GB2382042A

GB2382042A - Centrifugal brush filter apparatus

Info

Publication number: GB2382042A
Application number: GB0221725A
Authority: GB
Inventors: John Thorne
Original assignee: J B THORNE Ltd
Current assignee: J B THORNE Ltd
Priority date: 2001-09-19
Filing date: 2002-09-19
Publication date: 2003-05-21
Also published as: GB0221725D0; GB0122553D0

Abstract

A method of removing liquid droplets suspended in a gas stream comprises the step of feeding the gas stream into a rotating brush 6. The liquid droplets impinge on the fibres 6, travel along the them centrifugally and are flung outwards to a collection means 26,40. The inlet gas stream is along the axis and the outlet gas stream either is radial or is parallel to the axis at a larger radius than the radius of the inlet, whereby the rotating brush 6 acts as both an effective filter and a centrifugal fan. Also disclosed is a filter apparatus for removal of droplets in a gas stream and a method of deodorising/sterilising air.

Description

TITLE Filter apparatus DESCRIPTION Technical field The invention relates to the separation and removal of droplets of liquid suspended in a gas flow. The gas may be air and the liquid oil, but not necessarily so in either case.

Background There are many situations in which it is desirable to remove liquid droplets from a flow of gas.

One example is in the field of machine tools, in which the lubricating oil often forms a mist in the surrounding air. This oil must be removed before the air can be discharged to the environment, both because the oil is a pollutant and because a suspension of fine drops of a combustible material in air leads to the risk of an explosion. A similar need exists to remove oil and grease from the air that is extracted from kitchens of fried food restaurants.

Another example is in the field of dust extraction. One method of removing solid dust particles from air is to direct the flow of air against a water surface. Most of the dust comes into contact with the water surface, is wetted and settles out. However, in this process droplets of water can be entrained in the airflow, some of which may also contain particles of dust. It is desirable to remove the water droplets and the dust in a further filtration step.

There are also industrial chemical processes that result in a suspension of a liquid in a gas, where it is desirable either to purify the gas or to recover the liquid.

In hospital environments, there are great problems with airborne diseases spreading between patients. Patients with impaired immunity are especially at risk, for example bone marrow patients, AIDS sufferers and premature babies. Many disease-causing

bacteria and viruses are carried in tiny liquid droplets in the air so there exists a need to filter the droplets or the bacteria themselves from the air supply.

It is well known to filter a gas flow by passing the flow through a filter of fine mesh, open foam or other porous material. The principle is that most of the suspended droplets will come into contact with the large surface area of the filter and will stick, thereby being removed from the gas flow. However, as the filter is used, the pores gradually become clogged and the efficiency is reduced. Eventually the filter must be cleaned or replaced.

An alternative filtration method is to pass the gas flow through a convoluted path defined by baffles. The principle in this case is that the suspended droplets cannot change their trajectory as easily as the gas flow, so as the direction of the gas flow changes, the suspended droplets continue and collide with the baffles, from which the liquid can be collected and removed. Such filters require a long path in order to remove a high proportion of the suspended liquid and are less efficient for smaller droplets.

Patent GB 663194 discloses the use of a rotating screen across the air intake of a compressor for throwing off centrifugally any extraneous matter deposited on the screen from the passing air. The screen may consist of one or two annular rows of radially extending wires.

Patent DD 216169 Al discloses a rotary oil separator, which consists of a cylindrical brush that is turned by a gas stream directed against its periphery. Because the brush is driven only by the gas stream, no motor is required and the separator can be selfcontained within an oil tank. However, the gas stream must be input with sufficient energy to rotate the brush effectively and because the peripheral speed of the brush matches the speed of the gas stream, the separation effect of the brush is reduced.

Patent application EP 0331809 Al discloses a device for separating aerosols from waste gases, comprising one or more radial brushes mounted on a common axle. The

gas passes in an axial direction through successive rotating brushes and the aerosol particles coalesce on the brushes. The brushes are rinsed by a liquid supplied to the hubs of the brushes.

