GB2033247A - Filter Silencer for Compressed Gas Stream - Google Patents

Abstract

A filter silencer for oil contaminated compressed air is constructed to enable extremely high efficiencies to be obtained both in regard to filtering and silencing. The oil contaminated air passes through a tube 39 to the centre of a coalescing filter element 29 surrounded by an open pore drainage barrier 36 of foamed material. The element 29 having an oil-removal efficiency of at least 99.95% traps solid particles and coalesced oil drips from the barrier 36 to the bottom of a casing 44. The cleaned air passes through an intervening space to the inside of a cylindrical silencing element 32 through which the air is discharged to atmosphere. The elements 29, 32 may be made of borosilicate glass fibres bound by a synthetic resin. The air leaves tube 39 through ports 47 located for the air to undergo a change of direction throwing out some oil droplets. The silencing element may alternatively be located coaxially above the filter element with an intervening space. Where maximum silencing efficiency is not required the silencing element may be omitted so that the air after travelling across the intervening space escapes through large vents at the top of a lateral wall of a casing. <IMAGE>

Description

SPECIFICATION Filter Silencer for Compressed Gas Stream This invention relates to filter silencers for compressed gas streams, particularly compressed air systems, contaminated by atomised oil.

It is very rare for compressed air not to return to its original ambient conditions and often in industry when compressed air exhausts to atmospheric pressure through a control valve a great deal of noise is created. Not only is noise created but any contamination within the air line is exhausted into the atmosphere. The noise levels can be above 125 decibels which is very damaging to the sense of hearing and contamination such as oil aerosol can be damaging to the respiratory system. Health and safety organisations throughout the world are now implementing stringent controls covering working conditions and a particular interest is being taken in the reduction of noise and air pollution levels in industry.

For example when pneumatic equipment is used it is generally accepted that a lubricator should be fitted to ensure that the moving parts receive lubrication. As an example a pneumatic piston and cylinder assembly requires lubrication on the cylinder wall to stop the piston scoring the bore and increase the life of the piston seal. In the case of an oil lubricated air compressor, a certain amount of oil is atomised and carried down stream. This means a high oil level concentration in the exhausting air. Similarly on the exhaust side of an oil lubricated vacuum pump the air needs to be filtered and silenced.

In the past, when noise control has been necessary, attachments have been fixed to exhaust ports to reduce the noise level of the escaping air. However, these attachments do not reduce the contamination level and in some cases where sintered material is used, these increase this pollution by atomising lubricating oil so that it is sprayed into the atmosphere. Recently, however, filter silencers have been produced which provide a 30 to 35 decibel reduction in noise level. So far as can be ascertained, the filtration efficiency of these attachments by DOP methods is for the best 99.9% and for the worst less than 10%.

A main object of the present invention is to provide a filter silencer that is substantially more effective than any heretofore proposed in that it provides a DOP efficiency of at least 99.95% and generally in the order of 99.999% having an oil removal efficiency of the same order with a noise reduction that can be as great as 50 to 60 decibels. A further object is effectively to accommodate pulsating flow patterns that occur when the air supply is switched on and off.

According to the invention, a filter silencer for compressed gas streams contaminated by dirt, water, oil or atomised oil aerosol comprising a casing containing at least one vertical hollow filter element, and means for directing a gas stream to pass outwards from inside the filter element through the wall of the filter element whereby, when the filter silencer is in use, the noise level of the gas stream is reduced and solid dirt particles are trapped by the filter element while oil aerosol is coalesced and passes through the filter element as bulk oil with any water to the outside of the filter element to drain into a sump and for further directing the gas to atmosphere wholly through a silencing element or elements or through a casing wall having apertures sufficiently large to pass the gas therethrough substantially at atmospheric pressure, an intervening space of a size sufficient substantially to eliminate any carryover of oil to the silencing element or elements or casing wall being provided between the filter element and the silencing element or each silencing element, or casing wall. Very conveniently a cylindrical silencer element may be mounted coaxially with and either above or surrounding the filter element.

In order that the invention may be clearly understood and readily carried into effect, an example thereof will now be described with reference to the accompanying drawings, in which: Figure 1 is a vertical section through an air filter silencer; Figure 2 is a perspective view of a modification of the filter silencer of Figure 1; and Figure 3 is a vertical section through another air filter silencer.

Figure 4 is a vertical section through yet another air filter silencer.

Referring to Figure 1, a filter comprising a cylindrical filter element 3, sealed at its ends into end caps 10, 1 1 and supported by a perforated cylinder 12, is clamped between a flange 13 on a bobbin shaped casing member 14 and a nut 15, sealing rings 16, 17 being provided. The nut 15 is screwed onto a central tube 18 threaded at its upper end into an axial passage 1 in the member 14. The filter is surrounded with clearance by a transparent or translucent cylindrical casing wall 6 of plastics material located between a flanged member 21, screwed onto the bottom of the tube 18 and a ring 22.

