WO1997034680A1

WO1997034680A1 - Compressed air filter

Info

Publication number: WO1997034680A1
Application number: PCT/AU1997/000171
Authority: WO
Inventors: Mario Ragusa; Joseph Borg
Original assignee: General Pneumatics Pty Ltd
Current assignee: General Pneumatics Pty Ltd
Priority date: 1996-03-19
Filing date: 1997-03-18
Publication date: 1997-09-25
Anticipated expiration: 1998-09-19
Also published as: AUPN877296A0

Abstract

A compressed air filter includes an inlet (1) for compressed air, a pre-filter (2), an expansion chamber (3), a microfilter (4) and an air outlet (5). When compressed air enters the inlet (1), it passes through the pre-filter (2), expands in the expansion chamber (3), passes through the microfilter (4) and leaves through the air outlet (5). One or more routes (6, 10) are provided for oil trapped by the microfilter (4) to escape, so that the microfilter does not become clogged.

Description

COMPRESSED AIR FILTER This invention relates to a compressed air filter. It relates particularly to an air filter for removing particulate matter, water and oil, whether in the form of vapour or droplets, from compressed air systems such as air braking systems. Hot compressed air has a greater propensity for carrying water and oil vapour, suspended water and oil droplets and other particles than has non- compressed room temperature air. Contaminants such as exhaust fumes, oil, dirt and water can be drawn into a compressed air system at the compressor intake. Other contaminants such as metal, carbon and plastic particles can be introduced into the compressed air stream from the system itself.

Contaminants in a compressed air system can cause significant damage, depending upon the uses to which the compressed air is put. Contaminants typically block up small orifices, rendering pneumatic machinery and equipment ineffective. Where the air is to be breathed by a person, significant harm can be caused by contaminants. In air braking systems, contaminants may prevent brakes from closing properly. Contaminants, and in particular water, can cause corrosion and rust. It is therefore important for filtering apparatus to be provided for removing contaminants from compressed air systems.

There are numerous different types of filters available for compressed air systems. One common type is a filter which includes a porous filter medium which traps particles and droplets which are larger than the mesh size of the filter medium. Another type is an air cooler, which encourages precipitation of water and oil vapours. Another type is a desiccant dryer, which passes the air stream through a chamber filled with desiccant material. Water vapour is adsorbed by the desiccant. A further type is a coalescing filter for removing oil. It is common to place several filters of different types in series in an air stream. However, it is often inconvenient and expensive to use several filters in this manner, and each filter has an air pressure drop associated with it, and the amount of work required to be done by the air compressor increases as the total pressure drop brought about by filters increases.

Some filter units are available which incorporate more than one type of filter within a single unit. An example is provided in United Kingdom Patent Specification 1 ,236,396 in the name of CA. Norgren Limited. The filter comprises a hollow cylindrical coarse pre filter located within a larger diameter hollow cylindrical fine filter. The compressed air stream passes into the centre of the pre filter, out through the pre filter, and then out through the fine filter. The pre filter traps larger particles and oil and water drops, while the finer filter traps finer particles and droplets.

The advantage of the system described in United Kingdom Patent Specification 1 ,236,396 is that the surface of the fine filter does not become blocked by larger particles and droplets which are trapped by the pre filter. However, a particular disadvantage with filters which incorporate fine mesh fibre areas is that the performance of the filter deteriorates as more and more oil is trapped inside the filter fibres, and only oil which can migrate through to the external surface of the filter can escape by falling as droplets from the surface. As a result, the filter becomes saturated with oil from the bottom upwards, and the saturated portions do not allow the passage of air. The level of pressure drop across the filter increases as the available area for air passage decreases, and after a short time the filter element must be taken out and replaced.