Summary of the invention In a first aspect, the invention provides a method of removing liquid droplets suspended in a gas stream, comprising the steps of : driving a cylindrical brush, which comprises a plurality of generally radial fibres distributed over a cylindrical surface of a hub, to rotate about the axis of the hub; and feeding the gas stream into a region swept out by the fibres as the brush is rotated ; whereby the gas stream enters the rotating brush parallel to the axis, and is discharged from the rotating brush at right angles to the axis, and liquid removed from the gas stream travels centrifugally along the fibres.

In a second aspect of the invention, the gas stream enters the rotating brush parallel to the axis along the axis or at a small radius from the axis, and is discharged from the rotating brush parallel to the axis at a greater radius from the axis.

The invention also provides filter apparatus for removal of liquid droplets suspended in a gas stream using either of the methods previously described.

The rotating brush is an efficient way of removing droplets from the gas stream because it works in three ways. First, the many fibres in a brush have a very large total surface area so impingement of each droplet on a fibre is very likely (even if the brush were stationary). The droplets that impinge on the fibres wet them, agglomerate and are removed by centrifugal force. Second, the fibres of the brush are moving at high speed in a direction at right angles to the inflowing gas so are highly likely to impact upon any suspended droplets and remove them from the gas stream.

Third, the gas stream does not pass straight through the dense array of fibres in the brush but is diverted into a rotary motion, whereby any droplets of liquid remaining in

the gas stream are likely to be flung outwards by centrifugal force and thus separated from the gas.

This method and this apparatus have the advantage that by increasing the number of fibres the surface area of the filter can easily be made large. Additionally, the rotation rate can be increased to enhance the probability that each droplet will contact one of the fibres and to add to the centrifugal force that removes the liquid from the filter. In fact, a doubling of the rate of rotation leads to a quadrupling of the centrifugal force.

Because the liquid filtered out of the gas stream is continuously removed from the filter, the filter is effectively self-cleaning so it does not become clogged and the efficiency of the filter is maintained. If further cleaning should be required, a cleaning liquid may be introduced while the filter is operating and this will clean the fibres. For example, a filter for removing grease from the air extracted from a fried food restaurant might need to be cleaned of grease occasionally for hygiene reasons.

By spraying water or a detergent into the filter, the usual filtration action can be used to clean the apparatus.

The cylindrical brush is easy to manufacture using known techniques in a manner that is resistant to the high centrifugal forces involved. Such a brush is an especially compact arrangement for providing a large surface area in a confined space.

In the first method according to the invention, the gas stream enters the rotating member parallel to the axis, and is discharged from the rotating member at right angles to the axis, i. e. radially in the same direction as the liquid is discharged.

In the second method according to the invention, the gas stream enters the rotating brush parallel to the axis, either along the axis or at a small radius from the axis, and is discharged from the rotating brush substantially parallel to the axis at a greater radius from the axis (i. e. the emerging gas does not have any significant radial component of motion, though it may have been imparted with a circumferential

motion about the axis). Because the air and liquid are discharged from the brush in different directions, there is a strong separating effect.

Both methods share the inventive concept that the rotating filter will act as a centrifugal fan. The increase in radius of the emerging gas creates a static pressure rise, which will induce gas flow through the device and either avoid the need for a separate fan or reduce the power required for any separate fan.

In one embodiment of the inventive apparatus, the collection means comprises a volute-shaped housing surrounding the hub and the fibres. This maximizes the centrifugal fan effect of the filter.

In an alternative embodiment, the collection means comprises a generally cylindrical housing surrounding the hub and the fibres, the housing including at least one stationary member located on an inner surface of the housing to trap liquid travelling along the inner surface of the housing; and at least one orifice adjacent to each stationary member, through which liquid trapped by the stationary member can be discharged from the housing.

