GB2415647A

GB2415647A - A magnetic separator

Info

Publication number: GB2415647A
Application number: GB0414468A
Authority: GB
Inventors: Nicholas Mark Alford
Original assignee: Individual
Current assignee: Individual
Priority date: 2004-06-29
Filing date: 2004-06-29
Publication date: 2006-01-04
Also published as: GB0414468D0

Abstract

A magnetic separator comprises a housing 10 which defines a fluid flow chamber 13 having a fluid inlet 20 and fluid outlet 30. A tube 60 is positioned within the chamber and an array 50 of magnets is positioned within the tube 60. Magnets 51 in the array 50 are separated by pole pieces 52A, 52B, 52C of different lengths. Each length of pole piece causes a magnetic field of a different size/density. The magnetic field is larger, and lower density, nearest the fluid inlet 20 to attract large material from the fluid flow. Flow guides 171, 172, 173 guide fluid through the chamber 13, causing the fluid to pass between the tubes on a plurality of occasions. Material is removed from the fluid flow in a graded manner.

Description

241 5647 A Maunetic Separator This invention relates to magnetic

separation of material from a fluid flow.

Magnetic separators are used in a variety of industries to remove unwanted materials from a fluid flow. A fluid flow is introduced to a chamber via a fluid inlet and passed along a flow path. One or more tubes are positioned within the flow path, with magnets positioned inside the tubes. In use, the magnets serve to attract magnetic l 0 material in the fluid flow. Periodically, attracted debris is removed from the outside of the tubes during a purging process.

Magnetic separators are used in food processing applications to remove any unwanted magnetic debris from a flow of foodstuff. Magnetic separators are also used to cleanse coolant used in machining applications, with the separator attracting magnetic material that has become entrained in the coolant flow over a machine or The tube within a magnetic separator can house an array of magnets which are separated by pole pieces as shown, for example, in GB 2,390,315A. The pole pieces are of uniform length. It has been found that a separator of this type can become saturated with accumulated material within a relatively short period of time, and is not able to attract the smallest particles in the fluid flow.

It is desirable to remove as much material as possible from the fluid flow during a single pass through the separator. It is also desirable to maximise the amount of time between the occasions when the separator is purged of collected material.

The present invention seeks to provide an improved magnetic separator.

Accordingly, a first aspect of the present invention provides a magnetic separator for separating material from a fluid flow comprising: a housing which defines a fluid flow chamber, the chamber having a dirty fluid inlet for receiving dirty fluid and a cleaned fluid outlet for emitting cleaned fluid; a tube positioned within the chamber; and, a linear array of magnets positioned within the tube, with pole pieces separating the magnets, wherein the length of the pole pieces is non-uniform along the length of the array.

Providing pole pieces of different lengths has the effect of providing a different attraction profile at each of the pole pieces along the array. The longest pole pieces have a surface area which is large enough to hold captive large quantities of material but lack the field strength to entrap the smallest (e.g. micron) size particles. The shortest pole pieces have the ability to entrap the smallest particles but quickly become saturated with large particles due to their small surface area.

Preferably, the longest pole piece is positioned at the end of the array nearest the fluid inlet and the length of the pole pieces decreases in the direction towards the end of the array nearest the fluid outlet. In this manner the largest material is attracted first, as the large material is attracted by the relatively low density magnetic field. The large pole piece 'spreads' the flux across a large area and increases capacity of the array in this region. As the fluid flow progresses along the chamber progressively smaller material is attracted to the tube by the progressively higher density (more concentrated) magnetic field. As large material has already been removed from the fluid flow, the higher density (and shorter) sections of the array can attract the smaller unwanted material without risk of becoming clogged by the large material.

Preferably, the fluid flow is constrained to pass over the tube in the direction in which the pole pieces decrease in size. In a separator having a single tube and array, this can be achieved by a small clearance between the wall of the tube and the inner wall of the chamber. In a separator having multiple tubes, this can be achieved by guiding the flow of fluid through the chamber such that it is brought close to the tube on a plurality of occasions during the passage along the chamber. This can be achieved by flow guides, such as bulkheads, which are placed within the chamber.

