GB2201902A - Edge filters - Google Patents

Abstract

An edge filter comprises a stack of circular or rectangular members between which liquid is filtered, the stack being resiliently axially compressible to vary the effective pore size. In Fig. 8 the stack comprises alternate flat discs 39 and discs 33 with resilient spacing fingers alternately turned up and down around their inner peripheries. The stack in Fig. 1 comprises pairs of frusto-conical discs with radial slots round their inner edges, and in Fig. 4 the slots are replaced by spaced holes. The discs are threaded on to a central, axially movable rod 11, of flat section to allow passage of filtrate. In Figs. 11 to 15 there are filtrate channels formed by aligned holes in the rectangular members, distinct from the holes through which rods 11 extend. In Fig. 14 flat members have recesses accommodating frusto-conical springs which space them apart. <IMAGE>

Description

DESCRIPTION IMPROVEMENTS RELATING TO FILTERS.

The present invention relates to a filter having a variable flow-through cross-section.

The use of filters is an old and well-established practice and many filter designs can be traced back to the industrial revolution of the 18th and 19th centuries. All filters suffer from certain disadvantages, which restrict their applications and operating regimes. There are some disadvantages which afflict almost all filters, whilst various filters have their own peculiar limits. A simple example of a common disadvantage, is that virtually all filters have "fixed" pore sizes, i.e. the pore opening or pathway through the filter is of a fixed dimension.

Examples of particular limitations are (a) using a plastic filter for filtering hot gas (the filter element would simply melt) or (b) using a finely woven cloth filter element to filter strong acid (the element would rapidly disintegrate). Successful attempts have been made in recent years to eliminate the worst aspects associated with corrosive and thermal degradation on filter elements. Probably the most outstanding examples of such new filter technology are those of sintered stainless steel filter media and ceramic filter media. Both of these materials can operate in aggressive regimes that preclude most other filters. However, both these latter materials and all other filter media have "fixed" pore sizes and this can be seen and demonstrated to be, a major disadvantage.Having a filter media of a fixed pore size automatically dictates that different grades (pore sizing) of filter element must be used to suit different liquids or gases to be filtered, e.g., it would be useless to use a filter element with a rated pore size of 20 um to filter a liquid containing 5 pm particulate; all the particulate would pass through the filter.

Furthermore, having a fixed pore or pathway size, militates against the efficiency of any cleaning system that may be used to revitalise a "blinded" or "clogged" filter element. Most filter element media are made up of random labyrinthine pores and their micro appearance resembles that of a piece of coke.

This makes cleaning or unclogging of the element even more difficult and frequently impossible. When such a filter becomes clogged, the only recourse is usually to replace it. Such a recourse is time consuming; it stops the filtration process; it is expensive and in many circumstances, it can be quite hazardous.

If a highly durable material, whose pores or pathways were substantially flexible, could be instituted as a filter, this would present an excellent improvement in filtration technology. The advancement would be all the more profound if the new filter media could have its pores or pathways adjusted in situ, from a very small micron size to a relatively large macro size and set with precision at any predetermined intermediate opening. Such a filter system would bestow a most advantageous regime on the filtration requirements of industry at large. One filter element design could be used for any filtration duty, from, say 1 um to 500 pm, by simply adjusting it o to the required pore or pathway size. This would obviate the present necessity to purchase a multitude of differently rated filter elements to suit various filtration requirements.Furthermore, the new, variable pore filter could easily be adjusted in situ, for cleaning purposes; a feature not found on other filters. If the filter were made from a very obdurate material, e.g stainless steel or ceramic, then it would be able to perform in very hazardous and aggressive environments, for a long period of time.