A further aspect of the invention provides a method of deodorizing or sterilizing air, comprising the steps of : driving a first cylindrical brush, which comprises a plurality of generally radial fibres distributed over a cylindrical surface of a hub, to rotate about the axis of the hub; supplying a deodorizing or sterilizing liquid to the hub of the first brush so that the liquid travels centrifugally along the fibres of the brush; feeding the airstream into a region swept out by the fibres as the first brush is rotated, whereby the air and the liquid come into intimate contact; driving a second cylindrical brush to rotate about its axis; and feeding the airstream that emerges from the first brush into a region swept out by the second brush as it is rotated to remove droplets of the deodorizing or sterilizing liquid from the airstream.

The first and second brushes may be driven on a common shaft and it is clear that this aspect of the invention may be generalized to more than two cylindrical brushes, at least one of which is supplied with the deodorizing or sterilizing liquid.

The drawings Fig. 1 shows a brush for use in filter apparatus according to the invention.

Fig. 2 shows a first configuration of airflow through the brush of Fig. 1.

Fig. 3 shows a second configuration of airflow through the brush of Fig. 1.

Fig. 4 shows a third configuration of airflow through the brush of Fig. 1.

Fig. 5 is a cross section of a first embodiment of filter according to the invention, on line B-B of Fig. 6.

Fig. 6 is an axial section of the first embodiment, on line A-A of Fig. 5.

Fig. 7 is a cross section similar to Fig. 5 wherein the filter has a casing in the form of a volute.

Fig. 8 is an axial section of a second embodiment of filter according to the invention.

Fig. 9 is an axial section of a third embodiment of filter according to the invention.

Fig. 10 is an axial section of a fourth embodiment of filter according to the invention.

Fig. 11 is a cross section of the fourth embodiment of the invention, on line X- X of Fig. 10.

Fig. 12 is an axial section of a fifth embodiment of filter according to the invention.

Fig. 13 is an axial section of a sixth embodiment of filter according to the invention.

Fig. 14 is an axial section of an seventh embodiment of filter according to the invention.

Fig. 15 is an axial section of a eighth embodiment of filter according to the invention.

Figure I shows the basic construction of a cylindrical brush that may be used in filter apparatus according to the invention. A hub 2 is mounted on a shaft 3 for rotation about an axis 4. Arranged circumferentially about the hub 2 and axially along the hub 2 and firmly attached thereto are many fibres 6, which extend radially from the hub 2 to form the cylindrical brush. In one example, the cylinder has a diameter of 300mm and a thickness of 150mm.

The fibres 6 may be of any suitable material, such as nylon, polypropylene or another polymer, or a metal, that is sufficiently strong to withstand the centrifugal forces when the brush is spun at high speed, has suitable wetting properties for agglomeration of liquid droplets and is resistant to chemical attack by the gas and liquid in question. The fibres 6 can be arranged in tufts using conventional brush- making technology. The thickness of each fibre can be chosen to give the desired surface area within the available volume but is preferably less than 2 mm. It does not matter whether the fibres are flexible because they will straighten under the radial centrifugal force when the brush is rotated at high speed during use.

In operation, the brush is spun at high speed and a stream of gas containing suspended droplets of liquid to be removed is fed into the cylindrical region swept out by the fibres 6 during their rotation. It is highly likely that each droplet will impinge on one of the fibres 6 as it moves and will stick thereto, wetting the fibre and removing the liquid from the gas stream. As more droplets agglomerate on the fibre 6 to form a larger drop or liquid film, the liquid is carried around by the fibre 6 and experiences a centrifugal force, which urges the liquid radially outwards along the fibre 6. When the liquid reaches the end of the fibre 6, it is flung tangentially from the brush and can be collected (by means not shown in Figure 1).

Whereas the process just described would be effective for any rotating radial member, there is a further process specific to a brush, by which liquid droplets are removed from the gas stream. The large surface area of a brush causes the gas between the fibres to rotate with the brush, so that a rotary motion is imparted by the brush to the

gas stream. Thus, even liquid droplets that do not impinge on a fibre will be subjected to centrifugal forces that will tend to separate them from the rotating gas stream.

