GB2223187A - Centrifugal filters - Google Patents

Abstract

Fine particles are removed from air by drawing the air through inlet 23 and expelling it centrifugally through a band of paper 34 of 20 microns or less pore size wrapped round a rotor 12. In alternative constructions the rotor carries a plurality of concentric paper bands or a spirally wound band separating respective inlet and outlet chambers, which are otherwise separated by concentric or spiral impervious walls, or the rotor comprises a tube rotating about a transverse axis with paper discs closing its ends. <IMAGE>

Description

APPARATUS FOR FILTERING VERY FINE PARTICLES FROM AIR This invention relates to the extraction of dust and other particles by filtration from air and other gases.

The use of a filter to remove dust from air is a very common practice. Conventionally, a filter is placed in a suitable static housing, with ductwork to convey the air into and away from the filter, and a fan is provided to force the air through the filter.

The invention is aimed at allowing filters to be used of much greater fineness than has been commercially practical hitherto. The invention lies in mounting an element of the very fine filter material on a spinning rotor, and in harnessing centrifugal force to urge the air through the fine filter without causing an undue back pressure.

The performance of a filter may be quantified in terms of the size of the particles which can pass through the filter.

Thus, a 10-micron filter will only allow particles having overall dimensions less than 10-microns to pass through. A 10-micron filter is effective to prevent substantially all particles 10 microns and larger from traversing through the filter in a single pass.

The design of a fan is such that the fan cannot increase its output pressure: if there is a resistance to airflow, such that pressure might tend to build up, a fan will simply allow air to leak back past its blades. This limitation applies to any fan which permits back leakage, including both centrifugal and axial fans.

By contrast, in a positive-displacement air-pump device, such as a piston-type air compressor, the output pressure of the air can be very much higher because, in a positivedisplacement pump, back-leakage is prevented. However, positive displacement pumps are considerably more expensive than fans.

The fact is, however, that in order to pass an adequate volumetric flow the finer filters do require the kinds of pressure which, although they might be easily generated by an air compressor, can hardly be generated at all by a fan.

In fact, in a fan-driven system, if the flow is to be substantially useful, it is not practically and economically possible, using ordinary filters, to filter out from an airflow particles of less than about 30 microns. However, in exceptional cases, filtration down to 10 microns, or even less has been achieved (albeit very inefficiently) in fan-driven systems.

The pressure-drop created by a filter to a through-flow of air depends on the volumetric flow rate of air being forced through the filter. The greater the pressure head available to drive the air, the greater the flow rate that can be forced through the filter. But even with only the very small pressure head that is available from a fan, it is still possible to achieve some flow of air through the filter. The invention should therefore be regarded not as permitting flow to take place through filters that are so fine that a fan simply would not drive air through the filter at all, but rather as creating conditions in which a reasonably economically viable volumetric flow rate can be achieved through these fine filters, without having to resort to expensive high-pressure pumps.

Hitherto, therefore, it has not been economically practical to extract particles smaller than the range of about 30 to 10 microns efficiently from air, for the above-described reasons. A filter that is fine enough to extract those particles from a flowing air stream of any appreciable volume requires more pressure than can be readily generated by a fan. Thus, whether the requirement is for filtering air in an industrial factory, or in a clean room, or in the home, or in a hospital, or in any other case, it has not been economically practical to extract particles below the range of about 30-10 microns, although as mentioned some very expensive systems have gone lower than this.

Many pollutant particles, however, are much smaller than -30 microns, and it is, if anything, these smaller particles that are the ones that should be removed from air that is to be breathed.

One answer to this problem of the high air-pressure required for fine filtration, is the use of an electrostatic filter.

These units are highly effective at removing very small dust particles, and they do create very little back pressure.

However, the drawbacks of electrostatic filter units are that they are almost as expensive as positive-displacement pumps, and also that they are soon clogged, in that once a coating of ionised dust particles has established itself on the charged surfaces of the electrostatic unit, the capacity of the surfaces to attract further particles is much reduced.

It has been found, in the invention, that if an element of a fine filter medium is mounted for rotation, and i#s spun at a considerable speed, the resulting centrifugal force acting on the molecules of air within the filter medium drive the air through the filter: it has been found that it is possible to drive air through a very fine filter, just using centrifugal force in this manner, without the use of a separate fan or pump of any kind (although a separate fan may be provided for handling the bulk flow of air if desired).

The present invention is not the first occasion on which it has been proposed to spin a filter element, as opposed to the more normal case where the filter remains stationary.