The bulkheads also serve to shield the shorter pole pieces from the fluid flow arriving at the dirty fluid inlet. The only fluid reaching the part of the tube housing the shorter pole pieces will have first flowed over the parts of the tube housing the larger pole pieces. The flow guide can be inclined with respect to the longitudinal axis of the chamber or perpendicular to the chamber.

The magnetic separator can be purged of collected material during a purging cycle which is either 'wet' or 'dry'. In a wet purging process the array of magnets is moved along the chamber or withdrawn completely and the chamber is flushed using dirt-laden fluid. In a dry purging process the chamber is emptied of dirt-laden fluid and air is blown through the chamber, or the chamber is pressurised to dry accumulated material and the dried material is then expelled.

The separator can be scaled as required. There may be only a single tube within the chamber or a plurality of tubes positioned within the chamber, each having a magnet or array of magnets positioned within them.

The separator can be used as a primary filtering stage which receives contaminated fluid or as a secondary filtering stage which receives the fluid effluent from an upstream separation or filtration stage.

Another aspect of the invention provides a magnetic separator for separating material from a fluid flow comprising a plurality of separating stages through which a fluid passes sequentially, each separating stage comprising: a housing which defines a fluid flow chamber, the chamber having a dirty fluid inlet for receiving dirty fluid and a cleaned fluid outlet for emitting cleaned fluid; a tube positioned within the chamber; and, a linear array of magnets positioned within the tube with pole pieces separating the magnets, wherein the length of the pole pieces in the total set of arrays is non-uniform.

Preferably the length of the pole pieces decreases in the direction of flow through the separating stages. The pole pieces within each separating stage can be of uniform size, or they can be of non-uniform size, with the pole pieces decreasing in size in the direction towards the cleaned fluid outlet of each separating stage. Each separating stage can comprise one or more of the preferred features of the first aspect of the invention.

A further aspect of the invention provides a method of separating material from a fluid flow using any of the above magnetic separators.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure I shows a side view of a first embodiment of a magnetic separator; Figure 2 shows a magnet array for use in the separator of Figure]; Figures 3A and 3B show cross-sections through a tube of the separator of Figure 1; Figure 4 shows a second embodiment of the magnetic separator; Figure S shows a cross-section through the separator of Figure 4; Figure 6 shows a third embodiment of the magnetic separator; and, Figure 7 shows a fourth embodiment of the magnetic separator in which there are multiple separating stages.

Figure I shows a first embodiment of a magnetic separator. A cylindrical housing 10 defines a fluid flow chamber 13 with an inlet 20 and an outlet 30. The inlet is directed perpendicularly or tangentially to the longitudinal axis 1 S of the housing and serves to encourage fluid flow to swirl about the longitudinal axis as it passes from the inlet 20 to the outlet 30 along flow path A-D. A valve 22 is positioned upstream of the inlet 20. The valve is movable between port 23, through which dirty fluid is received, and port 24 through which compressed air can be received. This will be described more fully below. A further valve 32 is positioned downstream of the outlet and is movable between three positions: port 33 through which cleaned fluid is emitted, port 34 which forms a waste path and an off position. The housing 10 has a tube 60 mounted within it, aligned with the longitudinal axis 15. Mounted within tube is a linear array of magnets shown generally as 50. The linear array of magnets is a sliding fit within the tube 60 and is movable along the tube by application of air pressure to ports 61, 62 at each end of the tube 60. Tube 60 is preferably a thin-walled stainless tube and the magnets can be neodymium magnets. A controller 90 controls operation of valves 22, 32 and valves which apply air pressure to ports 61, 62 at each end of tube 60.