Figures 2 to 4 illustrate three possible configurations for gas flow through a filter brush. These configurations are achieved by suitable inlet and outlet surfaces in the filter, which are not shown in Figures 2 to 4. In every case, the removed liquid is discharged centrifugally in the radial direction.

In Figure 2, the input gas stream containing suspended droplets of liquid is fed axially into the brush as indicated by arrows 10. The output gas stream, from which the liquid droplets have been removed, emerges radially relative to the rotating brush as indicated by arrows 12. The rotating brush exerts centrifugal forces on the gas stream, thereby naturally acting as a centrifugal fan, so the configuration in Figure 2 tends to draw the gas through the brush without the need for an external fan.

However, because the outlet gas and separated liquid both emerge in the same, radial direction, the separation force of the filter is reduced.

In Figure 3, the input gas stream is fed axially into the brush as indicated by arrows 14. The output gas stream emerges axially from the opposite side of the brush as indicated by arrows 16. In this case, the rotating brush has no tendency to induce the gas flow through it so the flow must be induced by external means such as a fan. However, because the outlet gas and separated liquid emerge in different directions, at right angles to one another, the separation force of the filter is greater than in Figure 2.

In Figure 4, the input gas stream is fed radially into the brush as indicated by arrows 18. The output gas stream emerges axially from the brush as indicated by arrows 20.

In this case, the centrifugal fan effect of the rotating brush tends to counter the radial gas flow into it so the flow must be induced by external means such as a fan of sufficient power to overcome the centrifugal forces. However, because the separated liquid emerges in exactly the opposite direction to the inflowing gas, the separation effect of the filter is greater than in either Figure 2 or Figure 3.

Figures 5 and 6 illustrate a first embodiment of filter apparatus in accordance with the invention. The brush comprises a hub 2 mounted on a vertical shaft 3 for rotation by an external motor (not shown). A large number of fibres 6 radiate from the hub 2.

Arranged coaxially with the shaft 3, an inlet 22 feeds air containing suspended droplets of oil upwards towards the brush. The airstream passes axially into the brush and travels outwards by centrifugal force to emerge radially from the brush, as indicated by the arrows in Figure 6. The configuration of airflow is thus the same as in Figure 2. A plate 24 prevents the induction of air into the brush from the downstream side. The plate 24 may be attached to and rotate with the brush or be stationary and fixed to the casing 26 or the driving motor (not shown).

The brush is surrounded by a generally cylindrical canister 26, which is closed at the lower end by an end wall 28, and which constrains the air emerging from the brush to exit from the apparatus at the upper end of the canister 26. Because the brush acts as a centrifugal fan, drawing air in axially and ejecting it radially, the described pattern of airflow naturally occurs and no other fan is necessary to drive it.

As the airstream containing the suspended oil droplets passes through the brush, droplets impinge on the fibres 6, agglomerate and flow radially outwards along the fibres 6 under centrifugal force. Upon reaching the ends of the fibres 6, the droplets are flung tangentially from them, as shown by the arrows in Figure 5. They collect on the inner wall of the canister 26 to form a liquid film 30. Under the influence of the swirling airflow in the canister 26, the oil film 30 continues to move circumferentially around the canister wall. At one point around the wall is fixed a vertical barrier 32, which prevents further progress of the oil around the wall. Adjacent to the barrier 32 is an vertically extending orifice 34, through which oil trapped by the barrier 32 flows into a drain pipe 36. The oil descends the drain pipe 36 under gravity, to be collected for recovery or disposal.

The flow of air in the canister 26 tends to be helical, with both a rotating component induced by the brush and an upward component towards the exit. Accordingly, the

flow of the oil film 30 on the canister wall may also acquire an upward component and adopt a helical pattern. For this reason, it may be more efficient to replace the vertical barrier 32 with a helical barrier (not illustrated) of opposite handedness to the helical flow of oil. Again, one or more orifices should be provided adjacent to the barrier for removal of trapped oil from the canister 26.