In US-3018896 (1962 GEWISS) US-3606735 (1971 BAIGAS) and US-4547208 (1985 OACE) for example, pleated filter elements are arranged to act, themselves, as ordinary centrifugal fan blades. There is no suggestion in these patents that mounting the filter material so as to harness centrifugal force would enable very fine filters to be used, and indeed, it is clear that pleating the filter element actually largely prevents centrifugal force from acting in the advantageous manner as proposed in the invention: that is, of directly driving the molecules through the filter.

Instead, these prior designs rely on the air flow created by the blades to create enough suction to draw the air through the filter. The limitations on the fineness of the filter that are posed by conventional centrifugal fans are still present in these devices.

It is recognised in the invention that the filter element ideally should be placed at a right angle to the radius of the axis of rotation -- that the element should NOT lie almost along a radius of the rotor as it does in the pleated versions: the greater the deviation from the right angle, the more the action of centrifugal force will tend to drive molecules deeper into the filter element, rather than through it.

Other prior patents however have shown rotating filter elements mounted for rotation, where centrifugal force does act, as it does in the invention, to drive air directly and at right angles straight through the filter medium. Examples are GB-532467 (1941, SADD) US-2272746 (1942, HOLM-HANSEN) US-3123286 (1964, ABBOTT) and GB-1037365 (GEC, 1966). In all of these, again however, there is no suggestion that the use of centrifugal force to drive air through the filter would allow very fine filter materials to be used without needing to resort to positive-displacement air-pumps.

Indeed, it appears to be a feature of these prior designs that the filter material itself acts like the blades of a centrifugal fan, the implication being that the filter material must have a considerable radial or annular width in order to create the radial flow of air. It is recognised, in the invention, that the filter may be paper thin, and yet, if mounted properly, still will create sufficient force to drive air through the finest of filters.

It is also known in the prior art to separate solid particles from air by the cyclone method, wherein particleladen air is caused to swirl vigorously around a chamber: the particles are thrown aside, and the air escapes through the middle. Examples are US-2110978 (1938, NEUMANN) US-2994407 (1961, DIEPENBROEK) and SU-1263313 (1986, KRASD POLY). The cyclone principle is different from that of the invention in that, in the cyclone method, the air flows inwards, ie against centrifugal force, and even apart from that difference it is known that cyclonic separation is of little use with fine particles.

In its broadest aspect, the invention lies in the recognition that air can be made to pass through a very fine filter, without back pressure, by causing centrifugal force to act directly on the air in the filter material.

A preferred aspect of the invention lies in arranging the fine filter material into a cylindrical band or hoop, and in arranging several such bands concentrically about the axis of rotation. As will be described later, this arrangement shows considerable savings in space and volume utilization, as compared with a unit having only a single, elongate, filter element.

The ability of the invention to provide very fine filtration of air, without the need for generating the large pressure differentials that have been thought necessary hitherto for fine filtration, is useful in a wide variety of applications. Examples of these applications are: 1. A unit for absorbing fumes at source, such as a unit for collecting welding fumes right at the torch.

2. A stand-alone air cleaner for a room.

3. A filter unit for building-in to a ducted air-movement ventilation system.

The invention is especially suited for concentrated particles. It is preferred that the air to be cleaned is passed through the centrifugally driven fine filter as soon as possible. Once the particles have been allowed to disperse into a large body of air, it is much more difficult (and expensive) to collect that large volume of air and extract the particles. The invention is not particularly suited to producing high-volume streams or flows of air: a task for which fans are eminently suitable. It is however possible to use the fine filter of the invention in conjunction with a fan, in which case the fan deals with the high volume flow, and acts to convey the air into and away from the rotating fine filter.

In cases where the airflow is already vigorous, the centrifugal filter of the invention may be arranged to be driven by the airflow itself. In the case of the intake of air into an automobile engine, for example, the inflow of air is amply vigorous for driving a turbine of sufficient power to rotate the filter. The net extraction of energy from the intake flow can in this case be less, with the invention, than with conventional static, passive filters.

The use of centrifugal force to drive air through a fine filter, as in the invention, does not preclude the use of other filters: for example, it will generally be advantageous to include a relatively coarse filter to take out larger particles before the air enters the fine filter.

The coarse filter may be static.

An activated carbon or charcoal filter element may also be incorporated into a filter unit which embodies the invention, if desired.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT By way of further explanation of the invention, exemplary embodiments of the invention will now be described, with reference to the accompanying drawings, in which: Fig 1 is a cross-sectional elevation of a filter apparatus which embodies the invention; Fig 2 is a side elevation of the apparatus shown in Fig 1; Fig 3 is a pictorial view of one of the components of the apparatus of Fig 1; Fig 4 is a cross-sectional view of a portion of a rotor of another apparatus which embodies the invention; Fig 5 is a general cross-section of the said other apparatus; Fig 6 is a view corresponding to that of Fig 4 of an alternative arrangement; Fig 7 is a pictorial view of yet another apparatus which embodies the invention; Fig 8 is a graph which is used to illustrate one of the principles upon which the invention is based.

