WO2014174323A1

WO2014174323A1 - Filter and method of manufacture

Info

Publication number: WO2014174323A1
Application number: PCT/GB2014/051312
Authority: WO
Inventors: Andrew Mark Burns; Robert Neil Burns; Darren TRAVIS
Original assignee: Croft Filters Limited
Priority date: 2013-04-26
Filing date: 2014-04-25
Publication date: 2014-10-30
Also published as: GB2527996B; GB201307613D0; GB2527996A; GB201518612D0

Abstract

This invention relates to a filter and to a method of manufacturing the filter. The invention provides a two-part filter having a support portion and a filter portion, the support portion providing a substantially rigid support structure, the filter portion having pores of a predetermined size. The filter portion comprises a number of filter strands, the filter strands being integral with the support structure. The filter is made for example by an additive manufacturing process.

Description

FILTER AND METHOD OF MANUFACTURE FIELD OF THE INVENTION This invention relates to a filter and to a method of manufacturing the filter.

BACKGROUND TO THE INVENTION Filters (which may be referred to alternatively as "strainers") are in widespread use, for example to remove contaminants in the form of particulate matter from a fluid flow. The present invention is expected to find its greatest utility in the filtration of a liquid flowing along a pipe or other conduit and the following description will therefore relate to such an application. It will nevertheless be understood that the filter and method of manufacture are suitable for other filtration applications.

In industrial applications, filters are often used to remove solid particulate matter from a liquid. The liquid may for example be a fuel in which case the solid particulate material should be removed prior to the combustion process. Alternatively, the fluid may be steam which has been generated to drive a turbine, the solid particulate matter being removed to protect the turbine from damage.

Filtration is typically undertaken as the fluid passes along a pipe or other conduit, the filter spanning the pipe so that all of the liquid or gas passes through the filter. The filter will have a defined pore size corresponding to the minimum size of the solid particles which it is designed to capture, the finer the filter the smaller the pore size and the smaller the particles of solid material which are captured. There is an inevitable pressure drop within the fluid as it passes through the filter. Energy is therefore consumed by the filtration process to produce the pressure head for the fluid upstream of the filter. The filter designer will typically seek to minimise the pressure drop and thereby minimise the energy usage. However, in general the smaller the pore size of the filter the greater the pressure drop across it, and so there is often a compromise between seeking to reduce the pore size so as to capture all or more of the contaminants, without leading to an unacceptably large pressure drop across the filter.

The filter will gradually become blocked by the captured solid matter during use. This results in fewer pores or pathways for the fluid through the filter, and increases the pressure drop across the filter. Most industrial processes will include means to measure (either directly or indirectly) the pressure drop across the filter, and when a predetermined threshold for the pressure drop is reached the filter is removed for cleaning.

It is desirable to increase the number of pores within the filter, i.e. to increase the area of the filter through which the fluid can pass. This results in an increased period for which a filter can be used before the threshold pressure drop is reached. To achieve this, the area of the filter will typically exceed the cross- sectional area of the pipe. Alternatively stated, the filter is typically not planar and mounted simply to span the conduit. Instead, the filter is typically of three- dimensional form and extends both along and across the pipe. Filters of frusto- conical form are therefore typical of industrial filtration applications. Our copending international patent application WO2012/120252 describes a filter having a typical frusto-conical form, and Fig.1 of the attached drawings reproduces a drawing from that earlier application. As shown in Fig.1 , the filter 10 has a base 12 which is typically solid and is designed to be clamped between adjacent sections of pipe, or otherwise be secured to the pipe. In this embodiment the base is circular so as to span a pipe of circular cross-section, but other shapes may be provided for non-circular pipes. The base 12 plays no part in the filtration process. The filter 10 has a conical wall 14 and a substantially planar end wall 16, both of which have pores 18 defining multiple pathways for the fluid through the filter. The size of the pores 18 defines the minimum size of solid particles which can be captured by the filter 10. The conical wall 14 and end wall 16 of the filter 10 comprise respective single sheets of material (e.g. metal) through which the pores 18 are formed. Since there is a minimum size of the pores 18 which can be formed in a sheet of material, filters such as that of Fig.1 are typically used for removing relatively large solid particles, i.e. they provide a relative coarse filter.

