GB2563814A

GB2563814A - Filter element testing

Info

Publication number: GB2563814A
Application number: GB1706262.1A
Authority: GB
Inventors: Gunter Alexander Waltl Hans; Nicholas Maria Waltl Hans; Chochrane Lloyd
Original assignee: PSI Global Ltd
Current assignee: PSI Global Ltd
Priority date: 2017-04-20
Filing date: 2017-04-20
Publication date: 2019-01-02
Also published as: GB201706262D0

Abstract

A method of testing a tubular filter or filter element, includes supplying light to a lumen of the filter or filter element such as a filter for air/oil separation having a molded coalescing layer of borosilicate glass microfibers. Light distribution from an external cylindrical surface of the filter or filter element is observed and depending on the observation of regions of anomalous light emission from the external cylindrical surface the filter or filter element is accepted or rejected. A further disclosed invention is directed to an alternative method of testing a tubular filter or filter element where heat is supplied to a lumen of the filter or filter element and the distribution of heat from an external cylindrical surface of the filter or filter element is observed. The method enables identification of filter defects thus reducing failures being passed into the manufacturing process and supplied to marketplace.

Description

FILTER ELEMENT TESTING

FIELD OF THE INVENTION

This invention relates to the testing of tubular filters, particularly though not exclusively coalescing filters, and to filter manufacturing methods including such testing. A coalescing filter may in some embodiments be used 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 airline.

BACKGROUND TO THE INVENTION

Historically PSI have used aerosol challenge (mg/m3) to test the integrity of their final air/oil separator at the end of the manufacturing process. This was designed as a final proof test the overall assembly. During this process, the separator is subject to a challenge of aerosol at low flow rate. The concentration downstream of the separator is then quantified into a pass or a fail.

SUMMARY OF THE INVENTION

Both light intensity and thermal imaging techniques have been identified within the moulding process of air/oil separators to identify defects which may occur during manufacture of a filter layer, more particularly a moulded layer.

It has now been appreciated that it may be desirable to provide a process check of the moulded tube before they reach the production line. As part of the manufacturing process it is expected that the packing density of the glass fibres will never be 100% homogeneous. Sections of the cured moulded glass fibre tube may have voids or channels that arise during the manufacturing process, and these defects may cause excessive aerosol to pass through the separation media in application which fall outside of market and environmental requirements of l-3mg/m3 downstream of the air/oil separator The ability to identify clear process defects due to channels or voids is important to reduce failures being passed into the manufacturing process and being supplied into the marketplace.

In an embodiment, the invention provides a method of testing a tubular filter or filter element, comprising: supplying light or heat to a lumen of the filter or filter element; observing the distribution of light or heat from an external cylindrical surface of the filter or filter element; and accepting or rejecting the filter or filter element depending on observation of regions of anomalous light or heat emission from the external cylindrical surface.

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 that may be tested according to the invention;

Fig. 2 is a view of the filter of Fig. 1 in vertical section;

Fig. 3 shows probes including LEDs as light sources for testing filters as shown in Figs 1 and 2;

Fig. 4a is a photograph of apparatus for testing a tube by thermal imaging and Fig. 4b is a thermal image of the tube of Fig. 4a under test;

Fig. 5a shows visible light emerging through an impregnated glass fiber molded tube sub assembly; Fig 5b is similar but shows light emitted by a defective impregnated final separator assembly; Fig. 5c is similar to Fig. 5a and shows a passed impregnated glass fiber molded tube sub assembly and Fig. 5d is similar to Fig 5b but shows a passed impregnated final separator assembly;

Fig. 6a is a thermal image of a defective impregnated glass fiber tube final molded assembly and Fig. 6b is a plot of measured temperature against axial position for the tube of Fig. 6a;

Fig. 7a is a photograph of a test rig for a final molded tube assembly and Fig. 7b is a thermal image of an acceptable final molded tube assembly.

DESCRIPTION OF PREFERRED EMBODIMENTS

As previously explained the present invention provides a process for testing tubular filters based on pleated or moulded filter media.

Filters to be tested

The invention relates generally to the testing of tubular filters which may be for in-line filters, filter silencers, separators for vacuum pumps, separators for screw or rotary vane compressors or other tubular filters for air/oil separation.