The structures shown in the accompanying drawings and described below are examples of apparatus which embody the invention. It should be noted that the scope of the invention is defined by the accompanying claims, and not necessarily by features of specific embodiments.

The filtering apparatus shown in Fig 1 includes a housing 10, a rotor 12, and an electric motor 14 fixed to the housing 10, for driving the rotor 12.

As shown in Fig 3, the rotor 12 includes a back plate 16, to which the shaft 18 of the motor is attached, and a front plate 20, which is provided with a large aperture 23. The housing 10 is enclosed except for an inlet port 25 and an outlet port 27.

As shown in Fig 3, the periphery 29 of the rotor 12 is right-cylindrical in shape, and is skeletal in construction, in that the periphery consists of bars 30 which run parallel to the axis of rotation of the rotor 12. The front and back plates 20,16 are provided around their outside edges with grooves 32.

A strip 34 of filter paper is wrapped around the periphery 29 of the rotor 12, and thus takes on the cylindrical shape of the periphery. The resulting band or hoop 36 of filter paper is held in place at the periphery 29 of the rotor 12 by means of retaining rings 38, which cooperate with the grooves 32. Alternatively, a suitable adhesive may be used to attach the strip 34 of filter paper to the rotor 12.

The filter paper used, although conventionally termed "paper" in fact is of substantially mineral constituents, comprising glass micro-fibres. Other synthetic materials are also available which provide comparable filtration.

The band or hoop 36 of filter paper extends all around the periphery 29 of the rotor, in a substantially airtight manner. The cylindrical band 36 defines an inlet plenum chamber 40, and the band separates the inlet plenum 40 from an outlet plenum chamber 43. The outlet plenum 43 is spiral scroll shaped, as shown in Fig 2, and communicates with the outlet port 27.

in accordance with the invention, the filter paper comprising the cylindrical band 36 is very fine, ie less than 20 microns, and preferably less than 1 micron.

In operation, with the motor 14 running, dirty air enters into the inlet plenum 40 via the inlet port 25. The air then flows through the band 36 of filter paper, into the outlet plenum 43, from which the filtered air is discharged via the outlet port 27.

No rotating vanes or blades are required to drive the air through the filter, because, as is recognised in the invention, centrifugal force acting on the molecules of air trapped in, and rotating with, the filter paper is enough to drive those molecules right through the filter paper. It is sometimes advisable, however, to mount blades on the rotor, outside the band 36, to make sure the filtered air is properly discharged from the outlet plenum chamber 43. This helps to reduce the possibility of a spurious back-pressure building up outside the band 36 of filter paper which, if it were present, might interfere with the free passage of air through the filter paper.

It should be noted however, that when such extra blades are added, for the purpose of creating a bulk air flow, in the manner of a normal centrifugal fan, that any pressure differentials created by such blades would have little effect on the volumetric through-flow of air passing through the filter. It is recognised, in the invention, that the agency which acts to move those molecules through the filter is centrifugal force acting on the molecules of air that are trapped in, and carried round by, the filter. This agency should be distinguished from the conventional outside pressure source acting to force air through the filter by brute force.

The unit shown in Figs 1 and 2 may be used as a general purpose air cleaner, for incorporation into ductwork, or to stand alone in a room. In the latter case, the housing need not be fully enclosed, as shown, but may be replaced by a cage, the main function of which is simply to prevent contact with the moving parts.

Figs 4 and 5 show another embodiment of the invention, in which several cylindrical bands of filter paper are arranged concentrically.

In this embodiment, the rotor 50 is constructed in the following manner. A strip 52 of thin sheet metal is bent over along one edge to form a bead 54. The strip 52 is bent round to form a circle 56: as shown on Fig 4, the strip 52 lies at an angle, and so the resulting structure is tapered, or conical. About ten or so circles 56 of the strip 52 are formed, each of progressively larger diameter.

The circles 56 are mounted on a series of wire spokes 58, which radiate from a hub 60. The circles 56 are attached to the spokes by a suitable means, such as by welding. The spokes 58 constitute a front plate 63 of the rotor 50.

A back plate 65 of the rotor is formed in a similar manner from wire spokes 67. Circular channels 69 are welded to the spokes 67. When the front plate 63 and back plate 65 are brought together, the top flanges 70 of the tapered circles 56 on the front plate 63 engage into the channels 69 on the back plate 65.