Figure 7 illustrates a variation of the first embodiment, in which the part of the casing 26 that surrounds the brush is shaped as a volute 40. This enhances the centrifugal fan effect of the filter.

Figure 8 illustrates a second embodiment of filter apparatus according to the invention, which is similar to that in Figure 6 but has a different pattern of flow through it. Corresponding parts have been given the same reference numerals as in Figure 6 and their description will not be repeated here.

In Figure 8, the motor 5 that drives rotation of the cylindrical brush 2,6 about a vertical axis is shown adjacent to the plate 24 at the downstream side of the brush, so the filter apparatus is more self-contained than in Figure 6. There is also shown a conventional stationary filter 46 at the outlet of the casing 26. However, the most important feature by which the third embodiment of the invention differs from the first is the annular wall 48, which surrounds the plate 24. The plate 24 and the annular wall 48 define between them an annular outlet 50 between the axis and the outer radius of the cylindrical brush 6.

The annular outlet 50 constrains the gas to emerge from the cylindrical brush 6 in a generally axial direction (perhaps with an element of swirl about the axis but with no significant radial component) and at a radius greater than the radius of the inlet 22.

This creates a flow pattern that is a hybrid between the patterns shown in Figures 2 and 3 and combines advantages of both those patterns. Because the gas stream both enters and leaves the rotating brush in a generally axial direction, the circumferential forces on the liquid droplets are acting in a different direction from the direction of flow of the gas and there is a strong separation effect. On the other hand, the

increased radius at which the gas stream emerges from the rotating brush, compared with its inlet along the axis, creates a centrifugal fan effect much like Figure 2, so the gas is drawn through the filter without the need for a separate fan.

In the embodiment of Figure 8, or any other embodiment in which the gas flow through the brush has a component parallel to the axis, the properties of the brush may be varied in the axial direction. Preferably, the brush has fewer, thicker fibres near to the inlet side and more, finer fibres near to the outlet side. This allows larger liquid droplets to be removed rapidly from the gas stream as it enters the brush, while the larger surface area of fibres towards the outlet side removes any remaining fine droplets. A brush with varying properties in this way will filter more efficiently, while presenting less resistance to the flow of gas into the brush from the inlet.

Figure 9 illustrates a third embodiment of filter apparatus according to the invention, which has a pattern of flow similar to that in Figure 8. The filter is mounted so that the cylindrical brush 2,6 is rotated by motor 5 about a horizontal axis. In this embodiment, the brush 6 is surrounded by a casing 41 in the form of an annular channel, on which the oil flung from the brush 6 collects as a film. The oil flows along the surface of the casing 41 until it reaches the drain pipe 36 and flows away under gravity. Flanges 42 overlap the edges of the annular channel 41 to prevent re- entrainment of the collected oil into the airstream.

The filter of Figure 9, like that of Figure 8, includes a plate 24 at the downstream side of the brush 6. The plate 24 and the flange 42 of the casing define between them an annular outlet 50 at a radius between the axis and the outer radius of the brush. This constrains the gas to follow the same"hybrid"flow pattern as previously described in relation to Figure 8, with the same advantages.

Figure 10 illustrates a fourth embodiment of filter according to the invention. It is similar to the embodiment of Figure 8 but comprises a further fan 52 to draw the gas stream through the filter, adding to the natural centrifugal fan effect of the rotating brush 6. The fan 52 is of conventional construction and is mounted on the same shaft

3 as the hub 2 of the cylindrical brush for common rotation therewith. A motor 5 mounted on a stationary mounting plate 54 drives the rotation of the shaft 3.

Because the gas stream tends to emerge from the rotating brush with a swirling motion about the axis, the efficiency of the commonly rotating fan 52 is much reduced. For this reason, a stationary flow straightener 56 is interposed between the brush 6 and the fan 54. As shown in more detail in the cross section of Figure 11, the flow straightener comprises vanes 58 that divert the gas stream back to the axis of the filter and reduce the circumferential component of its motion. This has been found to increase the efficiency of the fan 52 significantly.