In an embodiment, the invention is applicable to the testing of a coalescing filter or separator for compressed air which 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 30-150 mm, internal diameter 15-100 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 120°C 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 to which the invention is applicable 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 molded borosilicate glass microfiber and on the outer surface of the outer foraminous member 18 a drainage layer 22. The end caps 12, 16 preferably 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 22 comprises heat-fusible fibers or foam material 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 molded, 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, the coalescing layer is a molding in borosilicate glass microfibers as described in our GB-A-1603519 and US-A-4303472, the disclosures of which are incorporated herein by reference. These specifications disclose a process 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 molding process, the filter element comprises a mass of borosilicate glass microfibers 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 process 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 of the above mentioned process 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 process 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.

An alternative moulding process disclosed in EP-B-2456538 forms a fibrous layer within a foraminous tubular support, and comprises the steps of: (a) providing the tubular support for said fibrous layer; (b) providing forming mesh over the curved surface of the support and closing its ends against escape of fluid; characterized by (c) introducing fibrous slurry from a pressurized source into an annular molding space between a rotary molding torpedo and an inner surface of the support, the torpedo having at least one channel for slurry opening along a curved surface thereof, fibers in the slurry collecting within the forming mesh and within the support to form the layer.

In the above process, the tubular support may be of foraminous metal and it may be directed generally horizontally during the forming process.

In an embodiment of the above process, a bearing member at a headstock end of the molding torpedo supports the torpedo for rotation and is configured to meet and close a headstock end face of the support on insertion of the torpedo into the support to reduce or prevent escape of slurry at the headstock end of the support during molding; and a tailstock moves a disc to meet a tailstock end of the support, the disc being configured to close an end face of the support to reduce or prevent escape of slurry from an end surface of the support during molding. Half-molds are releaseably closed about the support to permit molding thereof, each half mold carrying forming mesh having a semi-cylindrical forming surface that meets the curved surface of the support when the half-molds are closed. Each half mold comprises (a) support plates for the forming mesh, the support plates being radially directed and spaced longitudinally apart in a closely spaced array, spaces between the plates permitting liquid from the slurry to flow from the forming mesh, and (b) a suction box within which the support plates and forming mesh are secured, suction lines leading from the suction box, and headstock and tailstock end plates of the suction box respectively sealing to the bearing member and to the tailstock when the half molds are closed about the support.

The pH required for stable microfibre dispersion may be significant for the timing of resin addition since phenolic resins, for example, polymerize in acid and cannot be added to a dispersion at pH 2.9 or the like. A molded filter can only be treated with phenolic resin after the molding operation has taken place. However, water-soluble thermosetting acrylic acid-based resin binders can be incorporated into aqueous dispersions of heat resistant inorganic fibers e.g. borosilicate glass micro-fibers and the dispersions can be used to mould filter elements, after which the molded filter elements can be dried and heat-cured. Unlike phenol-formaldehyde binders, the acrylic resins provide acid-tolerant binders that can be incorporated into the filter as-molded without post-molding drying and impregnation steps, and can be cured following the molding process to give a filter having similar physical properties and performance e.g. in oil coalescing to a phenol-formaldehyde bound filter but of improved appearance.

One preferred resin that may be used is Acrodur DS 3530 (BASF) which is an aqueous solution of a modified polycarboxylic acid and a polyhydric alcohol as crosslinking component and as supplied has a solids content of 50 wt%, a pH of 2.5-4, a molecular weight of about 12,000 and a Brookfield viscosity at 23°C of 150-300 mPa.s. A further more preferred resin is Acrodur 950 L (BASF) which is similar, but has a pH of 3.5, a molecular weight of about 80,000 and a Brookfield viscosity at 23°C of 600-4000 mPa.s. It has been stated to crosslink at temperatures as low as 180°C., with a recommended temperature of 200°C and is an aqueous solution of a substituted polycarboxylic acid. It contains a polybasic alcohol as the crosslinking agent. The polycarboxylic acid is a carboxylated acrylic polymer and the polybasic alcohol is triethanolamine. The preparation is presented as a 50% solids solution in water with viscosity of 1000-4500 cps and specific gravity of 1.2. It may be used in the dispersion at e.g. a concentration of about 40-80 g/liter depending on the desired mechanical properties of the filter, the effect of resin concentration in the dispersion on pressure drop of the resulting filter medium being relatively small. Waste aqueous liquid from the molding process may be recycled to the dispersion-forming tank giving economy of resin use. Cure temperatures of Acrodur resins are typically 130°C and 200°C.