Before the front and back plates are brought together, strips 72 of filter paper are formed into right-cylindrical hoops or bands 74, and assembled into the rotor 50. This is done by threading the band 74 into the bead 54, which leaves the top edge of the paper lying snugly against the top flange 70 of the strip 52. Thus, when the flange 70 enters the channel 69, the top edge of the paper also enters the channel. Once assembled, the front and back plates may be retained together by means of suitable clips (not shown).

As before, within the chamber that contains the filter paper, each filter paper circle 74 serves to delineate an inlet plenum 78 from an outlet plenum 80. Air molecules within the filter paper are driven outwards by centrifugal force, and enter the outlet plenum 80. Molecules of air from the inlet plenum 78 enter the filter paper 72, and are discharged in turn.

The rotor 50 is mounted on the shaft of an electric motor 81, upon which is also mounted an axial impeller 83. These components are mounted in a tubular housing 85. In the intake end of the housing 85, a static filter 87 of a comparatively coarse porosity may be incorporated, for the purpose of taking out larger particles present in the air before the air reaches the fine filter in the rotor 50. The static filter 87 is coarse enough that air may pass through it without needing a substantial gradient of pressure.

The arrangement as just described of the concentric cylinders of filter paper has considerable advantages, as will now be described.

It is often necessary, in dust-filtering systems, to expose the maximum possible area of filter material to the dirty air. Thus, when the design of the system is such that the filter element has to be cylindrical in shape, it is generally desirable that the element be either of a large diameter, or of a long axial length, or both.

It is recognised that in such a case, however, a great deal of the overall volume of the system often remains unused.

The arrangement of the filters in concentric circles, as described and as shown in Figs 4 and 5, provides a vastly more efficient utilization of the volume or space, enabling the total envelope required for the system, for a given exposed-area of filter, to be much reduced.

Arranging the filters in concentric short cylinders enables the volume of the envelope of the system to be reduced by a factor of about five, when compared with the volume of the envelope the system would require if the filter were to be a single long cylinder. In the arrangement of the invention, of course the outermost of the several short cylinders plays a more effective part than the innermost cylinder, but it is recognized that the inner cylinders can have a considerable subsidiary effect.

For example, consider the case where the innermost cylinder is one-half the diameter of the outermost cylinder, and where there are a total of ten concentric cylinders, all having the same axial length. Now, the total area of exposed filter in that case is the same as that of a single cylinder having the diameter of the largest cylinder but having a length seven or eight times longer. Since centrifugal force is linearly proportional to diameter, the inner cylinders will not create so powerful an airflow, but even when that is allowed for, the arrangement still is equivalent in performance to a single cylindrical filter element some five times as long.

In the embodiment shown in Fig 5, the flow through the unit is predominantly axial, and the unit is suitable, for instance, for insertion into a length of ducting. It might be arranged, alternatively, that the impeller 83 be replaced by a centrifugal drum, and the filtered air be led off tangentially. Instead of a drum, in that case, radially disposed blades or vanes may replace the spokes 58 on the outlet side of the rotor.

It is preferred to pack in as many concentric bands or hoops as possible, to maximise the throughput of the unit for a given size envelope. The inlet plenums 78 are important in gathering up the air from the inlet mouth, and in presenting the flow of air smoothly and evenly to the filter paper, along the whole axial width of the band. It is preferred that the plenum extend over the whole exposed area of the filter for this reason. The plenum should be roomy enough in the radial direction to permit incoming air to circulate easily over the whole axial width of the filter, but it is recognised that such radial depth of the plenum may be small, yet still be adequate. The same considerations apply also to the outlet plenums. In Figs 4 and 6, the steepness of the angles has been exaggerated for clarity, and in fact the hoops and strips may be packed quite tightly together.

Although separate individual cylinders have been described in relation to Figs 4 and 5, it would be equally acceptable to arrange the strip 52 into a single continuous spiral, and to use a single length 72 of filter paper, wrapped into a corresponding spiral, and the scope of the invention should be construed accordingly.

In the embodiment of Figs 4 and 5, the strip or strips 52 which delineate the chambers were attached to, and rotated with, the rotor. Whilst it is essential, in the invention, that the filter material rotates with the rotor, it is not essential that the chamber-delineating barriers should rotate with the rotor. Fig 6 shows strips 79, which are attached to a static structure 86, and hence are non-rotating. The rotor 89 carries hoops 74 of filter material, and plenums corresponding to 78 and 80 as in Fig 4 are thereby created. The air flows radially through the filter paper hoops 74.

