The present invention relates to methods and apparatus for fluid filtration.

In one aspect, the invention provides a fluid filter element comprising:

a plurality of filter stages; and

a fluid path which passes sequentially through the filter stages;

and wherein at least some of the filter stages have different filter characteristics.

The fluid path may pass through at least one relatively coarse filter stage and at least one relatively fine filter stage. The filter characteristics of the filter stages may be increasingly fine along the fluid path. The filter stages may be arranged in groups with substantially the same filter characteristics, and wherein neighbouring groups along the fluid path have different filter characteristics.

At least some of the filter stages may have different pore sizes. At least some of the filter stages may remove different filtrands.

At least one of the filter stages may be provided by a generally planar filter structure, the fluid path crossing the plane through the structure. A plurality of successive filter stages may be generally planar. A plurality of filter stages may be generally planar filter structures arranged at generally parallel planes, the fluid path crossing each of the planes through the corresponding structure.

There may be at least one partition member located between adjacent ones of the filter structures, the partition member having impervious regions and being dimensionally stable, and further having pervious regions through which fluid can flow, in use, from one filter structure to the adjacent filter structure. The partition member may be planar and may be perforated.

The generally planar structure may be a disc. At least one of the generally planar filter structures may have an aperture, the fluid path crossing the plane through the aperture, without filtration, and also crossing the plane through the filter structure for filtration. The at least one of the generally planar structures is a disc, and the aperture may be concentric with the disc. The fluid may cross the plane of the filter structure in opposite directions through the aperture and through the filter structure.

A plurality of filter structures having apertures as aforesaid may be arranged with aligned apertures to provide a fluid path leg through the apertures, without filtration, and a fluid path leg sequentially through the structures. There may be a conduit member defining a passage through the or at least one of the apertures. There may be a common housing around the filter structures, there being seal means arranged substantially to prevent fluid passing the filter stages without passing through the filter stages.

In another aspect, the invention provides a fluid filter comprising:

a housing having an inlet and outlet;

a fluid path from the inlet to the outlet; and

a fluid filter element according to any preceding definition, the fluid path of the fluid filter element forming at least part of the fluid path from the inlet to the outlet. In a further aspect, the invention provides a method of providing fluid filtration for a target fluid, comprising:

passing target fluid through a fluid filter element as defined in any of the definitions above;

analysing the filtrand captured, including identifying the filter stage or stages in which capture has occurred;

modifying the filter characteristics of the filter stages to cause filtrand to be captured more evenly across all of the filter stages.

The filter characteristics may be modified by replacing one or more filter stages with alternative filter stages having different filter characteristics.

Examples of the present invention will now be described in more detail, by way of example only, and with reference to the accompanying drawings, in which:

Fig. 1 is a schematic perspective view of a filter element;

Fig. 2 is a schematic perspective view of a filter stage of the element of Fig. 1 ;

Fig. 3 is a schematic perspective view of a filter including the element of Fig. 1 ;

Fig. 4 is a perspective view of a partition member for use in an alternative example of a filter stage;

Fig. 5 is a perspective view of a filter element including partition members as shown in Fig. 4; and

Fig. 6 is a simplified flow diagram of a process for designing a filter element. Overview

Fig. 1 illustrates one example of a fluid filter element 10. The element comprises a plurality of filter stages 12. In this example, the stages 12 are illustrated as layers of the element 10, which is constructed in a manner to be described below.

A fluid path is indicated by an arrow 14. The path 14 passes sequentially through the layers of the element 10 and thus, passes sequentially through the filter stages of the element 10.

At least some of the filter stages 12 have different filter characteristics, as will be described. This results in a staged filtration process in which the filtration being effected on the fluid passing through the filter element 10 ₁ changes at different positions along the path 14.

In this description, the material to be filtered will be called "fluid" and may be liquid or gas. The material which is removed from the fluid will be called the "filtrand". The clean fluid, after removal of the filtrand, will be called the "filtrate".

Structure of a filter stage

Fig. 2 illustrates one of the filter stages 12. The filter stage 12 is in the form of a generally planar disc-like filter structure, having a generally circular periphery 16 and a central aperture 18. In this example, the periphery 16 and the aperture 18 are substantially concentric. The fluid path 14 is shown crossing the plane in one direction through the structure, and in the opposite direction through the aperture 18. Other geometries could be used. The periphery 16 could have another shape. The aperture 18 could have another shape. The aperture 18 could be omitted.

The structure 12 may be one or more layers of filter material such as a woven or non-woven filter cloth. The filter material will have a filter characteristic which represents the ability of the material to remove filtrand from fluid. Filter characteristics may be described in various different ways. In one possibility, the filter characteristic is expressed as a micron rating representing the maximum particle size which can pass through the material. For example, a filter material with a 3 micron rating will pass particles smaller than 3 microns, but will block particles with a size of 3 microns or greater. A micron rating may also be called a "pore size", referring to the size of pore in a perforated structure which allows the same particle size to be passed. However, the pore size or micron rating may represent a measurement from an equivalent structure providing the same filter characteristic, rather than an actual measurement of a perforation, because many techniques of filtering particles are available in addition to simple perforated sheets, meshes or the like.