Such a filter has already been devised, tested and proved. This filter takes the form of a closely wound wire coil which is manufactured from fine, flat form stainless steel wire, and this filter is known as the 'VSP filter' - see our copending British Patent Application No. . Although the precision wound casing from which the filter elements are made is usually of stainless steel, any one of a variety of metals can be used in its manufacture, e.g. bronze, brass, monel metal, chrome vanadium, nimonic alloys, carbon steel, copper, etc. It is quite feasible to use such a filter in the most aggressive environments, at elevated temperatures, over a very long period of time.Given that the filter is equipped with an in-situ cleaning cycle, it is quite possible that it could function indefinitely in the most rigorous of environments, e.g. under intense radiation coupled with a highly acidic or alkaline fluid at elevated temperature. However, due to design criteria, the VSP filter element concept has a "wire thickness" limitation, below which the embodied principle becomes untenable. This wire thickness limitation governs the acceptable lower limit of rigidity of the completed filter element and is directly related to the outside diameter of the particular metal wound casing employed. The larger the outside diameter of the casing employed, so the larger the wire size required to wind the casing in order to achieve the rigidity criteria. Conversely, the smaller the outside diameter of the casing, so the smaller the wire size requirement.This criteria, therefore, automatically restricts the ratio of the amount of metal surface to available pore or pathway opening. The attainable open pathway for filtration in VSP wire wound elements is, therefore, restricted by the wire thickness necessarily employed in the construction of the element. It is possible to advantageously alter the ratio of pathway opening to wire thickness, by carefully selecting the sectional geometry of the wire used to wind the casing. Even so, though some gain is made, it is unlikely that the pathway length for a given diameter of spring can be more than doubled from present practice, before once again running into the critical region for rigidity. It is, therefore, a logical conclusion that a critical limitation of the VSP filter is that of wire thickness.Ultimately, all other design criteria being optimised, the wire thickness determines the open pathway length of a VSP filter element, which in turn regulates the volume of filtrate passing through the element.

The aim of the present invention is to provide a variable filter which can be used in hazardous and aggressive environments for long periods of time, the filter having an optimised flow through pathway for its size, which pathway can filter a greater volume-of fluid per unit length of filter, as compared to the VSP filter.

According to the present invention there is provided a filter having a variable flow-through cross-section, said filter being comprised of a number of superposed flexibly resilient members between which a fluid to be filtered can be passed.

In one embodiment of the present invention the flexibly resilient members are each in the form of an annular member which is circular and frusto-conical in shape. Alternatively, each annular member may be generally rectangular and frusto-conical, i.e. with inclined side faces. The annular members each have a series of equi-spaced-apart slots extending radially outwardly from the inner edge thereof. The annular members are arranged in pairs on an elongate centre core with the outer peripheral edges of the annular members of any one pair engaging each other; the inner peripheral edges of said pair of annular members engaging with the inner peripheral edges of adjacent annular members of the respective two adjacent pairs of annular members.

One end of the centre core is fashioned into an integral disc of a diameter a little larger than the outside diameter of the annular members. The centre core, over which annular members are fitted, is precisely engaged at its other end in an integral filtrate discharge plenum and stack head plate. With the stack of annular members mounted on the centre column, the column can be moved linearly in and out of the plenum/head plate housing, thus crushing or releasing the stack. Releasing the pressure on the stack causes the stack to expand, opening up the filtration gaps and obviously "coarsening" the filtration. Conversely, putting more load on the stack causes it to collapse slowly, thus closing down the filtration gaps and consequently "refining" the filtration.The adjustment mechanism is such that it can be set precisely at any point giving an exact filter gap setting of a predetermined value, between the totally closed and fully open stack positions. In a filter with radially slotted annular members, it is possible to set the filtration gap from about 1 pm or 2 pm up to about 250 pm, at any point between the two extremes.

Several advantages are gained by a filter constructed according to the present invention over the earlier VSP system. Perhaps the most important feature is a large increase in filter pathway length for a given surface area or linear length of stack.

The pathway length improvement over the earlier VSP format is quite dramatic and there is ample scope for further improvement. Whilst annular disc members of about 0.325 mm (0.0128") thickness were used in the prototype of the present invention, it would be entirely feasible to construct a filter with annular disc members of a similar diameter (27 mm) but having a thickness of about 0.127 mm (0.005"). Even with the prototype of the present invention, the filter pathway length was about 10 to 12 times that achievable with a VSP filter element of comparable outside diameter; with the thinner discs, it is 25 to 30 times more efficient. A massive gain in throughput, indeed.

Another advantage of the present invention, as compared to the VSP, is that it can be a "natural" all metal filter element, whereas the VSP has to be carefully "engineered" to achieve an all-metal format.