A cylindrical silencing element 7, supported by a perforated cylinder 23, is sealed between the ring 22 and a top flange 24 of the member 14.

The filter silencer of this example has an overall outside diameter of 105 mm and a length of 200 mm. The inlet of the axial passage 1 is 1/2 inch BSP. In operation, compressed air passes down the passage 1 and tube 18 to ports 25 in the tube 18, whence most of the air undergoes a sudden change in direction throwing out some of the larger oil droplets as it passes through to the interior of the filter and then through the filter element 3, which traps dirt, water, oil and atomised oil aerosol and causes oil particles to coalesce. The coalesced oil passes through the filter element 3 into an outer open port drainage barrier 4 of foamed material, from which the oil drips down under gravity into a quiet zone in a sump above the flange member 21. The filter element 3 also acts as a first stage silencer contributing to the overall silencing performance of the assembly.The cleaned air then flows upwards through the ring 22 to the silencing element 7.

The cylindrical wall of the silencing element advantageously consists of a solid pad of resin reinforced glass fibre. The thickness of the pad depends on the air velocity and the grade of fibre used. The silencing element is moulded to shape from a coarse grade borosilicate glass microfibre having a diameter range of 3.5 to 5.1 microns reinforced with a synthetic resin binder such as a silicone resin. In the example, a wall thickness of 2 centimetres with an outer circumferential face area of 70 square centimetres and a flow rate of 1000 litres per minute was found to give excellent results, the noise reduction being in the region of 50 decibels at 50 centimetres distance from the silencer.The pressure drop through the complete silencer for various flow rates is as follows: Flow Rate Pressure Drop (M3/HR) (Bar) 30 0.1 40 0.15 50 0.2 60 0.25 70 0.3 80 0.35 90 0.4 100 0.5 The filter element 3 which has a DOP efficiency of at least 99.95% may be constructed in accordance with the invention set forth in our cognate patent applications Nos 1364/76, 19083/76,46969/76 and 9680/77. Thus, it may consist of at least one cylindrical sheet of nonwoven borosilicate glass microfibres having 0.5 to 9.0 microns (inclusive) mean diameter and a length of between 1 and 2 mm (inclusive) and contain coarse fibres up to 6 mm length, the fibres being bound by a silicone resin and the sheet being 0.73 mm thick and formed with pleats over its entire area.The depth of the pleats may be approximately 0.6 cms, the sheet preferably being between 1 mm and 13 mm thick.

Alternatively, the filter element 3 may be constructed in accordance with our patent application No. 12228/76. Then the filter element comprises a supporting sheet of comparatively rigid material formed with apertures distributed substantially uniformly over its entire area and forming a total of open areas that is at least 30% of the total area of the sheet, and a unitary mass of amorphously arranged fibres, the sheet being embedded within the mass with portions of the mass of fibres passing through the apertures from one side of the sheet to the other side of the sheet and a silicone resin or polyurethane bonding all the fibres together and to the sheet. The supporting sheet may be embedded either inside the mass of fibres or on the surface thereof.As described in the said application 12228/76 the ends of this element may be fitted into the end caps by way of taper compression engagement between the said ends and grooves or recesses in the end caps.

In a further alternative, the filter element 3 may be constructed according to our Patent Application No. 2559/78 according to which a filter element comprises a mass of fibres compacted together and bonded together with a synthetic resin, a majority of the fibres being disposed so that they are directed approximately in parallel with one another. In this case also the supporting cylinder 12 is embedded in the outside surface of the filter element 3. Other types of filter element may be used provided they have an oil removal efficiency of at least 99.95% One such filter element is a pleated paper element made of glass, cellulose, man made fibre or combinations thereof.Another is made by wrapping or winding paper on a perforated cylinder or cylindrical porous material such as sintered plastics material or sintered metal, or sintered glass the paper again being of glass, cellulose or man made fibres or combinations thereof. A ceramic filter element can also be used. Yet again a filter element vacuum formed (moulded) from glass, cellulose, man made fibres, or combinations thereof may be used. Sintered plastics material or sintered metal can also be used for the whole filter element.

Oil collected on the flange 21 can be removed by opening a drain 5. The sasmgwan be dismantled to enable a filter and, if required, the silencing element to be replaced.

It will be clear to those skilled in the art that many modifications may be made to the device shown in the drawing without departing from the scope of the invention. For example, white the filter shown is of the replacement type, it can be permanently mounted when both the filter and casing or housing are disposable.

Instead of the untapered cylindrical filter element 3, a cylinder in the form of a truncated cone can be used with the largest diameter at the bottom. This cone shape has the advantage of forming an expansion chamber which reduces the exit air velocity between the casing wall 6 and the filter element. This reduction of velocity can reduce oil carryover especially during any initial surge condition. The burst strength of the filter element is normally greater than 5 bar, in order to withstand initial shock loads when exhausting high pressure compressed air.