A bulkhead 40 surrounds the tube 60 at a position adjacent the outlet 30. The bulkhead 40 extends fully across the housing, sealing against the inner wall of the housing 10 and against the outer wall of tube 60 with a fluid-tight seal, thus forming one end of chamber 13. The bulkhead 40 is inclined with respect to the longitudinal axis 1 S of the tube 60 and housing 10, with a lowermost side 41 aligned with the base of the outlet 30 and an uppermost side offset in the upstream direction. Bulkhead 40 is not aligned to the field generated by the pole pieces of the magnet array 50. An inclined bulkhead 40 reduces the force required to 'strip' attracted material from the outside of tube 60 and also serves to direct the material towards the outlet 30. Housing 10 and tube 60 have a total length which is at least twice the length of the magnet array 50. Bulkhead 40 divides the housing 10 into two parts 11, 12. The part of the tube 60 lying within part 11 of the housing is long enough to receive the magnet array 50 such that it is fully exposed to the fluid flow. The part of the tube 60 lying within part 12 of the housing is at least as long as the magnetic array 50 such that the magnet array can be fully withdrawn out of the fluid flow path. Part 12 of the housing includes a magnetic cloak 14, such as Ni Fe material, which surrounds the tube 60. Bulkhead 40 also incorporates shielding material 45 which serves to shield the outlet 30 from magnet array 50, when the magnet array is withdrawn into part 12 of tube 60. A separator of the above type is described more fully in co-pending UK Patent Application GB0400308.3.

Figure 2 shows the magnet array 50 in more detail. The array 50 is movable within the tube 60. The array 50 comprises a plurality of annular magnets 51 and pole pieces 52A, 52B, 52C which are threaded onto a rod 55 and secured at each end by a nut 56. A sealing O-ring 54 at each end is mounted around an annular end piece 57.

The magnets 51 are arranged such that similar poles face one another along the array.

Pole pieces 52A, 52B, 52C, which are formed of a ferrous material such as steel, fit between each pair of poles. Assuming that all of the magnets 51 in the array are of the same strength, it is the length of pole pieces 52A, 52B, 52C which determine the density of the magnetic field at each point along the array. The field density is inversely proportional to the length of the pole piece, with a short pole piece 52C providing a dense magnetic field 53C and a long pole piece 52A providing a less dense magnetic field 53A. The pole pieces 52A, 52B, 52C decrease in length from one end of the array to the other. In Figure 2 three discrete lengths of pole piece are shown: lOmm (0.5 Tesla pole), 5mm (0.75 Tesla pole) and 2. 5mm (1.0 Tesla pole). Each length of pole piece serves to attract particles within a particular range of sizes, with the lOmm pole piece attracting particles in the range 20-100 microns, the 5mm pole piece attracting particles in the range 5-20 microns, and the 2.5mm pole piece attracting particles in the range <5 microns. It will be appreciated that the field strength can be varied to a desired value by suitable selection of the pole piece length and magnet strength.

The array 50 is arranged within tube 60 such that the longest pole pieces 52A (less dense field 53A) are positioned closest to the dirty fluid inlet 20 and the shortest pole pieces 52C (strongest field 53C) are positioned closest to the cleaned fluid outlet 30. By arranging the pole pieces in this manner, it can seen that the largest material will be removed first, leaving the shorter pole pieces (more dense field) to attract the smaller material. As the large material has been removed in the first section of the lo chamber, the shorter (denser field) pole pieces will not become clogged with large material.

A further feature of the separator is a web 80 of non-ferrous material which extends radially outwardly from the wall of tube 60. The nonferrous material 80 can be a material such as Nylon, PVC or high density polypropylene (HDPP). It is preferred that web 80 is bonded to the outer wall of tube 60. Tube 60 has a thin wall to ensure the field produced by the magnet array 50 is as strong as possible. The purpose of web 80 is best shown in Figures 3A and 3B. Figure 3A shows a cross-sectional view through a conventional tube 60 without a web 80. In use, material 86 accumulates on the outside of tube 60. When a ring of material 86 has accumulated around the entire periphery of tube 60 this conducts magnetic flux and has the effect of shielding fluid flow in chamber 13 from the effects of the magnet array 50 within tube 60. Accordingly, the effect of the separator is diminished. In Figure 3B the web 80 extends radially outwardly from the wall of tube 60 and has the effect of preventing a ring of material accumulating around tube 60. Thus, the magnet array 50 within tube 60 continues to attract material from the fluid flow through chamber 13.

