets from an airstream, for example but not limited to an airstream from an oil-lubricated compressor or vacuum pump or in an air line.

BACKGROUND TO THE INVENTION

in air purification systems, primary separation filters (coalescing filters) are commonly provided downstream of an oil lubricated compressor (see US-A-423 1768, Pall Corporation). Coalescing filters are also commonly fitted to vacuum pumps for puri1'ing 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 0.01-50 Lm, though the filter may also be arranged for fluid flow in an out-to-in direction. The air stream is usually 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 fitter. 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 or horizontal.

Coalescing filters commonly spend their service lives wetted out with oil, and the problem of production of secondary aerosols from such filters is disclosed, for example, in GB-A-2177019 (Pall Corporation). One avenue of research has been to try to reduce oil-carry-over from a coalescing filter by improving the drainage layer.

A known drainage layer material with low carry-over is an open-celled polyurethane foam having about 60 pores/inch and an acrylic coating to provide PSI.025*P-IJK resistance to chemical attack. The layer may also be coloured with a dye or pigment to indicate the grade of filter. The material 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. However, in other respects the properties of the material are poor. Its maximum working temperature is 6OLC whereas for many applications an ability to withstand 12OLJC is necessary. 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. l0

Our WO 89/07484, the contents of which are incorporated herein by reference, discloses another solution of the oil carry-over 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 IS of both foams and felts is disclosed. A practical embodiment of that invention employs a drainage layer of an 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 fluorochemical 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 fibres are formed into a web which is subsequently heavily needled, after which it is dipped into an acrylic binder and finally passed through a fluorochemical 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 poor 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 60 C, and is resistant to light and to chemical attack is disclosed in our EP-B-O 177756, the contents of which are laso 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 microfibrous PSIO25.P.UK 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, characterised in that the drainage layer is a non-woven felt or wadding thermally bonded by fusible bi-component fibres.

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 carry-over 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 microfibrous material and a second layer of an oil drainage material located downstream IS 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, characterised in that the drainage layer comprises at least one inner layer of polyester fabric and an outer layer of different construction. 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.

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 purifring air from an oil lubricated compressor or vacuum pump which comprises passing the air through a coalescing filter as aforesaid.

BRIEF DESCRIPTION OF THE DRAWINGS

How the invention may be put into effect will now be further described, by way ofexarnple only, with reference to the accompanying drawings in which: Fig. I is a view in isometric projection and obliquely from above of a filter according to the invention; and PSI.025-P-UK Fig. 2 is a view of the filter of Fig. I in vertical section.

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 foraminous tubular member 18 e.g. of steel 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 being preferable end cap material.

Coalescing layer The coalescing layer 20 may be of glass microfibres or other inorganic material, e.g. borosilicate glass microfibres and may be moulded., wrapped or pleated. It may also be of organic microfibres e.g. polyester fibres. Preferably the coalescing layer is a moulding in borosilicate glass microfibres as described in our GB-A-1603519, the disclosure of which is incorporated herein by reference.

Inner drainage layer The inner drainage layer may comprise one or more e.g. two layers of non polyester fabric e.g. non-woven spunbonded polyester fabric of thickness 0.2-0.4 mm e.g. about 0.28 mm and of basis weight 10-50 gsm, more typically 25-35 gsm e.g. about gsm. A preferred inner drainage layer is one or two layer Reemay 2014 spunbonded polyester fabric of filament of trilobal-section, of thickness about 0.28 mm and of basis weight about 34 g.s.m. available from Reemay, inc., Old Hickory, Tenn., USA.

PStO25-P-1JK Outer drainage layer The outer drainage layer may have a weight of lO0-300g/mm2, typically about 200g/mmz, and a thickness of about 2-7mm, 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, whercas if there are more than 25% the bonded fbric 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 PS.025-P-Uk by capillary action. Polyester fibres have been 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, arid low final retention mass leading to a low final wet-band height when the resulting filter is in use.

The bicomporient fibres 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 200 C and a sheath which melts at about 110-175 C. They may comprise about 10 wt % of the fibres of the drainage layer. The felt or wadding may be obtained by forming a loose web of the fibres, and passing the loose web between heated rollers so as to form a structure of an intended thickness and porc size, and it need not contain fluorocarbon. The minority bi-component fibres may be of the same chemical composition as the majority fibres of the drainage layer, or they may be of different composition. They may be of the same diameter as the majority fibres, 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-coniponent thermally bondable fibres may be polyester fibres of the same diameter as the remaining fibres. A suitable drainage layer may be made from a 200 g/m2 thermally bonded 17 d.tex polyester needlefelt (available from Lantor (UK) Limited) crushed to a thickness of 5mm and formed into a sleeve 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 130-140 C 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 165-175 C available from EMS Griltex.

(c) PES 4.4 d.tex x 50mm bi-component T91 Terital fibres fusing at 110-120 C 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 PSI,025.P-%JK carded, crossfolded, needled, sprayed by means of a spray line successively with nitrile rubber (Synthomer 5046), resin (BT 336, Beatle) and colourant (Arti)ene Red PBL, Clariant), and passed through an oven to cure the resin.

An embodiment of the invention will now be described in the following

Example.

Example

A tubular microfibre coalescing element was made based on glass microfibres of diameter 0.5-I 0.tm and aspect ratio 500:1 -4000:1 using the moulding procedure of our GB-A-1603519. The element had an inside diameter of 75mm, an outside diameter of 95mm and a length of 250mm. It was impregnated with a phenolic resin and cured in an oven, after which the ends were sanded flat. An inner drainage layer was 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 XS1 I (polyester bi-component non-woven fleece material). The outer drainage layer, when inspected by eye, had a uniform appearance without visible holes from needling. The resulting tubular structure was fitted with steel end caps to form a filter element for in-to-out air or gas through-flow. The filter element was placed with its axis horizontal in a filter housing and challenged with air from an oil-lubricated rotary vane compressor. 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 showed particularly effective results with even below 0.5 ppm aerosol air contamination which has been proposed as a potential new limit in the US. The filter also exhibited good pressure drop at "cold start", good wet flow pressure drop, and relatively low oil spitting. The filter may be used with its axis vertical instead of horizontal, e.g. as described in GB-B-2261830.

PS 1,025-P-UK

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, characierised in that the drainage layer comprises at least one inner layer of polyester fabric and an outer layer of different construction.

2. The filter of claim 1, wherein the inner layer or layers is or are is of spunbonded polyester.

3. The filter of claim I or 2, wherein the inner layer or layers are of polyester filament of trilobal cross-section.

4. 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.

5. The filter of any preceding claim, which is tubular and has 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.

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

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

8. A process for purifying air from a rotary vane compressor or vacuum pump which comprises passing the air through the coalescing filter of any of claims 1-6.

PSI,025-P-LJK