As with the earlier VSP filter, the present invention may be constructed in a variety of metals, e.g. carbon steels/stainless steels/nimonic alloys/brass/copper/ chrome/vanadium, etc. Compared with most other filters, including the VSP, the present invention achieves an exemplary compactness for a given capacity or throughput. Quite tiny installations, constructed according to the present invention and little larger than a tea cup, filtering down to, say 5 pm and running at pressures of 40 psi - 60 psi, could give clear filtrate discharges of several litres per minute; a very difficult, if not impossible feat, for any other filter to achieve.

Due to the slotted design of the annular members in the above embodiment, said annular members have a "broken" interior profile which presents a multitude of crevices and crannies to any fluid being filtered, and though not as bad as a sintered metal or ceramic filter element, such a profile offers an anchoring point for particulate matter being removed from the fluid to be filtered. Furthermore, the annular disc member is seriously weakened by being of slotted design. This weakness can easily be observed by comparing two spring discs of identical dimensions, one being slotted and the other plain. Though both discs are of the same thickness, the slotted disc will flatten under substantially less load than the plain disc. This obviously implies that a much thinner plain disc can supplant a slotted disc for a given deflection.Using thinner discs considerably enhances the filtration efficiency of the present invention.

In order to take advantage of this situation, a modified annular member may be used. This modified annular member takes the form of a thin, shallow frusto-conical disc, whose "land" is pierced by a series of small equi-spaced holes. The number and size of holes in the "land" area of the spring disc can be varied to suit different sizes of discs and filtration requirements. The only proviso is that the holes must be equi-spaced in order to maintain even stressing on the disc peripheries. Uneven stressing on the internal or external rims of the disc will cause uneven deflection under load and consequently lead to malfunction of the filter. By maintaining the inner and outer peripheries of the annular member intact, a much stronger geometry is obtained, and a much smoother flow path.

This modified annular member design also presents another possibility when mounting the annular disc members in a stack. A multiplicity of holes in each annular disc member offers a surfeit of flow path area to the incoming fluid being filtered. Opposing pairs of redundant holes may be used as guide holes for guide rods passing through the stack. In this way the central guide column can be omitted, leaving the entire centre bore, formed by the multiple internal diameters of the annular disc members, free for fluid flow.

Depending upon the rigidity of the particular annular members being used in a stack, the number of annular members per stack can vary from a few tens to several hundred. As in the case of the VSP filter, where several elements could be mounted in a common plenum, so a multi stack filter can be constructed in accordance with the present invention. Each stack would be adjusted by a single, common, adjustment mechanism, mounted above the stack's head plate, on the filtrate side (clean side) of the system. An adjustment override mechanism would allow "super" extension of the stack, beyond the pre-set gap setting, in order to facilitate washing and cleaning of discs, e.g. by back or forward washing, and/or blowing.

It can be seen that in both of the above described embodiments the stacks essentially operate on a "paired" disc principle, each pair of discs presenting a closed pair of rims on its external periphery.

Thus, if each annular disc member is 0.005" thick, then each pair of annular disc members offers a closed periphery of 0.010" thickness to the observer. As far as filtration is concerned, this is a dead area, i.e.

a non-functioning filtration area. As previously stated, by going from the slotted annular disc member to the modified design, it is possible to reduce the thickness of each annular disc member by virtue of the greater strength of the latter type. It is indeed quite feasible to reduce the disc thickness by 50% by employing the modified design of annular disc member.

Thus, the "dead area" of rim periphery can be greatly reduced by using the modified design rather than the "slotted" design. In both designs the frusto-conical or "dished" annular disc members employed form the apex of a triangle at their outer rim contact points.

Within the stressed height of the stack (from minimum to maximum filtration gap opening), the rims are kept in intimate contact by virtue of the inherent "spring" pressure in the individual annular disc members. The filtration gap, formed between separate pairs of annular disc members, is set by varying the triangular geometry of rim contact points. At zero gap, the annular disc members are completely flattened and the triangles are eliminated. At a coarse setting with the stack extended, the rim contact points form an obvious triangle. The paired rims never separate within the spring stack height, i.e. within the designed filtration range of the stack. Thus, the "dead" area of a "dished" stack system is always that presented by the multiplicity of the intimately contacting rims of annular disc member pairs.