The separator is arranged to collect material from a fluid flow during a first, collecting, phased of operation and to purge the separator of collected material during a second, purging phase of operation. Flow is constrained by the outer wall of housing 10, ensuring that all flow is within the magnetic field exerted by magnet array 50.

Cleaned fluid leaves the chamber via outlet 30. During this state, a thick bed of magnetic and paramagnetic particles accumulates on the outer wall of tube 60 and these also entrap non-magnetic particles.

After a period of time, a point is reached where no further particles may be captured. This can be determined by a predetermined elapsed period of time or by other suitable means. One method of purging the separator of accumulated material is as follows. Compressed air is fed to the chamber 13. This air displaces the fluid in the chamber 13 until all fluid has been displaced through the outlet 30. This leaves the chamber 13 free of liquid and the rise in pressure to achieve this has the effect of squeezing moisture from material which has accumulated around tube 60. A valve at the outlet 30 is closed and the pressure within chamber 13 rises. After a period of time, which can be determined by measuring elapsed time, compressed air pressure/flow, or by using a pressure switch, the pressure within chamber 13 reaches the pressure of the compressed air supply, which is typically 5-7 Bar. The magnet array 50 is then moved along tube 60, from the position shown in Figure 1 to a position where magnet array 50 is located in the lower section 12 of the separator. In moving between these positions, the magnet array passes through the bulkhead 40 to ]5 the 'cloaked' position in portion 12 of tube 60. As the magnets move through the bulkhead, each magnet pole piece is stripped of its contaminant load, one pole piece at a time. The magnet array is moved along tube 60 by applying positive air pressure to port 61. Once magnet array 50 has reached the cloaked position within part 12 of tube the valve is opened to connect inlet 20 to waste outlet port. The debris which has been stripped from the tube 60 is ejected under chamber pressure to waste. The effect is similar to that of an airliner depressurising. Compressed air continues to be applied to port 24 to ensure complete removal of the waste.

The magnet array 50 returns to part 11 of tube 60 within chamber 13 by applying positive pressure to port 62. Any debris not ejected are retained on the surface of tube 60 and will be ejected in the next purge sequence.

In Figure 1 the separator has only one tube 60 and is suitable for smallscale applications. Fluid flow is constrained to pass close to the tube 60 along the entire length of the chamber 10 by virtue of the small clearance between the tube 60 and the wall of chamber 10. Figure 4 shows a further embodiment of the separator. The main differences are that the housing includes multiple tubes 160, arranged in a circular array (best shown in Figure 5) about the longitudinal axis 115 and the fluid inlet 120 and fluid/waste outlet conduit 130 are aligned with the longitudinal axis 115 of the housing 110. Each of the tubes 160 have the same form as previously described.

While the provision of multiple tubes 160 increases the capacity of the separator, the fluid flow is no longer constrained to pass close to the tubes 160. A further modification to the separator is the addition of three flow guides or bulkheads 171, 172, 173 which serve to cause fluid flow to pass around tubes 160 on multiple occasions as the fluid flow passes along chamber 110. Viewed together, the bulkheads 171, 172, 173 define a labyrinth which causes fluid flow to repeatedly pass between and around the tubes 160. Fluid flow through the separator will now be described.

Dirty fluid enters the separator via inlet 120 at A. Bulkhead 171 directs fluid on a radially outward path. A gap at B between bulkhead 171 and the wall of chamber 110 allows fluid to pass between the tubes 160. Bulkhead 172 directs fluid on a radially inward path C which causes the fluid to pass around tubes 160. Bulkhead 173 directs fluid on a radially outward path once more, with fluid passing D between bulkheads 172, 173. The fluid flows between tubes 160 again before passing through a gap E (185, Figure 5) between bulkhead 173 and the wall of chamber 110. The fluid must once again pass between tubes 160 before flowing F towards the outlet 130 of the chamber 113 and along the outlet passage G. The inclined bulkheads 171, 172, 173 shown in Figure 4 are advantageous during a dry purging cycle, as stripped material can be carried from the chamber by airflow through the chamber 113.