Finer filters are typically formed as a mat or mesh of thin strands (the term "strand" being used herein to refer to a wire, fibre, thread, filament or similar element), the strands being very closely spaced so that the pores between the strands are narrow. The pores may also be convoluted. It is a feature of such filters that they are usually not self-supporting, i.e. the mat will collapse or distort when fluid is passed through it. The likelihood of the fine filter mat distorting increases as the pores become blocked with captured solid material and the pressure drop across the filter increases. Any damage or distortion to the strands of the mat could lead to larger particles than desired being able to pass through the filter, and/or to an unacceptable increase in the pressure drop.

The distortion of a fine filter mat is typically prevented by mounting the mat upon a structural support which is sufficiently rigid and robust not to distort in use. The support necessarily engages the mat across the full area of the mat, and therefore spans the pipe or conduit. The support must also therefore have pores to permit the passage of the fluid, and may for example be of the form of the filter 10 of Fig.1 . Such a structure therefore provides a two-part filter, a support portion comprising the (rigid) support structure and a filter portion comprising the fine filter mat having the required pore size.

It is usually necessary to mount a two-part filter with the support portion downstream of the filter portion. This ensures that the passing fluid presses the fine filter mat against the support rather than forcing the mat away from the support. The tendency of the fine filter mat to become distorted is thereby resisted by the support. Locating the support downstream of the fine filter mat effectively avoids the support carrying out any filtering action, i.e. any solid matter of a size to be captured by the support will already have been captured by the fine filter mat.

Whilst such an arrangement avoids the possible benefit of the support carrying out an initial coarse filtration, it is generally desirable that the fine filter mat capture all of the solid particles, regardless of their size. This ensures that the large solid particles in particular are captured at the upstream surface of the filter, from where they can readily be removed during subsequent cleaning of the filter. Accordingly, a two-part filter is defined herein as having a support portion and a filter portion, notwithstanding that the support portion may be able to carry out a coarse filtering action in certain applications. The term "filter portion" will therefore refer exclusively to the fine filter. It is a disadvantage of two-part filters that not all of the pores through the filter portion will be aligned with the pores through the support portion. Alternatively stated, at least some of the pores of the fine filter mat will inevitably overlie a solid part of the support structure, so that at least some of the pores of the filter portion are blocked by the support portion.

It is another disadvantage of two-part filters that the fine filter mat can move relative to the support structure. With very fine filters (having pores of a few microns for example), even very small relative movements can alter the cross- sectional area of some of the pores. The size range of solid particles which are captured by the two-part filter can therefore change, slightly but significantly, if the filter portion moves relative to the support portion.

Notwithstanding the references to filter in this description, it will be understood that a filter such as that of Fig.1 can be used for the dispersal or dissipation of a fluid, typically a liquid. For example, a filter such as that of Fig.1 can be used adjacent to the outlet of a domestic water tap to dissipate and aerate the water flow. Notwithstanding that little or no filtering action is undertaken, the dissipating element will nevertheless be referred to herein as a filter because of its shared structure.

SUMMARY OF THE INVENTION

The inventors have sought to provide a filter which avoids or reduces the above- stated disadvantages. According to the invention there is provided a two-part filter having a support portion and a filter portion, the filter portion having pores of a predetermined size, the filter portion comprising a number of strands, the strands being integral with the support portion. Alternatively stated, the filter portion and the support portion together comprise a continuous and complete structure, or at least interconnected or joined structures, the respective portions having different functions and characteristics. Specifically, in common with prior art two-part filters the strands of the filter portion alone may be unable to withstand the operational pressure during use. The support portion on the other hand is sufficiently rigid to provide the support necessary to maintain the form of the filter portion during use.

By joining or attaching each of the strands of the filter portion to the support portion, the likelihood of the strands of the filter portion moving relative to the support portion is reduced or eliminated. The strands of the filter portion are thereby retained in their desired positions, and the size, shape and position of all of the pores through the filter are maintained (at least until they become blocked by solid matter during use). Desirably, the support portion comprises a grid-like structure constructed from a number of support strands and the filter portion comprises a number of filter strands. Desirably also, each of the filter strands is connected to at least one of the support strands and/or to at least one other filter strand. The support strands can be linear, curved or convoluted as desired. Thus, depending upon the fluid and the solid matter to be captured it may be desirable to form the support portion from a large number of interconnected linear support strands, or from a number of support strands which are curved or bent around each other.