Fig 7 shows another version of the invention, which comprises a device for testing the pollution count of a sample of air.

In the device of Fig 7, disks 90 of filter paper are mounted in the ends of a length of tube 92. An inlet port 94 is formed halfway along the length of the tube 92, and the tube is mounted for rotation upon the shaft of an electric motor 96. An airflow meter 98 is included in the inlet port 94.

To take a pollution count, first the mass of a pair of clean disks 90 is measured. The disks are attached into the ends of the tube 92. The motor 96 is activated, and the tube starts to spin. Centrifugal force drives air through the disks of filter paper, drawing more air in through the inlet port 94 and through the airflow meter 98. When a measured quantity of air has passed through the device, the motor is switched off. The disks 90 of filter paper are removed and weighed, from which the pollution count may be determined.

Although the fine filters described have been made of paper, this should not be construed as a limitation of the invention. The invention is applicable to filters having a fineness of less than 20 microns, of whatever construction, on the basis that such fineness hitherto has required a pressure differential across the filter of a value greater than can be generated by a fan.

As has been stated, the use of a rotating or centrifuging air filter per se to generate a radial airflow has been previously known. However, the manner in which the idea has been put into practice has been such that the full potential performance of the centrifuging air filter has not been realised.

The principle of mounting the filter for rotary centrifuging is known; the invention lies in the hitherto-unknown application of that principle to very-fine-filtering.

The previous approaches to very-fine-filtering have been based either on creating a very high air-pressure differential across the very-fine-filter by means of a vacuum pump or compressor, or on ionising the tiny dust particles and attracting the particles by electrostatic means.

Compared with the conventional ways in which the problem of very-fine-filtering has been addressed, it is recognised in the invention that the centrifuging air filter approach provides a huge improvement, not only (1) in the dust extraction capability of the filter, but (2) in the length of the effective service life of the filter elements, (3) in the efficient use of motor energy, and (4) in the overall cost of the whole filtration apparatus.

It is recognised in the invention that the excellent results of the centrifuging approach to very-fine-filtering could hardly have been predicted from an assessment of the performance of centrifuging a filter when the filter is of the more usual, relatively coarse, porosity.

A theoretical explanation as to why this is so will now be attempted. The explanation introduces the concepts of two different mechanisms which are responsible for producing air flows, termed the 'paddle mechanism and the centrifugal 'field mechanism. First, a comparison of the ways in which airflow is generated in the two mechanisms will be explored, and then a comparison of the manner in which a filter resists the generated flows will be explored.

An ordinary centrifugal fan has blades or paddles mounted on the rotor. When the rotor rotates, these blades strike the molecules of air, with an impact which depends on the speed of the blade. The energy of the impact imparts a radially outward velocity to the molecule, which then flies radially outwards, and out of the fan. The larger the area of the fan blade, and the faster it moves, the greater the number of molecules that will be hit by the blade, and the greater the volume of the resulting airflow.

Thus, in a conventional centrifugal fan, the blade or paddle acts to "blast" the molecule out of the fan by brute force.

It may be noted that the energy of the impact is not applied directly and equally to all the molecules: the energy of the struck molecule is transferred to neighbouring molecules, and in turn to other molecules, such that the energy of the impact is soon dissipated. This process, of causing the whole body of molecules to move by striking just some of the molecules, is inherently inefficient.

If the outwardly-flying molecules should encounter some resistance, in the form of a filter, the molecules are slowed down even before they strike their neighbours, and an even greater energy input from the blades is required to move a worthwhile flow of air, through the rotor, if a filter is present on rotor.

It may be noted that the use of fan blades or paddles to force air through a rotating filter is hardly different from the use of fan blades or paddles to force air through a stationary or static filter.

For the purposes of this explanation, the above-described mechanism, by means of which the air molecules are flung out of the rotor, may be termed the paddle effect, and it is the mechanism by which all bladed centrifugal fans generate an airflow. Essentially, in this mechanism, the impact of the moving blade gives to the molecules it strikes a radially outward component of velocity, and it is this radially outward velocity which drives the molecules through the filter.

It is recognised that there is also another mechanism (in addition to the paddle machanism) acting to drive air molecules through a centrifuging filter. This mechanism may be termed the centrifugal "field" mechanism, and it arises in the following manner.

Once a molecule of air has become entrapped in the pores of the spinning centrifugal filter, that molecule is carried around with, and acquires the same angular velocity as, the filter itself. Centrifugal force, in that case, therefore acts directly on the rotating air molecule. If and when the molecule appears at the outside circumference of the filter, the molecule is able to leave the filter, and will simply fly off at a tangent. The molecule need have had no radial component of velocity imparted to it in order for the molecule to leave the filter by this means.