The separator shown in Figure 4 also has a web 180 of the type previously described in relation to Figure 1. Figure 5 shows the array of tubes 160, each with their respective web 180. It is preferred to mount the web on to the outermost face of tubes 160 as this aids assembly of the separator. In this embodiment the web 180 is formed in three separate sections 180A, 180B, 180C, with each section fitting between a respective pair of bulkheads. The webs are mechanically fastened to the bulkheads.

Figure 5 also shows the close spacing of the circular array of tubes 160. Each time fluid flow passes from the outside of the chamber 113 to the inside, it is forced between the tubes 160 and thus within a controlled distance of the tubes 160.

Figure 6 shows a further embodiment of separator. This embodiment is a 'wet purge' separator which is intended to be continuously supplied with dirty fluid. In a normal, separating mode of operation, the magnet arrays 150 are housed in the lower end of tubes 160. Dirty fluid flows along path A-G in a similar manner to that previously described. During a purging mode of operation the magnet arrays are moved to the upper end of tubes 160. As the magnet arrays 150 are moved, accumulated material is removed from the outside of tubes 160. As dirty fluid continues to be applied to the separator, the material removed from the tubes 160 is washed towards outlet 130. This embodiment also includes bulkheads 174, 175, 176 which act as flow guides in the same manner as those shown in Figure 4. Although inclined bulkheads could be used in this embodiment, they are not as important as the washing effect of the fluid flow removes material during the purging operation.

Bulkheads 174, 175, 176 are planar. Bulkheads 174, 176 are circular discs with a diameter smaller than that of the housing 110. Each disc has a set of holes to accommodate the tubes 160. Bulkhead 175 is an annular plate with an outer diameter the same as that of the housing 110. The plate has a similar set of holes to accommodate the tubes 160. A central hole in bulkhead 175 allows fluid to pass through, and forms the only passageway at that point within the chamber 113.

In the above embodiments the pole pieces of an array 50, 150 within a single chamber 13, 113 decrease in size along the array. The same principle can be applied to multiple separating chambers. Figure 7 shows a set of separators 210, 220, 230 arranged sequentially. Each separator 210, 220, 230 has the same general form as the separators previously described. The direction of flow through the separators 210, 220, 230 is shown by arrow 250. Dirty fluid is applied to inlet 211 of the first separator 210. Fluid from the outlet of separator 210 is applied to the inlet of separator 220, and fluid from the outlet of separator 220 is applied to the inlet of separator 230.

Cleaned fluid emerges from outlet 235 of separator 230. Each separator 210, 220, 230 includes one or multiple arrays 50, 150 of magnets. The pole pieces decrease in length in the general direction 250. In a first variant, the pole pieces within each separator 210, 220, 230 are of uniform length, with separator 210 having longer pole pieces than separator 220, and separator 220 having longer pole pieces than separator 230. In a second variant, the pole pieces within each separator decrease in size in the direction 250. Thus, the pole pieces within separator 210 have a first range of sizes, the pole pieces within separator 220 have a second range of sizes (smaller than in separator 210), and the pole pieces within separator 230 have a third range of sizes (smaller than in either of separators 210, 220.) Bulkheads are employed in each unit to separate the different pole sizes and flow sequential over the different pole pieces. This principle can be applied to any number of separators. Each separator 210, 220, 230 can be individually purged (back-washed) of accumulated material using ports 212, 222, 232.

In the above embodiment the separator is shown in a vertical orientation.

However, the separator can be used in other orientations, such as a horizontal orientation or some orientation between horizontal and vertical. It is preferable that the orientation, together with the position of the bulkhead 40 allows gravity to aid the movement of removed material towards the outlet. Thus, a vertical orientation, together with an outlet near the base of the separator, or a horizontal orientation with an outlet directly below the bulkhead 40 is preferred.

In the embodiment shown in Figure 1 the inlet 20 through which the working fluid is received is also used as the inlet for compressed air during the purging process.

Similarly, the outlet 30 through which the cleaned working fluid is emitted is also used as a waste outlet. However, it is possible to use either the inlet 20 or the outlet 30 as an inlet for compressed air. Thus, air can be injected via the fluid outlet 30 and waste expelled through the fluid inlet 20, with the bulkhead 40 positioned adjacent the fluid inlet 20.