Filter materials with many different micron ratings are available. For example, materials with micron ratings as high as 100 microns are available for filtering fluids.

Filter characteristics which refer to particle sizes represent filter materials which remove the same filtrand (particulates) but more coarsely or more finely. Other examples of filter characteristics exist. For example, different filter materials may have different characteristics representing the different filtrands which they are capable of removing from a fluid. Thus, one type of filter material may be able to remove particulates, while another type of filter material may be able to remove water from an oil. In this document, references to "different filter characteristics" are intended to encompass any change in characteristic which results in a change in the filtrand or filtrate produced.

Structure of the filter element

Returning to Fig. 1 , the element 10 is formed from a stack of planar filter stages 12 arranged at generally parallel planes to form a cylindrical structure, as shown. The apertures 18 of the stages 12 are arranged to be aligned and are therefore centred on the cylindrical access of the structure. This results in a fluid path leg 15 available through all of the apertures 18, without filtration, and a fluid path leg 14 which returns back along the cylinder and sequentially through the filter material of each of the stages 12, in turn. Impervious outer surfaces (shown partially cut away in Fig. 1) may be provided around the peripheries 16 and around the apertures 18 to prevent fluid leaking from the path leg 15 into the stages 12, or leaving the stages 12 before passing along the whole of the path leg 14.

Fluid passing along the path leg 14 will undergo a filter process which depends on the filter characteristics of the various filter stages 12. At least some of the filter stages have different filter characteristics. For example, the fluid may first encounter at least one relatively coarse (relatively high micron rating) filter stage 12. This would remove relatively large particulate matter from the fluid. The fluid may subsequently encounter at least one relatively fine (relatively low micron rating) filter stage. This would remove relatively small particulate matter from the fluid. In a practical example, the use of a large number of stages 12 allows the micron rating of the filter stages 12 to reduce progressively along the leg 14. In one example, the micron rating of the stages 12 may reduce in the sequence: 100 microns; 50 microns; 25 microns; 10 microns; 5 microns and finally 3 microns. Using filter characteristics which are increasingly fine along the fluid path will result in particles of various different sizes being captured in various different stages 12, at various different positions along the length of the path leg

14.

In other examples, the various stages 12 may be chosen to have different filter characteristics in relation to the filtrands they are capable of removing, so that various different filtrands are captured in various different stages 12, at various different positions along the length of the path leg 14.

The filter characteristics of the filter stages 12 may change between each filter stage 12 and the next filter stage 12 along the fluid path. Alternatively, the filter stages 12 may be arranged in groups with substantially the same filter characteristics, and with neighbouring groups along the fluid path having different filter characteristics.

Typical filter materials for a wide range of common filter tasks may have a thickness in the region of 1 mm or 2 mm, allowing an element to be constructed with as many as 60 stages within a relatively small volume. This allows the filtration process to take place in a graduated set of 60 steps, or in a smaller number of steps provided by groups of filter stages.

Structure of a filter using the filter element

The filter element 10 is shown in Fig. 3 incorporated in a filter 20. The filter 20 comprises a housing 22 having an inlet 24 and outlet 26. A lid 27 (shown almost completely cut away in Fig. 3) closes the housing 22 so that the housing 22 is substantially fully enclosed except at the inlet 24 and outlet 26.

Fluid path means in the form of a central pipe 28 define a fluid path from the inlet 24 to the outlet 26, in the following manner. The housing 22 is cylindrical and the pipe 28 is concentric with the housing. The filter element 10 is installed in the housing 22 by placing it around the pipe 28, that is, by placing the apertures 18 over the pipe 28. Fluid which enters the inlet 24 passes along the length of the pipe 28, to the other end of the filter element 10. The fluid can then return to the first end of the filter element 10 by passing through the filter stages 12 and then leaving the housing 22 through the outlet 26. This completes the fluid path from the inlet 24 to the outlet 26 and it is apparent that the fluid path through the fluid filter element 10 forms part of the fluid path from the inlet 24 to the outlet 26, so that the fluid entering the inlet 24 is filtered before leaving the outlet 26, leaving filtrate flowing from the outlet 26, and filtrand captured within the filter stages 12. Fluid is prevented from passing between the periphery of the filter element 10 and housing 22, by appropriate seal 30.

Many other geometries could be used for creating a filter path which passes sequentially through a plurality of filter stages with different filter characteristics. Another example (not illustrated) uses filter stages similar to those of Fig. 2, but with no central aperture. These can be arranged in a housing similar to the housing 22, but without a central pipe 28, and with the inlet and outlet at opposite ends of the housing, so that the fluid path through the filter is provided by the fluid path 14, sequentially through the filter stages 12. In this example, the fluid flow direction remains generally the same (as compared with the example of Fig. 3, in which the direction reverses from 15 to 14). This example filter can conveniently be used for in-line filtering applications.

