WO2008146045A1

WO2008146045A1 - Improvements in coalescing filters

Info

Publication number: WO2008146045A1
Application number: PCT/GB2008/050387
Authority: WO
Inventors: Deborah Michele Spence
Original assignee: Psi Global Ltd
Priority date: 2007-05-29
Filing date: 2008-05-29
Publication date: 2008-12-04
Also published as: GB0710151D0; GB2449846A

Abstract

A filter is provided for coalescing droplets of oil in a stream of gas, comprising a n 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 p ath for oil to flow by gravity from the filter. For ease of manufacture and low oil carry - over, the drainage sleeve has a butt - welded seam e.g. of width 1-3mm formed using an ultrasonic welding machine.

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 0.01-50 μm, 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 filter. The coalescing layer may be of borosilicate glass microfibers (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 a PVC coating to provide resistance to chemical attack. The layer may also be colored 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 6O⁰C whereas for many applications an ability to withstand 12O⁰C 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.

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 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 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 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 6O⁰C, and is resistant to light and to chemical attack is disclosed in our EP-B-0177756, 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 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 is a non-woven felt or wadding thermally bonded by fusible bi-component fibers. The formation of a welded seam in a polyester layer of a filter is disclosed in

GB-A- 1519992. However, the polyester layer (whose construction is not disclosed) is not required to be of a construction such that oil can drain circumferentially through it, the intended use of the filter for membrane filtration being remote from the field of oil coalescence and the intended flow through the polyester layer being purely radial. The polyester layer is sandwiched between a cellulose acetate membrane and an outer supporting sleeve, and there is no disclosure or suggestion that the polyester layer should be an unsupported outer layer or that there should be used a material in which the structure is held together by a small proportion of fusible fibers incorporated into the web.

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, in particular a drainage layer which is easy to manufacture and has low oil carry-over.

That problem is solved, according to the invention, by providing a tubular 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 is a butt-welded tubular structure. 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. It yet further provides a drainage layer for the above filter, comprising fabric at least partially of heat-fusible material butt-welded to form a tube.

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 flowchart showing the steps of manufacturing a filter using a butt- welded drainage sleeve of thermoplastics material.

DESCRIPTION OF PREFERRED EMBODIMENTS

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).

A filter 10 according to an embodiment 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. First and second foraminous tubular members 17, 18 e.g. of steel have between a coalescing layer 20 of pleated borosilicate glass microfiber (see GB-A- 1557821, the disclosure of which is incorporated herein by reference) and on the outer surface of the outer foraminous member 18 a drainage layer 22. 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. The foraminous tubular members, coalescing layer and drainage layer are attached to the end caps by adhesive 24, 26 and the upper end cap is formed with a groove for receiving an O-ring 28 for sealing to a filter housing when the cartridge is screwed into position. The drainage layer comprises heat-fusible fibers butt -welded as at 30.

Coalescing layer

In general, the coalescing layer 20 may be of glass microfibers or other inorganic material, e.g. borosilicate glass microfibers and may be moulded, wrapped or pleated. It may also be of organic microfibers e.g. polyester fibers. In the disclosed embodiment the coalescing layer is of pleated glass microfiber, but in alternative embodiment it may be a molding in borosilicate glass microfibers. 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.

Drainage layer

The drainage layer is normally an unsupported outer layer of the filter. It may have a weight of 100-300g/m², typically about 200g/m², and a thickness of about 2- 7mm, typically about 5mm.

The fibers of the drainage layer advantageously have minimal intra-fiber and inter- fiber 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 fibers (which absorb water) are not used and the drainage layer comprises inert e.g. polyester fibers only. 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 . 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. For satisfactory dimensional stability it has been found in an embodiment of the invention that typically about 10-15 wt% of the fibers of the drainage layer should be fibers which are wholly or partly fusible, e.g. bi-component thermally bondable fibers. If the proportion of bi-component or other fusible fibers 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. In other 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 bind er fibers for bonding the fabric is that there need be no added chemical binder in the resulting spunbonded fabric.

The drainage layer material may on its intended outer face be subjected to a conventional treatment intended to reduce outwardly projecting fibers 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 fibers of the drainage layer may comprise polyester fibers of more than about 6 d.tex, and suitable fibers 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 fiber size giving a smaller pore structure in which oil may be retained by capillary action. Polyester fibers 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, 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 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 fibers on the overall pore structure of the drainage layer being significantly less than that of the majority fibers. For example the bi-component thermally bondable fibers may be polyester fibers 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 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 fibers which are smaller than the majority fibers may be suitable:

(a) PES/PROP 2.2d.tex x 40 mm fibers fusing at temperatures of 130-140⁰C and sold under the trade name Damaklon ESC fusible bi-component by Damaklon

Europe Ltd.

(b) PES 5.5 d.tex x 60 mm bi-component fibers fusing at 165-175⁰C available from EMS Griltex.

(c) PES 4.4 d.tex x 50mm bi-component T91 Terital fibers fusing at 110- 12O⁰C and available from TBM.