To reduce this "dead" area and consequently allow a substantial improvement in filter throughput, the actual outer rim of the annular members may be used instead of using slots or holes as the filter flow path, the "dead" area then being comprised of the rims of individual annular members, not pairs of rims. For a given thickness of annular member, this effectively halves the dead area on a stack, and consequently doubles the filter throughput.

In one embodiment of such a design, two types of generally planar annular disc member may be employed, in order to achieve the precision levitation or elevation required for an efficient filtration device.

One annular disc member has an unbroken, parallel outer rim and a broken (slotted) inner rim. The slotted inner rim is comprised of an even number of sprung prongs" , with alternate prongs projecting up and down from the plane of the parallel rim. This "duplex", planar annular disc member is, of necessity, precision made. All upward sloping prongs are set precisely, so that their uppermost points achieve exactly the same elevation from the flat rim plane.

Conversely, all downward sloping prongs are set to project, to an identical dimension to the upward sloping set, from the flat rim plane. Such an annular disc member, when sat down on its lower prongs on a horizontal surface, would have its rim truly parallel to the horizontal surface. A truly flat annular disc member then placed on the upper prongs, would in turn be truly parallel to the duplex disc rim and the lower horizontal surface. By stacking alternate duplex spring disc members and plain flat disc members in a stack, a multiplicity of parallel openings is achieved. By mounting such a stack on a specially designed centre column, a variable, all metal, parallel disc member stack filter geometry is achieved.Due to both the duplex spring discs and the plain discs being of a similar thickness (or even less) to the previously described modified design annular disc member, then, by comparison, the flow path area of the filter is at least doubled.

The height of prong elevation determines the "settable" filtration gap. In turn, prong elevation is determined by loading the stack. Full load will flatten the stack, by parallel deflection. Gradually releasing the load will cause precise parallel separation of the annular disc members, with precision parallel gap setting, at any desired point, between the fully loaded (flattened) stack state and the zero loaded (fully relaxed) stack state.

This latter design permits a very wide range of "relaxed" prong elevations to be achieved, thus permitting parallel disc stack filters to operate from say, O.5#m (or less) to 300O#m (or more) with one stack. However, as previously stated, since most filtration problems in industry lie in the range of say, O.5#m to 100cm, then this parallel planar disc member stack filter would be designed accordingly.

In contrast to'the annular flexibly resilient members described hereabove, said resilient members may be of any desired configuration. For example, in one still further embodiment the flexibly resilient members are each generally planar and rectangular with a central guide aperture and spaced-apart drain or filter holes. The periphery of the guide aperture is provided with alternate upwardly and downwardly projecting resilient tongues in similar fashion to the above last-mentioned annular embodiment. Again, as per the last-mentioned annular embodiment a planar member of like configuration but without said tongues, is located between adjacent tongued members with a central core extending through the aligned guide apertures. Additionally a guide rod extends through aligned guide bores in said members to thus align said drain holes.In use, axial movement of the central core can vary the spacing between adjacent members, the tongues being flexed as the members are compressed together or released to thus vary the filter grade.

In a modified form of the latter embodiment the central guide aperture of each resilient member is combined with the drain holes in an elongate slot with the resilient tongues formed along the sides of the slot. The central core has flattened sides which engage and align the slots of superposed members; obviating the need for a guide rod.

Whilst the resilient tongues in the above generally rectangular member embodiments are integral with said planar members, they can be replaced by disc springs of frusto-conical configuration, said members being then planar with oppositely facing, interengaged disc springs located over the central core and between adjacent planar members. In this case all planar members are identical.

The advantage of using generally rectangular members or plates, i.e. elongate members, in the -filter stack over annular or circular members, is that for a given filter chamber designed to receive a stack of annular plate members, two elongate member filter stacks can be substituted to provide for increased throughput.

In any of the possible embodiments of the present invention the flexibly resilient filter members may, with advantage, be made of any suitable metal.