The filter strands can also be linear, curved or convoluted as desired. Preferably, each filter strand is connected to the support portion in at least two spaced locations. For example, in the case of linear filter strands, each filter strand can be connected adjacent to its respective ends to one or more of the support strands.

Preferably, each end of some or all of the filter strands is connected to another filter strand or to a support strand. Preferably also, each end of some or all of the support strands is connected to another support strand or to a filter strand. In this way, the filter portion and the support portion comprise a contiguous grid-like structure of strands with a large number of nodes defined by the junctions between the ends of many or all of the strands. In such an arrangement few if any of the strands terminates at anything other than a node and the ends of each strand are supported by at least one other strand. The likelihood of any projecting strand being bent or deformed and thereby affecting the local pore size is reduced or avoided. The material of the filter strands is ideally identical to the material of the support strands. The cross-sectional area of the support strands can be thicker than the cross-sectional area of the filter strands. In some embodiments at least some of the filter strands are continuations of respective support strands. The density of the support strands (in terms of the number of strands per unit volume) can be lower than the density of the filter strands, the cross-sectional area and density of the filter strands determining the pore size of the filter portion. Preferably, the filter portion is embedded within the support portion, i.e. the support portion spans the full thickness of the filter. The filter portion may also span the full thickness of the filter. There is also provided a method of making a filter as herein defined, the method comprising additive manufacturing. Additive manufacturing (sometimes called "additive fabrication", "additive process", "additive layer manufacturing", layer manufacturing" or "freeform fabrication") is a process for making three- dimensional solid objects from successive layers of material. Typically, a thin layer of powdered material is placed onto a build platform and fused into a structure of the desired form by a laser. A subsequent layer of powder is then added and the process repeated to build up the three-dimensional object. The powder may be a metal so that the resulting product is metallic. The described additive manufacturing process therefore has some similarities to 3-D printing, which is a method of manufacturing three-dimensional objects by forming a solid in multiple layers, the layers being formed by extruding a plastic material from a "print" head. In fact, for the purposes of this description, the term "additive manufacturing" will encompass 3-D printing, since it is possible to make a filter from a plastic material, although metallic filters are more typical.

When the filter is made by additive manufacturing the filter portion and the support portion necessarily comprise a unitary structure. The word "integral" as used herein is not, however, limited to such structures and will also embrace structures made by other manufacturing methods in which the filter portion is joined to the support portion to form a complete structure. Accordingly, the invention also embraces filters in which the filter portion and the support portion comprise component or constituent parts which are permanently joined together to form the structure of the filter. BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

Fig.1 shows a prior art filter;

Fig.2 shows a sectional view of a first embodiment of filter according to the present invention;

Fig.3 shows an enlarged view of part of the filter of Fig.2;

Fig.4 shows a sectional view of a second embodiment of filter according to the present invention;

Fig.5 shows an enlarged view of part of the filter of Fig.4;

Fig.6. shows a sectional view of a third embodiment of filter according to the present invention; and

Fig.7 shows an enlarged view of part of the filter of Fig.6.

DETAILED DESCRIPTION A description of the prior art filter of Fig.1 is set out above and will not be repeated.

Whilst the overall shape of the filter is not critical to the present invention, the embodiments of Figs. 2-5 share the frusto-conical form of the prior art filter of Fig.1 , as that is a typical shape for many of the filters which will be made according to the invention. The filter 1 10 of Figs 2 and 3 therefore shares the feature of a circular base 1 12, a conical wall 1 14 and a substantially planar end wall 1 16.

As better seen in Fig.3, the conical wall 1 14 (and similarly the substantially planar end wall 1 16) comprises a support portion 120 and a filter portion 122. The support portion is rigid (where "rigid" in the context of this application means that it is sufficiently strong to be both self-supporting and to maintain its shape when subjected to the forces involved during use). The support portion 120 comprises a grid-like structure of interconnected support strands 124, each of the support strands 124 in this embodiment being substantially linear. The support strands 124 are attached to one another at their junctions, so that the support strands 124 together provide a unitary grid or matrix. In embodiments using the method of additive manufacturing, the matrix of support strands is contiguous in that the support strands together form a continuous structure having multiple linear portions interconnected at multiple junctions.