The casing or housing can be designed in a variety of ways to suit different requirements. For example, instead of providing the air inlet at the top of the assembly with the exhaust round the side, the inlet may be arranged at the side with the exhaust at the top and/or the side. This alternative can be made to suit multi-inlet port systems. Normally the silencing element 7 is arranged above the filter element 3 and there may be an expansion chamber between them. The cylindrical casing wall 6 may be provided with a sight gauge.

The silencing element 7 may consist of other types of fibre held together by a binder, such as polypropylene or acrylic or other plastics material, cellulose, ceramic or zirconia, alumina and silica oxides or other refractory grade fibres or glass/cellulose combinations however certain ma7-made fibres e.g., polypropylene can be heat bonded instead of using a binder. Alternatively a reticulated plastic foam can be used either alone or in combination with a fibrous pad. The shape of the silencing element depends on the shape of the exhaust port, which is cylindrical as shown in the drawing, but may take other forms, for example, circular or rectangular mass.Figure 2 shows a variation in which the filtered air, after flowing upwards between the drainage barrier 4 and casing wall 6, enters a rectangular hood 26 from which it escapes to atmosphere through opposed rectangular filter elements 27 supported by perforated plates 28.

OSHA which is a health and safety organisation in the U.S.A. recommend a maximum continuous noise level of 90 decibels.

When the inlet noise level is 125 decibels, the device disclosed has a noise reduction of 50 to 60 decibels and therefore reduces the sound level to well below the recommended standard. The table below gives some idea of the difference of noise levels: 130 Db Pain Threshold 125 Db Feeling Threshold 90 Db Foot of Niagara Falls 75 Db Street Corner Traffic (Large City) 60 Db Typical Office.

Referring to Figure 3, a cylindrical filter element 29 supported by a perforated metal cylinder 30 is mounted, with an intervening cylindrical space 31, coaxially with a cylindrical silencing element 32 supported by a perforated metal cylinder 33. The upper ends of the filter element 29 and silencing element 32 are sealed for example, by an epoxy resin, into a channelsection annular cap 34. The lower end of the filter element 29 is sealed into a channel section annular cap 35 and it is surrounded by an open pore drainage barrier 36. The lower end of the silencing element is sealed to an angle-section annulus 37. The filter and silencer assembly is held between a flange 38 on a diffuser tube 39 and nut 40 screwed onto a lower tubular extension 41 of the diffuser tube 39, sealing rings 42, 43 being provided in contact with the caps 34,35.

A transparent drainage bowl 44, which may be made from a polycarbonate plastics material, has on its base an internal cylindrical flange 45 screwed onto the tubular extension 41, the upper edge of the bowl 44 being formed so as to locate a sealing ring 46 against the annulus 37. A drain valve 47A can be screwed upwards to the position shown in full lines to permit the escape centrally through the valve of oil that has collected in the bowl 44.

In operation air contaminated by atomised oil enters the top of the diffuser tube 39 and travels down to ports 47 through which it enters the lower end of a cylindrical space 48 between the filter element 29 and diffuser tube 39. The tapered form of the diffuser tube 39 results in a slightly higher pressure being maintained at the bottom of the space 48 than at the top of that space and the sudden change in direction of the air stream as it leaves the ports 47 to rise in the space 48 results in larger oil droplets that cannot negotiate the change in direction being thrown to the bottom of the space 48, whence the oil escapes through the filter element 29 and drainage barrier 36 to drip into the bowl 44.

It will be seen that the assembly comprising the filter element 29, silencing element 32, and end caps 34, 35 is readily replaceable as in integral unit.

It will be appreciated that a variety of modifications of the filter silencer of Figure 3 are possible. For example, while the length of the silencer element 32 is shown as being about 70% of that of the filter element 29, the silencing element can have a length varying between 10% and the total length of the filter element depending on design parameters. The filter element 29 may take any one of a number of different forms, in particular the forms referred to above in relation to Figure 1. The silencing element 32 is a fibrous element that may be constituted by any of the materials mentioned above in relation to Figure 1. The metal support cylinder 33 can be replaced by a perforate tube of plastics materials, such as poly-propylene or can be omitted altogether for certain types of silencer.A melamine resin can be used to bind the fibres to one another and to the support cylinder. For certain applications the plastics drainage bowl 44 can be replaced by a metal bowl. While the diffuser tube 39 has been shown as having a cross-section that decreases towards its lower end, in some alternative designs the diameter may increase towards its lower end or may remain constant over its length. Further, the air, instead of being discharged through the ports 47, may simply be discharged through the lower end of the tube 39. However, the length of the tube 39 should be at least 20% of the length of the filter element.