Thus, it is recognised that, without any induced pressuredifferential across the filter, molecules of air do experience a force which acts directly on each molecule individually, tending to make them leave the filter.

This force acts equally on all the molecules, and the whole centrifugal effect may be termed a centrifugal force field (by analogy to a gravitational or magnetic force field). It is recognised that in this centrifugal "field" mechanism for generating airflow, the molecules within the filter alreadY have been given the capacity to leave the filter; the molecules DO NOT need to receive any impacts, nor do the molecules need to have any radially outward component of velocity at the moment the molecule is about to leave the filter.

In the field mechanism, the molecules are not blasted outwards through the filter by brute force. That is not to say that in the field mechanism the molecules do not move outwards -- of course the molecules do move outwards and through the filter -- but rather that their outward movement is a result of, rather than the cause of, molecules continually leaving the outer surface of the filter. In the field effect, the molecules move smoothly, steadily, and much more gently, than they move under the paddle effect.

Thus, the two mechanisms have been compared as to the manner in which each drives the molecules through a centrifuging filter. The two mechanisms will now be compared from the standpoint of the effect of filter-induced resistance to that generated flow.

Under the paddle mechanism for generating airflow, it is well known that the finer the filter, the smaller the airflow, all else being equal.

It is recognised in the invention that, under the field mechanism for forcing molecules through the filter, the volume of the air flow is, by comparison with the paddle mechanism, hardly dependent at all on the fineness of the filter. In other words, under the field mechanism, the volumetric rate of the airflow depends upon the angular velocity of the rotor, and upon the radius of the rotating filter: but the volumetric rate of flow depends hardly at all on the pore size of the filter.

It is recognised, in the invention, that when a filter of the ordinary, relatively coarse, type is rotated, the paddle effect quite swamps the centrifugal field effect, to the extent that the field effect may be quite ignored, but that when the filter is of the very-fine kind, the field effect predominates over the paddle effect. It is recognised that the two effects or mechanisms are always both present in any rotating filter system: but it is recognised in the invention that it is only when the centrifuging filter is of the very-fine-filter type, not the coarse type, that the centrifugal field effect overtakes the paddle effect.

Fig 8 shows the paddle effect and the field effect compared on a graph. Curve A is the paddle curve, and shows the well known relationship between throughflow and microporosity when the throughflow is generated by the paddle effect. It may be noted that this curve is substantially the same whether the filter is static, or rotating. Curve A illustrates the commonly-observed effect that as the filter becomes very fine the flow rate induced by the paddle effect dwindles to virtually nothing.

Curve B is the field curve, and shows the relationship between throughflow and microporosity when the throughflow is generated by the centrifugal field effect. The field curve is shown as substantially horizontal, which is not quite true absolutely: however, the main purpose of Fig 8 is rather to compare the two curves, and to show how they interact relatively.

Fig 8 shows that at typical normal air filter fineness ranges, ie down to about 20 microns, or even lower, the centrifugal field effect is negligible, and designers have been right to ignore it. The small extra throughflow generated by the field effect is quite swamped by the throughflow generated by the paddle effect.

When the filter is coarse, by far the larger proportion of the throughflow is generated by the paddle effect. The throughflow generated by the paddle effect is largely unaffected by whether the filter spins or not.

It is recognised in the invention that there exists a changeover point; numeral 100 in Fig 8, at which the field effect starts to become more significant than the paddle effect. It has been found that this changeover point occurs approximately at the transition between what are traditionally thought of as cheap, bulk-throughflow filters, and laboratory" filters, which are so fine that they could only be used with expensive pumping equipment. In other words, this changeover point occurs in the fineness range of between 20 microns to 1 micron, approximately.

It is recognised in the invention that there is hardly any advantage to be gained from centrifuging the filter when the filter is coarser than this changeover point. The filter might just as well be of the normal statically-mounted type as far as throughflow is concerned. But it is recognised, for the first time in the invention, also that when the filter is finer than the changeover point, there is a huge benefit to be gained from centrifuging the filter.

The paddle effect is very inefficient at producing flow through a very-fine-filter, whether the filter spins or not.

It is recognised that the centrifugal field effect is much more suited than the paddle effect to producing a good flow rate when the filter is very fine.

It should be emphasised again that when rotating filters have been proposed previously, the designers have sought to make use not of the field effect but of the paddle effect.

Thus, it has been proposed that the filter material be pleated concertina-fashion, the folds or pleats to act as paddles to drive air through the filter, as in GEWISS etc, for example. In such a case, the flow rate will be related to the filter fineness along the lines of curve A, even though the filter is spinning.