In the illustrated fourth embodiment, the plate 24 that is located immediately downstream of the brush 6, and which defines the annular outlet 50 from the brush, is stationary and forms part of the flow straightener 56.

Figure 12 illustrates a fifth embodiment of filter according to the invention. The filter comprises two cylindrical brushes 6 of the kind shown in Figure 8, both mounted for rotation on a common shaft 3 by motor 5. Associated with each brush are collection devices 32,36 for recovery of the separated liquid. Because of the annular outlet 50 from each brush, each brush acts as both a filter and a centrifugal fan. In order to prevent the rotation of the gas stream that emerges from the first filter stage from reducing the efficiency of the second filter stage, a flow straightener 56 is interposed between the stages, as previously described with reference to Figures 10 and 11.

Besides its use as a straightforward double filter, the fifth embodiment of the invention can be adapted for sterilizing or deodorizing the gas stream that passes through it. Means (not shown) are provided for introducing a sterilizing or deodorizing liquid to the surface of the hub 2 of the first rotating brush. The liquid travels outwards under centrifugal force, thoroughly coating the fibres 6 of the first brush with the liquid. Because of the large surface area of the brush, through which the gas stream must pass, the gas stream comes into intimate contact with the liquid on the fibres and the gas will be thoroughly sterilized or deodorized by the time it

emerges from the first brush. Most of the sterilizing or deodorizing liquid will be flung from the tips of the fibres 6 of the first brush and can be collected by means 32,36 for recycling or disposal.

Small droplets of the sterilizing or deodorizing liquid may remain suspended in the gas stream when it emerges from the first rotating brush. These will be removed by the second rotating brush in the second filter stage in the manner previously described.

Figure 13 illustrates a sixth embodiment of the invention. It is generally similar to the filter shown in Figure 6, whereby gas flows axially into a first brush 6 mounted for rotation on the hub 2 and is constrained by a plate 24 (which may be stationary or rotating) to flow radially out of the brush 6. A second brush 58 of smaller diameter than the brush 6 is also mounted on the hub 2 mounted for rotation with the first brush 6. An end plate 60 on the housing constrains the gas to flow radially into the second brush 58, as shown by the arrows, finally exiting in a generally axial direction.

The first brush 6 has a relatively low separating effect because the gas and the liquid droplets exit the brush in the same, radial direction, in the pattern of Figure 2.

However, the second brush 58 has a strong separating effect because the gas flows into the brush in the opposite direction from the expelled liquid droplets, in the pattern of Figure 4. Because the diameter of the first brush 6 is greater than that of the second brush 58, the positive fan effect of the first brush 6 outweighs the negative fan effect of the second brush 58.

Figure 14 illustrates a seventh embodiment of the invention, similar to the filter shown in Figure 8, with an axial gas outflow from the brush at a radius greater than the radius of the axial inflow. In this embodiment, the casing around the brush 6 is in the form of an annular channel 61 lined with a porous material 62 such as an open-cell foam. The purpose of the porous material 62 is to trap the collected liquid droplets

flung from the brush 6 and to inhibit re-entrainment of the finest droplets into the rotating gas flow around the brush. Evidently, the porous material 62 must not prevent the collected liquid flowing around the channel 61 to the drain pipe 36.

Finally, Figure 15 illustrates an eighth embodiment of the invention, similar to that of Figure 14 but with a radial outflow from the brush 6. In this embodiment, the gas flows through the porous material 62, which thereby doubles as a second stage filter.

Claims

1. A method of removing liquid droplets suspended in a gas stream, comprising the steps of : driving a cylindrical brush, which comprises a plurality of generally radial fibres distributed over a cylindrical surface of a hub, to rotate about the axis of the hub; and feeding the gas stream into a region swept out by the fibres as the brush is rotated; whereby the gas stream enters the rotating brush parallel to the axis, and is discharged from the rotating brush at right angles to the axis, and liquid removed from the gas stream travels centrifugally along the fibres.