Attached to (or mounted upon) the support strands are a large number of filter strands 126 which together form another grid-like structure of interconnected strands within the matrix of support strands 124. Since many of the support strands 124 span the filter 1 10 they provide a supporting function throughout the filter, and also provide a part of the filter portion. The filter strands 126 are arranged to be closer together than are the support strands 124, so that the pores between adjacent filter strands 126 (and also the pores between filter strands 126 and the adjacent support strands 124 within the filter portion 122) are small, and in particular small enough to prevent the passage of solid particles larger than a predetermined size.

A typical filter 1 10 may have a pore size of 50 microns for example, in which case the pores between the filter strands 126 are sufficiently small (and perhaps also sufficiently convoluted) to capture particles having a dimension greater than 50 microns. It is nevertheless expected that filters having a pore size significantly smaller than 50 microns can be made according to the invention (as can filters having a pore size significantly larger than 50 microns, if desired).

It will be seen from Fig.3 that the support strands 124 extend across the full thickness of the filter 1 10, i.e. the filter strands 126 are embedded within the matrix of support strands 124. In other (less preferable) embodiments the two parts of the filter could be more distinct, with the support strands being located in a first part of the filter and the filter strands being located in another part of the filter. Such an arrangement would more closely replicate the known two-part filters, but is not preferred since it is believed to be beneficial to embed the filter strands within the matrix of support strands so as to maximise the number of connections therebetween, and thereby maximise the structural support for the filter strands. The filter strands 126 are ideally of smaller cross-sectional area than the support strands 124. Such an arrangement takes advantage of the fact that the filter strands are attached to and supported within the matrix of support strands, and therefore do not need to be thick enough to be rigid on their own. Alternatively stated, the filter strands 126 do not need to be thick enough to be self-supporting when subjected to the forces imparted by a fluid flowing through the filter, either individually or when interconnected with other filter strands.

In the embodiment shown, each of the filter strands 124 is connected to one or more of the support strands 126 and/or to one or more other filter strands 124 in at least two separate locations. For example, the filter strand 126a is connected to the support strand 124a at the junction 130, to the support strand 124b at the junction 132, to the filter strand 126b at the junction 134, to the support strand 124c at the junction 136, and to the filter strand 126c at the junction 138. The second embodiment shown in Figs. 4 and 5 differs from the first embodiment only in the location of the filter strands 226 within the matrix of support strands 224. In this embodiment the filter strands 226 are located adjacent to the outside of the conical wall 214 and end wall 216 rather than the inside as in the first embodiment. The respective embodiments are therefore designed for different fluid flow directions F.

It will be understood that in other embodiments the filter portion could be embedded within the matrix of support strands with the support strands projecting to both sides of the filter portion. Regardless of the fluid flow direction, such a filter design would result in solid particles passing between support strands before being captured by the filter portion. The subsequent removal of those particles may be more problematic than with the first or second embodiments shown, so that such embodiments are unlikely to be widely used.

The third embodiment of Figs. 6 and 7 is similar to the first and second embodiments of Figs. 2-5 in that the filter 310 comprises a matrix of support strands 324 to which is connected a matrix of filter strands 326. Unlike the earlier embodiments, however, the ends of most of the strands 324, 326 are connected to other strands, the junctions of the ends forming nodes within the filter 310. Since there are few (or in some embodiments no) projecting strands the likelihood of damage or distortion of the strands is reduced, as is the likelihood of a pore becoming larger or smaller than intended.

It will be seen from Figs 6 and 7 that three different types of node are present within the filter. A first type of node 40 is formed between the respective ends of two filter strands 326 (a node 40 is shown in Fig.7). A second type of node 42 is formed between the respective ends of two support strands 324. A third type of node is formed between the respective ends of a support strand 324 and a filter strand 326 (though there are no such nodes visible in Figs. 6 and 7).