Another multiple stage type of air filter is described in International Patent Publication WO93/04761 , in the name of Guiseppe Ragusa and Jimmy Bajada. That publication, the contents of which are incorporated herein by reference, describes a filter having a pre filter chamber and an expansion chamber. Compressed air passes through a pre filter in the pre filter chamber before passing through Venturis into the expansion chamber. During expansion, water droplets precipitate and oil droplets coalesce, and the contaminants accumulate in a pool at the bottom of the filter unit before being discharged by a drain valve. The filter unit is highly effective in removing water, oil and other contaminants from a compressed air stream, but for some applications such as applications in which the air is to be breathed by a person, or when instrument quality air is required, a greater degree of contaminant removal is required. It is an object of the present invention to provide an air filter which overcomes some of the disadvantages described above. According to the present invention there ts provided a compressed air filter including. an inlet for compressed air; a pre filter; an expansion chamber; a micro filter; and an air outlet; wherein compressed air enters the inlet, passes through the pre filter, expands in the expansion chamber, passes through the micro filter and leaves through the air outlet; wherein further the micro filter includes oil escape means allowing droplets of oil trapped by the micro filter to fall off the surface of the micro filter

In preferred arrangements, the air filter may include impaction means located between the pre filter and the micro filter, whereby air is directed at high speed against a surface, encouraging precipitation of water and oil.

In particular embodiments, the optional impaction means may be provided by a baffle plate of the type illustrated in International Publication WO93/04761 , in combination with Venturis. Alternatively, or additionally, impaction means may include means for directing the air stream to rotate at a rapid rate so that water and oil entrained in the air stream is directed by centrifugal force against the walls of the air filter or expansion chamber or some other surface or surfaces provided for the purpose.

In a preferred arrangement, the micro filter is substantially in the form of a hollow cylinder whose walls are made of a micro-porous material arranged such that air passes from the centre to the outside of the cylinder, and the oil escape means includes an open bottom to the hollow cylinder, permitting droplets of oil trapped on the internal surfaces of the cylinder to fall off those surfaces

In such an arrangement, further oil escape means may be provided at the bottom of the external surfaces of the cylinder The further oil escape means may comprise one-way valves which may be opened when there is a pressure drop within the air filter for a short time while a dram valve is activated to discharge accumulated contaminants from the filter; the rest of the time, the one-way valves are kept closed.

In such an arrangement it is especially preferred that the contaminants trapped by the expansion chamber, by the oil escape means, and by the further oil escape means all accumulate in a single pool at the bottom of the air filter, and are all discharged through a single drain valve.

According to another aspect of the invention, there is provided a compressed air filter including: an inlet for compressed air; a pre filter; an expansion chamber; means for directing the air in a circular path so that a centrifugal force is applied to particles and droplets entrained within the air; a micro filter in the form of an open bottomed substantially hollow cylinder whose walls are made from a micro porous material; and an air outlet; wherein compressed air enters the inlet, and passes sequentially through the pre filter, expansion chamber, and means for directing air in a circular path, before passing upwards through the open bottom of the centre of the hollow cylindrical micro filter, through the micro filter walls, and out through the air outlet.

It will be appreciated that this specific embodiment of the invention may incorporate any or all of the optional features described with reference to the broader statement of the general principles of the invention.

The invention may optionally further include a desiccant chamber incorporating a desiccant material, through which air passes after passing through the micro filter, before proceeding to the outlet.

The invention may optionally further provide one or more meshes, baffle plates, or other devices for trapping entrained droplets or encouraging precipitation, located in the expansion chamber. The invention may optionally further include an air flow separator such as a sieve, located towards the bottom of the filter, allowing oil, water and other contaminants to pass through and accumulate below the air flow separator, but preventing or reducing the likelihood of contaminants accumulated at the bottom of the filter becoming re-entrained in the air flow.

The invention will hereinafter be described in greater detail by reference to the attached drawings which show an example form of the invention. It is to be understood that the particularity of those drawings does not supersede the generality of the preceding description of the invention.

Figure 1 is a schematic cross-sectional view of one embodiment of the present invention.

Figure 2 is a detailed view of an altemative arrangement for a one-way valve.

The compressed air filter includes inlet 1 for compressed air, pre filter 2, expansion chamber 3, micro filter 4 and air outlet 5. When compressed air enters inlet 1 , it passes through pre filter 2, expands in expansion chamber 3, passes through micro filter 4 and leaves through air outlet 5. Micro filter 4 includes oil escape means 6, which allows droplets of oil trapped by micro filter 4 to fall off the surface of the micro filter.