Dependent upon the metal or other flexibly resilient material selected for the filter members, the filter can be highly resistant to corrosion, andlor radiation, and/or may have a wide thermal operating range. Further, the filter may be very compact and easy to clean in situ with a very long, maintenancefree life. Further, as the filter is adjustable, it can be used for a multitude of different filtration problems with there being no need for an industrial user to have stacks of many different grades, i.e.

pore sizes, of filter. The filter of the present invention may be used as a normal pressure filter, a cross-flow filter, a vacuum filter or even as a flow control or safety valve on clean gases and liquids.

Whilst the filter of the present invention may be constructed as a compact unit, it may, of course, be any required size, and it may be used in a single element or multi-element, i.e. multi-stack, filter, the filter being variable over a wide range.

The present invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Fig.l is an axial cross-sectional view taken through one embodiment of a filter constructed according to the present invention; Fig.lA shows the filter of Fig.l in a flattened condition; Fig.2 is a plan view of one of the annular members used in the filter of Fig.l; Fig.3 is a side view of the annular member of Fig.2; Fig.4 is an axial cross-sectional view of a further embodiment of a filter constructed according to the present invention; Fig.5 is a plan view of one of the annular members used in the filter of Fig.4; Fig.6 is a side view of the annular member of Fig.5;; Fig.7 is an axial cross-sectional view of a multistack filter constructed according to the present invention, and using the annular members of Fig.6; Fig.8 is an axial cross-sectional view of a still further embodiment of the present invention; Fig.9 is a plan view of one of the annular members used in the filter of Fig.8; Fig.10 is a sectional side view of the annular member of Fig.9, taken along line A-A in Fig.9; Fig.ll is an axial cross-sectional view of a still further embodiment of the present invention; Fig.12 is a plan view of one of the rectangular members used in the embodiment of Fig.ll; Fig.13 is a plan view of a modified form of elongate/rectangular filter member; Fig.14 is a partially exploded axial crosssectional view of a still further embodiment of the present invention; and Fig.15 is a plan view of one of the rectangular members used in the embodiment of Fig.14.

The filter constructed according to the present invention and illustrated in Figs. 1 and 1A of the accompanying drawings, comprises a filter stack 1 comprised of a number of flexibly resilient members, each of which takes the form of an annular disc-like member 3 (see Fig.2) of dished or frusto-conical configuration (see Fig.3). Each annular disc-like member 3 is made of a flexible and resilient metal and has a series of equi-spaced-apart slots 5 extending radially outwardly of the annular disc-like member 3 from the inner periphery 9 thereof.

In the filter stack 1 the annular disc-like members 3 are arranged coaxially with respect to each other, in pairs, so that the outer peripheries 7 of the annular disc-like members 3, in a pair, engage one another, with the inner peripheries 9 of the members of said pair, engaging the inner peripheries of members of adjacent pairs. An elongate central core 11 which is generally cylindrical with parallel flattened side faces extending along its length, extends through the annular disc-like members 3 forming the filter stack 1. This central core 11 has an enlarged diameter disc-shaped end region 13 on which the filter stack 1 rests, the other end region 15 of the central core 11 extending snugly, though axially slidably, through a bore in a guide member 17, against which the filter stack 1 is pressed.This other end region 15 of the central core 11 is connected to a tie rod 19 which can be adjusted axially to compress or release the flexibly resilient annular disc-like members 3, to thus vary the filter flow-through cross-section. The completely flattened state is illustrated in Fig.lA. In use, with the tie rod 19 adjusted to give the required filtration path, the fluid to be filtered flows between adjacent pairs of said annular disc-like members 3, and through the slots 5. The filtered fluid then passes between the filter stack and the central core 11, and between the flat side faces of the central core 11 and the wall of the bore in the guide member 17, to discharge.

The radial slots 5 in the above-described embodiment of the present invention, provide a tortuous path for the filtered fluid, with a large number of recesses and edges which can attract the build-up of filter cake. To provide for a smoother filtration path with a less rapid filter cake build-up, and to also provide for a thinner flexibly resilient filter member, the annular disc-like member 21 can be modified as shown in Figs. 4, 5 and 6 of the accompanying drawings. The same reference numerals as used in Figs. 1 to 3 are used in Figs. 4 to 6 solely for the same component pacts. The modification lies in the fact that annular disc-like member 21 has continuous inner and outer peripheries 23,25 with a series of equi-spaced-apart holes 27 providing the necessary fluid path.As in the embodiment of Figs. 1 to 3, the annular disc-like members 21 are dished or frusto-conical and are similarly-arranged in pairs.