It will also be observed that in addition to the nodes which are formed at the junctions between the ends of respective strands, other interconnections or junctions between strands are present within the filter 310. Thus, an end of some of the strands is joined to a point between the ends of another strand (see for example the junction 44). Also, all of the strands are connected between their ends where they cross other strands (see for example the junction 46). Whilst Fig.6 shows the respective ends of all of the strands 324, 326 terminating at nodes, that is not necessarily the case, and in other embodiments some of the ends are disconnected (as is the case with the first and second embodiments of Figs. 2-5).

It will be understood that the difference between the pore size of the support portion (120) and the filter portion (122) can be significantly greater than that shown in the representations of Figs. 2-7. Whilst the figures show a filter 1 10, 210 in which the pore size of the filter portion is around half the pore size of the support portion (and a filter 310 in which the pore size of the filter portion is around one quarter the pore size of the support portion), in a typical filter the pore size of the support portion may be a factor of ten or more times greater than the pore size of the filter portion.

It is not necessary that the support strands, nor the filter strands, be linear as in the representations of Figs. 2-7, and the respective strands can be curved, looped (for example circular, oval, or polygonal), or convoluted (for example serpentine or of zig-zag form), as desired.

The method of additive manufacturing is ideally suited to making a filter of the form described, since the size, shape and location of every filter strand and every support strand can be predetermined. The location of every interconnection, junction or node between the respective strands can also be predetermined. It may be desired, for example, to provide strands, especially filter strands, having non-circular and/or non-uniform cross-sections, the shaping of the strands being somewhat dependent upon their location within the filter and being designed to minimise the turbulence of the fluid flowing through the filter. Whilst some turbulence is inevitable, it is recognised that reducing the turbulence will typically reduce the pressure drop across the filter. The large number of interconnections for each strand adds strength and rigidity to the filters 1 10, 210, 310, because each of the strands is supported by its interconnections to other strands at a number of spaced locations along its length. Whilst the filters 1 10, 210 and 310 each have a clearly defined filter portion embedded within the support portion, with the pore size changing significantly at the border of the filter portion, in other embodiments there may be a gradual change in the pore size across the filter, or the pore size may change in a plurality of discrete steps.

Alternatively, the embedded filter portion may extend across the full thickness of the support portion, with the small pores of the filter portion being present throughout the filter. In such alternative embodiments the separate support portion and filter portion are not clearly defined, but these embodiments are nevertheless two-part filters because of the different structure and characteristics of the material making up the support portion and the filter portion.

Claims

1 . A two-part filter having a support portion and a filter portion, the support portion providing a substantially rigid support structure, the filter portion having pores of a predetermined size, the filter portion comprising a number of filter strands, the filter strands being integral with the support structure.

2. The filter according to claim 1 in which each filter strand is one of: linear, curved and convoluted.

3. The filter according to claim 1 or claim 2 in which each filter strand is joined to the support structure and/or to at least one other filter strand.

4. The filter according to any one of claims 1 -3 in which each filter strand is joined to the support structure and/or to at least one other filter strand in at least two spaced locations.

5. The filter according to any one of claims 1 -4 in which each filter strand has a first end and a second end, and in which the first end of each filter strand is joined to the support structure or to another filter strand.

6. The filter according to claim 5 in which the second end of each filter strand is joined to the support structure or to another filter strand.

7. The filter according to any one of claims 1 -6 in which the support portion comprises a number of interconnected support strands.

8. The filter according to claim 7 in which each support strand is one of: linear, curved and convoluted.

9. The filter according to claim 7 or claim 8 in which each support strand has a first end and a second end, and in which the first end of each support strand is joined to another support strand or to a filter strand.

10. The filter according to claim 9 in which the second end of each support strand is joined to another support strand or to a filter strand.

1 1 . The filter according to any one of claims 7-10 in which the material of the filter strand(s) is identical to the material of the support strand(s).

12. The filter according to any one of claims 7-1 1 in which the cross-sectional area of the support strand(s) is larger than the cross-sectional area of the filter strand(s).

13. The filter according to any one of claims 1 -12 in which the filter portion is embedded within the support portion.

14. A method of making a filter according to any one of claims 1 -13 utilising additive manufacturing.