The air filter as shown includes impaction means located between pre filter 2 and micro filter 4, for directing air at high speed against a surface, encouraging precipitation of water and oil. The embodiment illustrated includes two different types of impaction means. Impaction means 7 directs the air stream to rotate at a rapid rate so that water and oil entrained in the air stream are directed by centrifugal force against walls 9 of expansion chamber 3.

The embodiment illustrated also includes a baffle plate 8 onto which air from pre filter 2 is directed at high speed, either through Venturis 11 of the type described in International Patent Publication WO93/04761 or standard openings 12.

Referring now to the particular details illustrated in the drawings, pre filter 2 is made from any suitable material or materials. An especially preferred type of material is sintered bronze. In the embodiment illustrated, pre filter 2 is in a frusto-conical shape.

Pre filter 2 is protected by pre filter guard 13. Pre filter guard 13 traps large objects caught up in the compressed air stream, preventing them from blocking the surface of pre filter 2, and reducing the amount of wear caused on the surface of the pre filter.

In the embodiment illustrated, pre filter 2 is housed within pre filter chamber 14. Air which has passed through pre filter 2 leaves pre filter chamber 14 either through Venturis 11 or openings 12. Although both Venturis and openings are shown in figure 1 , actual embodiments will normally include one or the other type of opening, not both.

The air strikes baffle plate 8 at a rapid rate, and must pass around the outside of baffle plate 8 to move further into expansion chamber 3. Water and oil droplets in the air stream strike against baffle plate 8 and the walls 9 of expansion chamber 3, with the result that droplets of oil and water fall down the sides of expansion chamber 3, accumulating in pool 15 at the bottom of the filter housing.

Further precipitation and coalescence is encouraged in expansion chamber 3 by meshes 16, which may include gauzes, baffles or other devices for trapping entrained droplets or encouraging precipitation.

After passing through meshes 16, the air stream encounters means for directing the air stream to rotate at a rapid rate. This may include a spiral coil, turbine or cyclonic arrangement, imparting centrifugal force to oil and water droplets and encouraging coalescence on expansion chamber walls 9 and the walls of bowl 17.

Circulating air in the lower region of expansion chamber 3 creates turbulence in lower region 18 of the filter. Water, oil and other contaminants also accumulate at the bottom of the filter, in pool 15, and such contaminants can be re-entrained in the air flow by turbulent air. Accordingly, an optional separator 19 may be installed above pool 15 to protect pool 15 from the turbulence of air in lower area 18. Separator 19 should of course allow free passage of water, oil and other contaminants down into pool 15. Separator 19 may be a sieve-type device. The air stream then passes up into the inside of micro filter 4. Micro filter 4 may be made from many suitable materials. Suitable materials include glass fibre and borosilicate fibre. In conventional micro filters, the air stream enters the micro filter from the top and passes out through the side walls. In such an arrangement, the bottom must be closed to ensure that air does not escape without passing through the filter. The largest quantities of oil are trapped at or near the inside surfaces of the micro filter, and in conventional arrangements, because the bottom of the filter is closed, the trapped oil builds up from the bottom of the filter (wet-zone), and air is no longer able to pass through those parts of the filter which are clogged up with oil. After a relatively short time, the filter must be replaced.

In the embodiment illustrated, air flows into the centre of the micro filter from its open bottom 6, which also acts as oil escape means. In this case the top of the filter is closed to ensure that all air must pass through the filter walls. Oil which is trapped at or near the inside surfaces of the filter can trickle down those inside surfaces (i.e. no blocked wet zone) and fall down towards pool 15 at the bottom of the filter. Oil which is trapped at or near the outside surfaces of micro filter 4 can trickle down the outside surfaces of the micro filter to the bottom of those surfaces, where it accumulates until one-way valves 10 are opened, allowing trapped oil to escape and fall towards pool 15.

Pool 15 may periodically be discharged through a drain valve (not shown) located at the bottom of the filter. When the drain valve is opened, the pressure inside the filter is reduced, and this reduction in pressure is sufficient to activate the opening of one-way valves 10. When the drain valve is reclosed, pressure builds up again and one-ways valve 10 are reclosed.