In a practical example, a radial gap of 15 mm between the inner diameter of the silencing element 32 and the outer diameter of the drainage barrier 36 has given good results with very little oil carryover to the silencing element.

However, this gap can be reduced and it is believed that the lowest practical limit is 2 mm.

For the example, of Figure 3, the relationship between flow rate and pressure drop has been found to be substantially the same as for the example of Figure 1. Also the noise reduction has been found to be in the 50 to 60 decibel region.

However, the device can, of course, be adapted for lower noise reductions for example, 40 decibels, if required.

It has been found that a noise reduction of 50 decibels is possible with an effectively designed filter silencer in which the silencing element or elements are omitted. Such a filter silencer is shown in Figure 4, in which reference numerals of Figure 1 are applied to equivalent parts in Figure 4. The filter element 3 is sealed to a cap 50 to which is bonded an outer casing 51 providing a sump for the extracted oil., The air after leaving the drainage barrier 4 and travelling across the space between this barrier and the casing wall, which space is wide enough to prevent any carryover of oil, escapes through large apertures or vents 52 which are sufficiently large to pass the air therethrough substantially at atmospheric pressure. The casing 51 protects the filter from damage and the apertures 52 are located well above the bottom of the barrier 4 to ensure further that the outside of the casing remains clean. While the filter element 3, cap 50, casing 51 and cap 11 are bonded together as a unit replaceably mounted on the inlet tube 18, the design may be modified for the independent replacement of the filter element.

Multi-element filter silencers may also be constructed within the scope of the invention, i.e., one or more filter elements may each be used with one or more silencer elements or, as in Figure 4, with no silencer element.

Claims (20)

Claims

1. A filter silencer for compressed gas streams contaminated by dirt, water, oil or atomised oil aerosol comprising a casing containing at least one vertical hollow filter element, and means for directing a gas stream to pass outwards from inside the filter element through the wall of the filter element whereby, when the filter silencer is in use, the noise level of the gas stream is reduced and solid dirt particles are trapped by the filter element while oil aerosol is coalesced and passes through the filter element as bulk oil with any water to the outside-of the filter element to drain into a sump and for further directing the gas to atmosphere wholly through a silencing element or elements or through a casing wall having apertures sufficiently large to pass the gas therethrough substantially at atmospheric pressure, an intervening space of a size sufficient substantially to eliminate any carryover of oil to the silencing element or elements or casing wall being provided between the filter element and the silencing element or each silencing element, or casing wall.

2. A filter silencer according to Claim 1, in which when in use under full flow conditions effects a noise reduction of at least 40 db.

3. A filter silencer according to Claim 1 or Claim 2, in which the width of the said intervening space is at no point less than 2 mm.

4. A filter silencer according to any one of the preceding claims, in which the filter element is covered on its outer surface by an open pore drainage barrier.

5. A filter silencer according to any of the preceding claims in which a cylindrical silencing element is mounted above and coaxially with the filter element.

6. A filter silencer according to any one of Claims 1 to 4, in which a cylindrical silencing element surrounds and is mounted coaxially with respect to the filter element.

7. A filter silencer according to Claim 6, in which the axial length of the silencing element is between 1 0% and the total length of the filter element.

8. A filter silencer according to Claim 6 or Claim 7, in which the filter element and the silencing element are sealed to a common end cap closing the upper end of the said intervening space.

9. A filter silencer according to Claim 8, in which the said two elements and the end cap constitute an interchangeable unit.

10. A filter silencer according to any one of the preceding claims, arranged for the contaminated gas stream to be introduced through a central tube extending through the filter element frorn one end thereof to a location at which the gas stream is discharged such that the stream undergoes a change in direction on being discharged resulting in some of the atomised oil being thrown out of the stream.

11. A filter silencer according to Claim 10, in which the length of the tube is at least 20% of the length of the filter element.

12. A filter silencer according to Claim 10, in which the cylindrical filter element is conical with the largest diameter at the bottom so as to provide between the filter element and a casing wall an expansion chamberleading to a silencer element.

13. A filter silencer according to anyone of the preceding claims, having a-silencing element supported by an external perforated member.

14. A filter silencer according to any one of the preceding claims, having a filter element which comprises a mass of borosilicate microfibres held together by a synthetic resin binder.

1 5. A filter silencer according to any one of the preceding claims, in which the silencing element or each silencer element consists of coarse grade borosilicate glass microfibres reinforced with a synthetic resin binder.

1 6. A filter silencer according to any one of the preceding claims, in which the filter element has an oil removal efficiency of at least 99.95%.

17. A filter silencer substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

18. A filter silencer substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.

19. A filter silencer substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

20. A filter silencer substantially as hereinbefore described with reference to Figure 4 of the accompanying drawings.