Thus a fabric for a drainage layer may be made from 85 wt % of 17 d.tex polyester fibers and 15 wt% of any of the fibers (a) to (c) above, the fiber mixture being carded, crossfolded, needled, sprayed by means of a spray line successively with nitrile rubber (Synthomer 5046), resin (BT 336, BIP Resins) and colourant (CI. pigment Red 101), and passed through an oven to cure the resin. The fabric may be formed into a tube for pulling over the interior portions of a filter using an ultrasonic welding machine e.g. a Pfaff Seamsonic 8310 (Trade Mark) machine.

Butt welding

Ultrasonic seam welding of thermoplastic materials has been known at least sunce the publication in 1966 of US-A-3242029 (Deans). That specification discloses ultrasonic sealing means in the form of a transducer element having a rotating tool operatively coupled thereto, the rotating tool beiung adapted to cooperate with an anvil which may be stationary or may rotate in synchronism with the rotating tool. The apparatus is disclosed as being suitable for use with films of plastics and also of synthetic fabrics e.g. of nylon.

Application of ultrasonic seam welding in relation to woven textile materials made wholly or partly of thermoplastic fibers in the manner of a sewing machine is further discussed in US-A-3666599 (Obeda). In an ambodiment there is provided an ultrasonic seaming apparatus comprising a seaming station including an anvil, an electro-acoustic transducer having a resonator disposed opposite the anvil and defining with the anvil a nip through which a workpiece to be seamed is fed, means for feeding the workpiece through the seaming station at adjustable feed rates, and means couopling the feeding means to a source of electrical energy for controlling the velocity of the resonator in response to the feed rate of the workppiece. US-A-3852144 (Parry) discloses a seaming machine which ultrasonically fuses or bonds two or more layers of thermoplastic material together in a manner similar to a sewing machine and includes means for cutting and fusing the material in a marginal portion adjacent the seam. In the apparatus, ultrasonic means are provided to effect a longitudinal seam and additional means are provided to simultaneously cut and fuse the material adjacent the seamed area. In this manner it is possible to fabricate completely finished articles wherein the folded seam or hem usually deemed necessary to provide an acceptable finished appearance can be omitted.

The state of the art with reference to ultrasonic machines for joining fabrics is further illustrated in US-A-6517651 (Azulay) and 6099670 (Louks et al). To the best of the applicant's knowledge, ultrasonic butt welding has not hitherto been applied to the manufacture of drainage socks for filters.

One machine that may be used is a Pfaff 8310-042 ultrasonic seam welding machine in which a workpiece is held between the sonotrode and the anvil wheel and welded continuously under pressure. When welding continuously by the ultrasonic method, t he material to be welded will be subjected to rapid changing pressure vibrations. The heat develops because of molecular vibrations beneath the material surface, for thin materials within the immediate vicinity of the actual weld. The machine further combines the physical procedures of welding and cutting using a V-wheel, which seams and cuts in a single operation. The material itself will be fed from a roll via a 'folder' (a metal, teardrop shaped device which takes the flat material, and doubles it over to present it to the welding machine with the two edges together ready for welding).

It has been found that the present arrangement provides in production quicker and more accurate seams for the drainage layer, and the seams are smaller than sewn seams and neater in appearance so that the appearance of the resulting filter is improved. Because of the reduced bulk of a welded seam compared to a sewn seam, the drainage layer can be inserted into a filter end cap over a greater range of sizes of filter than with a sewn seam. Oil carry-over is in most instances comparable to that with a sewn seam.

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

Example

A tubular microfiber coalescing element is made based on glass microfibers of diameter 0.5-10μm and aspect ratio 500:1 - 4000:1 using the moulding procedure of our GB-A- 1603519. 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. A drainage layer is formed by pulling over an outer drainage layer constructed from Lantor 7239 (polyester bi-component non- woven fleece material) and formed into a tube by an ultrasonic butt weld. This gives a drainage tube with a very narrow seam width (l-3mm e.g. about 2 mm), and also simultaneously cuts along the seam, hence removing excess material in the same operation. The drainage layer, when inspected by eye, has a uniform appearance without visible holes from needling. 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 is 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, it is found that the prototype described herein shows effective results. The filter may also be used with its axis vertical instead of horizontal, e.g. as described in GB-B-2261830.

Claims

1. A tubular 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, characterised in that the drainage layer is a butt- welded tubular structure.

2. The filter of claim 1 , wherein the outer drainage layer is a non-woven felt or wadding thermally bonded by fusible bi-component fibers.

3. The filter of claim 1 or 2, wherein the butt- weld in the drainage layer defines a seal of width l-3mm.

4. The filter of claim 1 , 2 or 3, wherein the coalescing layer is of glass micro fibers or other inorganic material.

5. A compressor or vacuum pump provided with an oil-coalescing filter as defined in any of claims 1-4.

6. 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 1-4.

7. A drainage layer for the filter of any of claims 1-4, comprising fabric at least partially of heat- fusible material butt- welded to form a tube.