In order to maintain continuous flow during the period when purging occurs, a number of units may be employed on a manifold, with each unit being purged at a different time.

The functionality of the controller for controlling operation of the valves can be implemented entirely in hardware or as software which is executed by a processor, as will be well understood by a skilled person.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The words "comprising" and "including" do not exclude the presence of other elements or steps than those listed in the claim. Where the system/device/apparatus claims recite several means, several of these means can be embodied by one and the same item of hardware.

Claims

Claims 1. A magnetic separator for separating material from a fluid flow

comprising: a housing which defines a fluid flow chamber, the chamber having a dirty fluid inlet for receiving dirty fluid and a cleaned fluid outlet for emitting cleaned fluid; a tube positioned within the chamber; and, a linear array of magnets positioned within the tube with pole pieces separating the magnets, wherein the length of the pole pieces is non-uniform along the length of the array.
2. A magnetic separator according to claim I wherein the length of the pole pieces decreases in the direction towards the end of the array nearest the cleaned fluid outlet.
3. A magnetic separator according to claim 2 wherein the fluid flow is constrained to pass over the tube in the direction in which the pole pieces decrease in size.
4. A magnetic separator according to claim 3 wherein the chamber is substantially cylindrical and wherein the tube and the linear array of magnets are substantially aligned with the longitudinal axis of the chamber.
S. A magnetic separator according to claim 3 wherein there are multiple tubes within the fluid flow chamber, the separator further comprising at least one flow guide positioned within the chamber which cause the fluid flow to flow between the tubes on a plurality of occasions during the passage of the fluid flow along the chamber.
6. A magnetic separator according to claim 5 wherein the flow guide comprises a bulkhead which causes the fluid flow to flow in a radial direction, the bulkhead extending between the longitudinal axis of the chamber and a position radially inwardly of the inner wall of the chamber, or between the inner wall and a position radially short of the longitudinal axis of the chamber.
7. A magnetic separator according to claim 5 or 6 wherein at least one of the flow guides is inclined with respect to the longitudinal axis of the chamber.
8. A magnetic separator according to any one of claims 5 to 7 wherein the pole pieces have a plurality of distinct lengths, with each distinct length occurring between adjacent pairs of flow guides.
9. A magnetic separator according to any one of claims 5 to 8 wherein the tubes are arranged in a circular array, with the flow guides causing the fluid flow, in use, to pass between the tubes in the array on a plurality of occasions.
10. A magnetic separator according to any one of the preceding claims wherein a web extends radially outwardly from the tube along at least part of the length of the tube, the web being formed from non-ferrous material.
11. A magnetic separator according to claim 10 wherein the web extends radially outwardly at least for a distance which is substantially equal to the depth of material expected to accumulate on the tube.
12. A magnetic separator according to any one of the preceding claims wherein the array is movable along the tube between a separating position.
13. A magnetic separator according to any one of the preceding claims wherein the array is movable along the tube in response to differential pressure across the array.
14. A linear array of magnets for use in a magnetic separator according to any one of the preceding claims.
15. A magnetic separator for separating material from a fluid flow comprising a plurality of separating stages through which a fluid passes sequentially, each separating stage comprising: a housing which defines a fluid flow chamber, the chamber having a dirty fluid inlet for receiving dirty fluid and a cleaned fluid outlet for emitting cleaned fluid; a tube positioned within the chamber; and, a linear array of magnets positioned within the tube with pole pieces separating the magnets, wherein the length of the pole pieces in the total set of arrays is non-uniform.
16. A magnetic separator according to claim 15 wherein the length of the pole pieces decreases in the direction of flow through the separating stages.
17. A magnetic separator according to claim 16 wherein the pole pieces are of uniform size within each separating stage.
18. A magnetic separator according to claim 16 wherein the pole pieces within at least one of the separating stages decrease in size in the direction towards the cleaned fluid outlet.
19. A method of separating material from a fluid flow using a magnetic separator according to any one of the preceding claims.
20. A magnetic separator or a method of separating material substantially as described herein with reference to and as shown in the accompanying drawings.