However, as a result of the annular members 21 having continuous inner and outer peripheries 23,25, they are considerably stronger than the slotted annular members 3 of Figs. 1 to 3. Thus annular members 21 can be made thinner than annular members 3, with no loss of resilient strength, and thus the available filtration path per unit length of the filter stack can be increased, as compared to the embodiment of Figs. 1 to 3.

In the embodiment of Fig.4, the annular disc-like members 21 forming the filter stack 1, are located on an axially movable central core 11, as in the embodiment of Fig.l. However, in an alternative embodiment (not shown) using annular disc-like members 21 to form a filter stack, the central core 11 is omitted and guide rods extend through a number of the aligned holes in the annular disc-like members, there being sufficient unused holes to provide the required filtration path.

Whilst Fig.4 illustrates a filter having a single filter stack 1 formed by pairs of annular disc-like members 21, Fig.7 illustrates a filter incorporating two such filter stacks 28,29, both of which filter stacks 28,29 are simultaneously controllable by selectively openable control means 31.

A still further embodiment of a filter constructed according to the present invention, is illustrated in Figs. 8 to 10 of the accompanying drawings. Again, the same component parts as used in the embodiments of Figs. 1, 4 and 7 are identified by like reference numerals. The main difference in the embodiment of Fig.8 is that the annular flexibly-resilient members 33, though disc-like, are not dished, but are basically planar, with the inner periphery provided with flexibly resilient tongues 35,37 projecting alternately upwards and downwards from the general plane of the annular member. In the filter stack 1 the annular members 33 are located over a central core 11, with further plain, planar disc-like annular members 39 located between each pair of annular members 33, these further annular members 39 engaging with the tips of the flexibly resilient tongues 35,37.The central core 11 extends through a bore in a guide member 17 against which one end of the filter stack 1 engages, the other end of the filter stack 1 being supported on an enlarged diameter end region 13 of the core 11. By axially moving the central core 11 the filter stack 1 can be compressed to decrease the filter size, or relaxed to increase the filter grade.

In the embodiments of Figs. 1 to 7 of the accompanying drawings, the disc-like annular members 3,21 are arranged in pairs and a triangular configuration is formed by the interengaging outer edges of each pair of annular members 3,21, the apex angle of the triangular configuration being dependent upon the compression of the filter stack 1. The thickness of the interengaged outer edges thus constitutes wasted axial length of the filter as regards available filtration path. This is optimised to an extent in the embodiment of Figs. 4 and 7 by virtue of the stronger, though thinner, annular members 21. However, the available filtration path is maximised for a given filter stack length in the embodiment of Fig.8, as it is only the thickness of each single annular member which constitutes wasted axial space.

Whilst the embodiment of Fig.8 utilises annular members 33, any other desired configuration can be selected for the filter members, using the principle of alternating upwardly and downwardly directed resilient tongues 35,37. For example, as shown in Figs. 11 and 12, the filter members 39 can be generally rectangular with a central guide aperture 41 and spaced-apart drain holes 43. The tongues 35,37 are provided around the periphery of the guide aperture 41 with a central core 11 projecting through aligned guide apertures 41 in superposed filter members 39. To align the drain holes 43, a guide rod 45 extends through aligned guide bores in the superposed filter members 39. Otherwise the embodiment of Figs. 11 and 12 operates in the same manner as the embodiment of Fig.8.

A modified form of generally rectangular or elongate, planar filter member 47, is illustrated in Fig.13. This filter member 47 is suitable for use in the manner shown in Fig.ll, and has an elongate slot 49 which replaces the guide aperture 41 and drain holes 43 of the embodiment of Figs. 11 and 12.

Alternate upward and downward tongues 35,37 are formed along the sides of the slot 49, and a central region of the slot 49 has parallel sides 51 which engage with flats formed on the central core 11 to thus align the superposed filter members 47 and slots 49, obviating the need for a guide rod as in the embodiment of Fig.ll. This filter member 47 is located between adjacent parallel members (not shown) again as in the embodiment of Fig.ll.