2. A method of removing liquid droplets suspended in a gas stream, comprising the steps of : driving a cylindrical brush, which comprises a plurality of generally radial fibres distributed over a cylindrical surface of a hub, to rotate about the axis of the hub; and feeding the gas stream into a region swept out by the fibres as the brush is rotated; whereby the gas stream enters the rotating brush parallel to the axis along the axis or at a small radius from the axis, and is discharged from the rotating brush parallel to the axis at a greater radius from the axis, and liquid removed from the gas stream travels centrifugally along the fibres.

3. Filter apparatus for removal of liquid droplets suspended in a gas stream, comprising : a hub mounted for rotation about an axis; means for driving the rotation of the hub; a cylindrical brush formed by a plurality of generally radial fibres distributed over a cylindrical surface of the hub ;

inlet means for directing the gas stream into a region swept out by the cylindrical brush as the hub rotates about its axis such that the gas stream enters the cylindrical brush parallel to the axis; outlet means for discharging the gas stream from the cylindrical brush at right angles to the axis; and collection means mounted radially outwards from the cylindrical brush for collecting the liquid removed from the gas stream.

4. Filter apparatus for removal of liquid droplets suspended in a gas stream, comprising: a hub mounted for rotation about an axis; means for driving the rotation of the hub; a cylindrical brush formed by a plurality of generally radial fibres distributed over a cylindrical surface of the hub; inlet means for directing the gas stream into a region swept out by the cylindrical brush as the hub rotates about its axis such that the gas stream enters the cylindrical brush parallel to the axis along the axis or at a small radius from the axis; outlet means for discharging the gas stream from the cylindrical brush parallel to the axis at a greater radius from the axis; and collection means mounted radially outwards from the cylindrical brush for collecting the liquid removed from the gas stream.

5. Filter apparatus according to claim 3 or claim 4, wherein the fibres of the brush are less than 2 mm in diameter.

6. Filter apparatus according to any of claims 3 to 5, wherein the brush has a greater density of fibres in a part of the brush remote from the inlet means than in a part of the brush close to the inlet means.

7. Filter apparatus according to claim 6, wherein the fibres in the said part of the brush remote from the inlet means have a smaller diameter than the fibres in the said part of the brush close to the inlet means.

8. Filter apparatus according to any of claims 3 to 7, wherein the collection means comprises a volute-shaped housing surrounding the cylindrical brush.

9. Filter apparatus according to any of claim 3 to 8, wherein the collection means comprises a generally cylindrical housing surrounding the cylindrical brush, the housing including: at least one stationary member located on an inner surface of the housing to trap liquid travelling along the inner surface of the housing; and at least one orifice adjacent to each stationary member, through which liquid trapped by the stationary member can be discharged from the housing.

10. Filter apparatus according to any of claims 3 to 9, wherein the collection means contains porous material.

11. A method of deodorizing or sterilizing air comprising the steps of : driving a first cylindrical brush, which comprises a plurality of generally radial fibres distributed over a cylindrical surface of a hub, to rotate about the axis of the hub; supplying a deodorizing or sterilizing liquid to the hub of the first brush so that the liquid travels centrifugally along the fibres of the brush; feeding the airstream into a region into a region swept out by the fibres as the first brush is rotated, whereby the air and the liquid come into intimate contact ; driving a second cylindrical brush to rotate about its axis; and feeding the airstream that emerges from the first brush into a region swept out by the second brush as it is rotated to remove droplets of the deodorizing or sterilizing liquid from the airstream.

12. A method according to claim 11, wherein the first and second brushes are driven on a common shaft.

13. A method according to claim 11 or claim 12, comprising the further step of passing the airstream through a flow straightener after it emerges from the first brush and before feeding it into the second brush.

14. Filter apparatus substantially as described herein, with reference to any of Figures 5 to

15.