Figure 2 shows an alternative arrangement for one-way valves 10. In this alternative arrangement, a resilient flange is provided around the bottom of micro filter 4, pressing firmly against the internal walls of bowl 17. Oil trapped near the external surface of micro filter 4 trickles down the external surface and accumulates on the resilient flange. When the drain valve is opened, the decrease in pressure is sufficient to allow the flange to fall downwards, permitting accumulated oil to fall down the gap created between the flange and the internal walls of bowl 17. When the drain valve is reclosed, the flange is restored to its position pressing firmly against internal walls of bowl 17. After air has passed through micro filter 4, it passes towards outlet 5. In the embodiment illustrated, an optional additional desiccant chamber is provided. Air which has passed through micro filter 4 passes upwards through air passages 18,19 to filtered air desiccant inlet chamber 20. From desiccant inlet chamber 20, air passes through perforated tubular cartridge cavity 21 , then through desiccant material in which water is adsorbed by the desiccant, and then through external desiccant cartridge wall 22, and then onto air outlet 5 via purified dry air path 23.

In the embodiment illustrated, the additional desiccant chamber is provided by means of a cartridge 24 which can be replaced at regular intervals as the desiccant material becomes saturated.

The main body of the filter may be engineered in any suitable manner. In the embodiment illustrated, the major pieces are: cap piece 25, which incorporates air inlet 1 , air outlet 5 and pre filtration chamber 14; insert piece 26, which is attached to cap piece 25, and to which are attached expansion chamber body 9, baffle plate 8, pre filter 2, pre filter guard 13 and micro filter 4; and bowl 17, which is attached to cap piece 25 and together with cap piece 25 defines the external housing of the filter. The filter may optionally further include a means for air regulation.

It is to be understood that various alterations, additions and/or modifications may be made to the parts previously described without departing from the ambit of the invention.

Claims

Claims . A compressed air filter including: an inlet for compressed air; a pre-filter; an expansion chamber; a microfilter; and an air outlet; wherein compressed air enters the inlet, passes through the pre-filter, expands in the expansion chamber, passes through the microfilter and leaves through the air outlet; wherein further the microfilter includes oil escape means allowing droplets of oil trapped by the microfilter to fall off the surface of the microfilter.

2. A compressed air filter according to claim 1 wherein the microfilter is substantially in the form of a hollow cylinder whose walls are made of a microporous material, arranged such that air passes from the centre to the outside of the cylinder, anr¹ the oil escape means includes or consists of an open bottom to the hollow cylinder.

3. A compressed air filter according to claim 1 or claim 2 further including impaction means located between the prefilter and the microfilter, whereby air is directed at high speed against a surface, encouraging precipitation of water and oil.

4. A compressed air filter according to claim 3 wherein the impaction means includes a baffle plate in combination with one or more Venturis.

5. A compressed air filter according to claim 3 or claim 4 wherein the impaction means includes means for directing the air stream to rotate at a rapid rate so that water and oil entrained in the air stream is directed by centrifugal force against walls of the expansion chamber or against another surface.

6. A compressed air filter according to claim 2 including further oil escape means at or near the bottom of the outside of the cylinder.

7. A compressed air filter according to claim 6 wherein the further oil escape means includes one or more one-way valves which are normally closed but may be opened when there is a pressure drop within the air filter. 8 A compressed air filter according to any one of claims 1 to 7 wherein oil and any contaminants accumulated in the expansion chamber, oil escape means and any further oil escape means collect in a single pool at the bottom of the air filter, and are discharged through a single drain valve. 9. A compressed air filter including: an inlet for compressed air; a pre-filter; an expansion chamber; means for directing the air in a circular path so that a centrifugal force is applied to particles and droplets entrained within the air; a microfilter in the form of an open-bottomed substantially hollow cylinder whose walls are made from a microporous material; and an air outlet; wherein compressed air enters the inlet and passes sequentially through the pre-filter, expansion chamber, and means for directing the air in a circular path, before passing upwards through the open bottom of the centre of the hollow cylindrical microfilter, through the microfilter walls, and out through the air outlet. 10. A compressed air filter according to any one of claims 1 to 9 further including an air flow separator located towards the bottom of the filter.