Equally, even when, as in SADD etc, the designer has not deliberately shaped the filter material to resemble paddle blades, nevertheless it is the paddle effect which is inherently being harnessed. The filters used in such cases have been ordinary relatively coarse filters, with substantial radial depth, which act to produce a throughflow when spun because of the paddle effect of the "blades (ie the fibres) striking the molecules and firing them outwards through the holes in the filter. Presumably this measure avoids the cost of providing real blades, but it is not the field effect.

Because the filters used in these previous designs were of the ordinary coarse type, the field effect can have played only an insignificant part in such designs. Until the invention, it had not been recognised that the field effect, as described herein, is ever of any significance in generating a throughflow through a filter. In previous filtration systems, it has been considered that really the only way to force air through a very-fine-filter was to apply more and more pressure differential across the filter, and it has never been considered that spinning the filter would help in such a difficult case. The importance of the field effect was not appreciated even by those designers who made the filter material into spinning paddles, in that the designers of those systems were still seeking to harness the paddle effect.

In the field mechanism for generating flow, there is no theoretical requirement for the filter to have radial (ie annular) depth, and the filter may comprise simply a layer of paper wrapped around in a circle. In fact, the filter may comprise several such layers of paper, still without the filter having any appreciable radial depth. It is recognised in the invention that the flow rate would be virtually unaffected by the addition of such extra layers, although of course the efficacy of the extraction of the fine dust particles would be somewhat increased.

Perhaps more important than that however is the recognition that the field effect still acts to generate throughflow even when the centrifuging filter carries, not extra layers of filter paper, but a layer of dirt. When the mechanism by which flow is generated is solely the paddle effect, as in previous filtration systems, whether the filter rotates or not, the dirt build-up on the filter has had to be compensated for by supplying an even greater air pressure to force the air through the filter.

When the filter is of normal coarseness, a large reduction in the throughflow follows from an increase in the fineness of the filter, and the question whether the filter rotates or not is relatively unimportant. The invention lies in the recognition that when the filter is of the fine "laboratory" level of microporosity, it makes a huge difference to the throughflow if the filter is made to centrifuge, and the throughflow is very much less dependent on the actual value of the fineness of the filter.

The flow created by the centrifugal field effect attributable to a very fine, say 0.3 micron, filter is much greater than the flow generated by the paddle effect attributable to such a filter. However, by comparison with the throughflow associated with normal relatively coarse, say 50 micron, filters, the flow induced by the field effect attributable to a 0.3 micron filter is still quite small.

As described, the total throughflow of the centrifuging very-fine-filter of the invention can be increased by arranging the filters in radially-stacked concentric or spiral layers.

Claims (23)