Instead of integrally forming the resilient tongues 35,37 with the filter members as in the embodiments of 8 to 13, planar, rectangular filter members 53 as shown in Figs. 14 and 15, can be separated by pairs of disc springs 55. Each disc spring 55 is a generally frusto-conical, annular member, and a pair of oppositely directed disc springs 55 is located over the central core 11 between each pair of superposed filter members 53. The disc springs 55 each engage snugly in a recess 57 in a filter member 53 and have a rectangular aperture (see Fig.15) through which a complementarily shaped central core 11 is passed. Thus the drain holes 43 in the filter members 53 are aligned with each other in a stack.

The planar filter members can provide advantages when they are of generally rectangular or elongate configuration, as two filter stacks of elongate filter members can be fitted into a planar chamber designed for a stack of annular filter members. Thus the throughput can be increased.

The present invention thus provides a simple filter which is adjustable over a wide range, and which can be made of a flexibly resilient material such as a suitable metal, so as to be highly resistant to corrosion, and/or radiation, and/or temperature.

Claims (21)

1. A filter having a variable flow-through cross section, said filter being comprised of members which are inherently flexibly resilient or resiliently mounted, between which members a fluid to be filtered can be passed.

2. A filter as claimed in claim 1, in which said members are each in the form of a flexibly resilient annular member which is frusto-conical in shape.

3. A filter as claimed in claim 2, in which the annular member is circular.

4. A filter as claimed in claim 2, in which the annular member is generally rectangular.

5. A filter as claimed in any one of claims 2 to 4, in which the annular members each have a series of equi-spaced apart slots extending outwardly from the inner edge thereof.

6. A filter as claimed in claim 5, in which the annular members are arranged in pairs on an elongate centre core with the outer peripheral edges of the annular members of any one pair engaging each other, and the inner peripheral edges. of said pair of annular members engaging with the inner peripheral edges of adjacent annular members of the respective two adjacent pairs of annular members.

7. A filter as claimed in claim 6, in which one end of the central core is provided with an integral disc on which the superposed annular members rest, the other end of the central core being axially movable in a bore in a filtrate discharge plenum, so as to compress or release the superposed annular members between the disc and discharge plenum, as desired.

8. A filter as claimed in any ore of claims 2 to 4, in which the annular members each have a series of equi-spaced apart holes, spaced around the annular member, between the inner and outer edges of the annular member.

9. A filter as claimed in claim 8, in which the annular members are stacked in pairs with the outer peripheral edges of the annular members of any one pair engaging each other, and the inner peripheral edges of said pair of annular members engaging with the inner perpheral edges of adjacent annular members of the respective two adjacent pairs of annular members.

10. A filter as claimed in claim 8 or 9, in which guide rods extend through a number of aligned holes.

11. A filter as claimed in claim 1, in which two types of generally planar annular members are employed, one type of annular member having an unbroken outer rim and a slotted inner rim, the slotted inner rim forming a number of resilient prongs with alternate prongs projecting in opposite directions from the plane of said one type of annular member, the other type of annular member being planar, said two types of generally planar annular member being stacked together alternately.

12. A filter as claimed in claim 11, in which the annular members are stacked on a central column which can be moved axially to flex the prongs and thus adjust the filtration gap between said annular members.

13. A filter as claimed in claim 11 or 12, in which said annular members are disc-shaped.

14. A filter as claimed in claim 11 or 12, in which both of said types of annular member are generally rectangular with a central aperture.

15. A filter as claimed in claim 14, in which a guide rod extends through aligned bores in said annular members.

16. A filter as claimed in claim 14, in which both of said types of annular member has an elongated central aperture. -

17. A filter as claimed in claim 16, in which part of the elongated slot engages snugly with a profiled region of a central column passing therethrough.

18. A filter as claimed in claim 1, in which said annular members are identical with each other and are stacked coaxially with each other with a pair of oppositely directed, frusto conical disc springs located between adjacent annular members in the central region thereof.

19. A filter as claimed in claim 18, in which the disc springs each engage snugly in a recess in an annular member.

20. A filter as claimed in claim 18 or 19, in which each disc spring has a rectangular aperture through which a copementaril shaped central core is passed.

21. A filter constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.