The above processes have 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 (pm) 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 180°C and a melting temperature of at least 250°C. 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 noncircular 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 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 40°C to 50°C 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.

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 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 200°C and a sheath which melts at about 110-175°C. 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.

Testing methods

Fig 3 shows a pair of light-source probes for testing tubular filters e.g. those described above. Each probe is configured to fit within an internal lumen of the tubular filter had has an axially extending array of LEDS e.g. a 4 x 40 array with spacing between adjacent rows of 4 LED elements being extended in the upper probe shown relative to the lower probe so as to provide a light source of greater length. The LEDs may be driven by 12 V DC and may emit visible e.g. white light of intensity 15K millicandelas. This intense light utilized in a dark environment can be used to identify sections of the cured molded glass fiber tube which may have voids or channels that occur during the manufacturing process.

The images in Figs. 5a-5d illustrate examples of defects on both the glass fiber molded tube sub-assemblies and the final separator assembly. The light intensity is higher and more detectable for the molded tube sub assembly as opposed to the final assembly. Testing is then carried out in application to determine the actual effects of the voids and channels based on remaining oil (mg/m3) downstream of the air/oil separator in comparison to marketplace and environmental requirements.

Within latent oil vacuum or compressor applications, the temperature can reach in excess of 80°C. This concept can be utilised as part of a tube integrity test to determine if a heat differential exists across the moulded tube from the internal high exposed temperature to the external section. In extreme cases where there may be a severe defect of the moulded tube, isolated areas can be clearly visualised which demonstrate an area of failure. An example of this technology can be seen below in Figure 4b which illustrates the heat profile which is generated as high temperature medium is passed through a standard air/oil separator. A uniform flow pattern can be determined as well as areas of isolated elevated temperature regions which can identify defects in the glass fibre moulded tube. In these areas, the flow medium will follow the path of least resistance where the defects occur which in turn will be represented by isolated high temperature regions.

The images in Figure 6a and 6b illustrate examples of defects on the glass fiber final separator assembly. The technology can be used to visualize the thermal differential regions where isolated failure regions exist which can then be cross sectioned to highlight the exact temperature transferred at any specific region. When compared to a non-defect molded tube it can be seen in Figure 7 that the heat transfer is evenly distributed which is expected for an acceptable manufactured molded glass fiber tube.

Claims

1. A method of testing a tubular filter or filter element, comprising supplying light or heat to a lumen of the filter or filter element; observing the distribution of light or heat from an external cylindrical surface of the filter or filter element; and accepting or rejecting the filter or filter element depending on observation of regions of anomalous light or heat emission from the external cylindrical surface.

2. The method of claim 1, wherein the filter includes a tubular glass fiber element.

3. The method of claim 2, wherein the fibers are of borosilicate glass.

4. The method of claim 2 or 3, wherein the glass filter element is a molding.

5. The method of claim 4, wherein the tubular filter element is the result 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.

6. The method of claim 4, wherein the tubular filter element is the result of forming a fibrous layer within a foraminous tubular support by the steps of (a) providing the tubular support for said fibrous layer; (b) providing forming mesh over the curved surface of the support and closing its ends against escape of fluid; characterized by (c) introducing fibrous slurry from a pressurized source into an annular molding space between a rotary molding torpedo and an inner surface of the support, the torpedo having at least one channel for slurry opening along a curved surface thereof, fibers in the slurry collecting within the forming mesh and within the support to form the layer.

7. The method of claim 5 or 6, wherein the glass filter element further comprises a stabilizing resin with which it is impregnated.

8. The method of claim 7, wherein the resin is an acrylate.

9. The method of any of claims 4-8, wherein the filter element is for a coalescing filter of which the glass filter element provides a coalescing layer and in which a drainage layer covers the exterior cylindrical surface of the coalescing layer.

10. The method of claim 9, performed on the coalescing layer alone.

11. The method of claim 9 or 10, wherein tubular glass filter elements exhibiting channels or voids detected by regions of anomalous light or heat emission are rejected prior to fitting of a drainage layer.

12. The method of claim 9, when carried out on a filter having a coalescing layer covered by a drainage layer.

13. The method of any preceding claim, including the step of inserting a light-emitting probe into the lumen of the filter or filter element and observing the distribution of light from the external cylindrical surface of the filter or filter element.

14. The method of any of claims 1-2, including the step of supplying a gaseous medium at elevated temperature into the lumen of the filter or filter element and observing the distribution of heat from the external cylindrical surface of the filter or filter element.