The geometries of the housing, filter stages and other components can be varied widely in order to change the geometry of the fluid path, and the outside geometry of the filter, including its size, and the number and locations of its inlet(s) and outlet(s), location of access for maintenance etc.

The use of filter stages 12 with different filter characteristics, as has been described, results in a staged filtration effect as fluid moves through the filter element 10, in which different filtrands are removed at different stages of the process and therefore at different positions within the filter element 10. This is expected to have a number of possible advantages. For example, when particulate filtrands of various different sizes are being removed, the use of increasingly fine filtering results in the first filter stages removing only the relatively large particulate filtrands, while relatively small particulate filtrands pass through to be removed by a later stage. This is expected to reduce problems of filter stages clogging. For example, the first filter stages are unlikely to clog with fine filtrands, which pass through, and the large filtrands will not be present to clog later stages which have finer ratings.

We have found that this staged filtration effect can result in a reduced pressure drop across the filter element and/or an increased flow rate through the filter element and/or an extended filter life.

Partition members

Figs 4 and 5 illustrate an alternative example in which partition members 44 (Fig. 4) are included in the filter element 10a.

Each partition member 44 is a planar disc made of an impervious material which is dimensionally stable, such as a synthetic plastics material. Regions 46 of the disc 44 are rendered pervious by the provision, in this example, of apertures in the material of the disc 44. The apertures 46 leave a continuous ring 48 around the outer circumference of the disc 44. A central aperture 50 is surrounded by a continuous inner ring 52. In this example, the rings 48, 52 are connected by spokes 54.

In the filter element 10a, several discs 44 are included in the stack of filter stages 12. The outer diameter of the discs 44 is approximately the same as the initial diameter of the filter stages 12. The diameter of the central aperture 50 is approximately the same as the initial diameter of the central aperture 18 in the filter stages 12. Accordingly, the discs 44 can be included in the stack of filter stages 12 by placing each disc 44 between two adjacent filter stages 12.

In this example, an inner cylinder 56 and an outer cylinder 58 resist fluid leaving the filter element 10a unless it has passed through the filter stages 12 by flowing through the filter stages 12 in a direction generally parallel with the cylindrical axis of the filter element 10a. In this example, the filter stages 12 are not sealed to the cylinders 56, 58. This reduces the risk of damage to the filter stages 12 if they shrink during use, which some filter media may do. Shrinkage of the filter stages 12 would open gaps between the filter stages 12 and one or both of the cylinders 56, 58, potentially leaving open a path through which fluid can bypass the filter stages 12. Gaps between the filter stages 12 and the cylinders 56, 58 could also arise for other reasons, such as production tolerances, inaccurate cutting, sizing or shaping of the material, or the like. However, the partition members 44 are dimensionally stable and have dimensions chosen to be a close fit to the cylinders 56, 58. Consequently, even if the filter stages 12 leave gaps, the gaps between the partition members 44 and the cylinders 56, 58 will remain small. The path of least resistance for fluid flowing through the filter element 10, across a partition member 44, will remain the paths through the apertures 46. Thus, each time the fluid encounters a partition member 44, it will be encouraged back to the main body of the filter stages 12, away from their edges, by virtue of the position of the apertures 46.

This is expected to increase the reliability of the filter element 10 by resisting fluid from flowing around the edges of the filter stages 12 even with wider manufacturing tolerances, and may increase the service life of the filter element 10, by continuing to resist this fluid flow even when ageing filter stages 12 begin to shrink. Design of the filter element

The elements and principles which have been described above can be used in the design of a filter element for providing fluid filtration for a target fluid, in a process illustrated simply in Fig. 4.

In a first step 32, a filter element 10 of the type described above is provided as a test element. Target fluid is passed through the test filter element 10 at step 34. This results in the target fluid being filtered by the filter stages 12, removing filtrand as has been described. The fluid is then stopped at step 36. The test filter element 10 is removed (step 38) and the captured filtrand is analysed (step 40). In particular, the filter stage or stages in which capture has occurred is identified for each filtrand (each size or range of sizes of particulate filtrand, or each filtrand material, for example).

The filter characteristics of the various filter stages 12 can then be modified in accordance with the outcome of the analysis. In particular, they can be modified (step 42) to cause filtrand to be captured more evenly across all of the filter stages. For example, if the analysis step 40 reveals that filtrand has been captured primarily in the early filter stages, the early filter stages can be modified (e.g. by replacement by a different filter material) to be more coarse so that more filtrand will be captured in later filter stages. If desired, the modified arrangement can then be tested further by reverting to step 32. This provides an ability to tune the filter element 10 to the particular task required.

Many variations and modifications can be made to the apparatus and methods described above, without departing from the scope of the invention. In particular, many different shapes, sizes, relative shapes and relative sizes of the various components could be used. Other geometries could be chosen for the filter stages, filter elements and flow paths. Many different materials could be used for filtration purposes according to the nature of the filtrand to be removed. Examples include materials for removing particulate filtrands, materials for removing water or otherwise separating different fluids, or materials for neutralising or otherwise treating chemical components in the filtrate.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.