1. CLAIM 1. Apparatus for filtering fine dust and other particles out of a gaseous fluid such as air, wherein: the apparatus includes a rotor, and a motor means for rotating the rotor; the apparatus includes filter material, which is mounted upon, and for rotation with, the rotor; the filter material is disposed, in a radial cross-section of the rotor, in two or more bodies of filter material; the apparatus includes barriers, which are mounted upon, and for rotation with,the rotor; the barriers lie disposed, in the radial cross-section of the rotor, in series, one radially outside another; the barriers are so disposed in relation to the radial cross-section of the rotor as to define chambers; each chamber includes a radially outer one of the barriers, which comprises an outside wall of the chamber, and includes a radially inner one of the barriers, which comprises an inside wall of the chamber; in respect of each chamber, the inner and outer barriers are so disposed that the chamber includes, located radially between the inner and outer barriers, an entry mouth and an exit mouth, so arranged that air may flow into and out of the chamber via the said mouths; each said body of filter material is contained in a respective one of the chambers; in respect of each chamber, the body of filter material is so located within the chamber that air, in passing from the entry mouth to the exit mouth, passes through the body of filter material; the barriers are so adapted as to substantially bar the passage of air in a direct radial direction from any point within the body of filter material in one chamber to any point within the body of filter material in the chamber radially outside it; and, in respect of each chamber, the entry mouth is, in substance, radially inside the body of filter material, and the exit mouth is, in substance, radially outside the body of filter material.
2. CLAIM 2. Apparatus of claim 1, wherein the bodies of filter material extend continuously and without interruption around the rotor.
3. CLAIM 3. Apparatus of claim 1, wherein the barriers are comprised of separate strips of inperforate material formed into concentric circles.
4. CLAIM 4. Apparatus of claim 3, wherein the bodies of filter material are comprised by respective hoops of material disposed between adjacent circles.
5. CLAIM 5. Apparatus of claim 1, wherein the barriers are comprised of a single strip of imperforate material formed into a spiral.
6. CLAIM 6. Apparatus of claim 5, wherein the bodies of filter material are comprised by a continuous layer of filter material located between adjacent turns of the spiral.
7. CLAIM 7. Apparatus of claim 1, wherein the bodies of filter material are in the form of thin sheets, and wherein, at substantially all locations in the bodies, the sheet lies substantially at right angles to the radius at that location.
8. CLAIM 8. Apparatus of claim 1, wherein: in respect of each body of filter medium, the chamber comprises a respective inlet plenum and a respective outlet plenum; the inlet plenum is in communication with the entry mouth, and the plenum is so arranged that air may enter the plenum at the entry mouth and may then pass through the inlet plenum to the said body of filter medium; the outlet plenum is in communication with the exit mouth, and the plenum is so arranged that air may leave the outlet plenum at the exit mouth, having entered the outlet plenum via the body of filter medium.
9. CLAIM 9. Apparatus of claim 8, wherein the entry mouth of the inlet plenum, and the exit mouth of the outlet plenum, are both annular in shape, and face in opposite axial directions relative to the rotor.
10. CLAIM 10. Apparatus of claim 7, wherein the fineness of the filter material is 20 microns or less.
11. CLAIM 11 Apparatus of claim 10 wherein the fineness of the filter material is 1 micron or less.
12. CLAIM 12 Apparatus for filtering very fine dust and other particles out of a gaseous fluid, such as air, wherein: The apparatus includes a rotor, and a motor means for rotating the rotor; the apparatus includes a filter medium; the filter medium is mounted on the rotor in such a manner that the filter medium lies in the said radial path of the flow of gaseous fluid; the rotor is open in construction, to the extent that the said gaseous fluid is able to flow in a substantially radial path through the rotor, from a point inside the filter medium to a point radially outside the filter medium; and the fineness of the filter medium is 20 microns or less.
13. CLAIM 13 Apparatus of claim 12 wherein the filter medium is in the form of thin sheet material.
14. CLAIM 14 Apparatus of claim 13 wherein: the thin sheet material has the shape of a cylindrical hoop or band; and the said hoop or band encircles the rotor, and is concentric with the axis of the rotor.
15. CLAIM 15. Apparatus of claim 14 wherein: the cylindrically shaped hoop or band of the thin sheet material defines an inlet plenum chamber, which lies radially inside the material; the apparatus includes an inlet means, for conveying the gaseous fluid to be filtered into the apparatus; the inlet means is effective to convey the incoming gaseous fluid into the said inlet plenum chamber; the cylindrically shaped hoop or band of the thin sheet material defines an outlet plenum chamber, which lies radially outside the material; and the apparatus includes an outlet means, for conveying gaseous fluid in the outlet plenum chamber, after having passed radially through the filter medium, out of the apparatus.
16. CLAIM 16. Apparatus of claim 15, wherein the filter medium is glass micro-fibre paper.
17. CLAIM 17. Apparatus of claim 15, wherein the fineness of the filter medium is 1 micron or less.
18. CLAIM 18. Apparatus of claim 15, wherein the cylinder that comprises the shape of the said hoop or band is a rightcylinder, and has substantially no taper or conical component to its shape: CLAIM
19. 19. Apparatus of claim 15, wherein the rotor includes a skeletal cage having a right-cylindrical periphery, and the filter medium is secured around the said periphery.
20. CLAIM 20. Apparatus for filtering very fine dust and other particles out of a gaseous fluid, such as air, wherein: the apparatus includes a rotor, and a motor means for rotating the rotor; the apparatus includes a filter medium, which is mounted on the rotor; the apparatus includes an inlet means for conveying gaseous fluid inside the rotor; the rotor includes a window, through which fluid is able to flow radially outwards, from the inlet means to a point outside the rotor; the filter medium is positioned in the said window, in such a manner that the said flow of air through the window passes through the filter material; the apparatus includes a motor means for rotating the rotor about such an axis that the filter material in the window is well spaced from the axis of rotation; and the fineness of the filter medium is 20 microns or less.
21. CLAIM 21. Apparatus of claim 20, wherein the filter medium is thin sheet material, of the glass micro-fibre type.
22. CLAIM 22. Apparatus of claim 20, wherein the fineness of the filter medium is 1 micron or less.
23. CLAIM 23. Apparatus of claim 20, wherein the rotor is in the form of a tube, and includes two of the said windows, one at each end of the tube, and includes respective portions of the filter medium, one portion to each window.