GB2409829A

GB2409829A - A magnetic separator

Info

Publication number: GB2409829A
Application number: GB0400308A
Authority: GB
Inventors: Nicholas Mark Alford
Original assignee: Individual
Current assignee: Individual
Priority date: 2004-01-08
Filing date: 2004-01-08
Publication date: 2005-07-13
Also published as: GB0400308D0

Abstract

A magnetic separator for separating material from a fluid flow comprises a housing 10 defining a chamber 13 having a fluid inlet 20 and a fluid outlet 30, a tube 60 positioned within the chamber, a magnet 50 positioned within the tube and a wiper 40 surrounding the tube wherein during a purging operation there is relative movement between the wiper and magnet to remove material from the tube. Preferably the wiper is a bulkhead inclined to the longitudinal axis of the tube and serves to remove attracted material from the surface of the tube and direct the removed material to the chamber inlet or outlet. Preferably the magnet is a linear array of neodymium magnets which is moveable within the tube which may be constructed as a thin-walled stainless steel tube. Advantageously the magnetic separator is used in food processing or machining applications.

Description

Magnetic Separators 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 material in the fluid flow. Periodically, attracted debris is removed from the outside of the tubes. 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 workpiece. The need to periodically clean the tubes requires the separation process to be interrupted for a period of time. It has also been found that the cleaning process is often ineffective, leaving an amount of material on the tubes or lying in the chamber.

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 fluid inlet for receiving dirty fluid and a fluid outlet for emitting cleaned fluid; a tube positioned within the chamber; a magnet positioned within the tube; and a wiper which surrounds the tube; wherein the wiper and the magnet are arranged to permit relative movement between them during a purging operation whereby to remove material from the tube, and wherein the relative movement is such that removed material is collected adjacent the fluid inlet or fluid outlet. \

The relative movement can be achieved by a stationary wiper, which is positioned adjacent the inlet or outlet, and a magnet which is movable within the tube.

In use, the magnet drags material along the tube until it reaches the wiper. Preferably, the wiper includes shielding material which serves to shield the material from the magnet during the purging operation.

Preferably, the wiper is directed towards the fluid inlet or fluid outlet so that any removed material is carried towards the inlet or outlet of the chamber.

Preferably, the wiper is inclined with respect to the longitudinal axis of the tube. This helps to guide the material towards the outlet and helps to collect material on the side of the tube nearest the outlet. The inclination of the wiper also reduces the force needed to move the magnet as the removal of material occurs, since the magnet is progressively stripped of material. A preferred way of moving the magnet is by differential air pressure across the ends of the tube, and the reduction in the force needed to move the magnet allows a lower differential pressure, and thus a thinner walled tube, to be used. This improves the separation efficiency of the separator.

Preferably, the tube comprises a first portion which lies within the chamber and a storage portion which lies outside of the chamber, the storage portion having a length which is sufficient to accommodate the magnet. In this way, the magnet is movable outside of the fluid flow chamber so that material cannot remain attracted to the tube.

Conveniently, the wiper can form a bulkhead at one end of the chamber and does not present an obstruction to flow within the chamber during normal use, or during a purging operation.

An alternative way of achieving relative movement between the magnet and wiper is by providing a stationary magnet and a wiper which is movable along the tube. Preferably, the wiper is movable into a position in which it lies adjacent the inlet or outlet, and suitably spaced or shielded from the magnet.

By causing material to be collected adjacent the fluid inlet or outlet, any material that is removed from the tube can be more reliably removed from the chamber during a purging process. It is preferred that material is collected adjacent the dirty fluid inlet. This has the advantage that any material which is not completely removed from the chamber during the purging process will be carried back along the flow chamber when fluid is readmitted into the chamber, and will be separated from the fluid once again by the magnet.

The purging process can use a fluid to flush the chamber, and preferably uses compressed air to raise the pressure inside the chamber. This process dries the accumulated debris. The dried debris can then be more easily stripped from the tube and ejected from the chamber when the chamber is depressurised. This process has an advantage of creating dry solid waste material which can be easily disposed or recycled.

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.

The separator is able to operate in the separation mode at full system pressure.

Where the fluid operating pressure exceeds the compressed air pressure that is readily available at the installation site, an air pressure booster can be used.

A second aspect of the 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 fluid inlet for receiving dirty fluid and a fluid outlet for emitting cleaned fluid; a valve at the inlet or outlet; and a controller for controlling operation of the separator, wherein the controller is arranged to perform a purging operation comprising closing the valve, admitting compressed air into the chamber whereby to pressurise the chamber and opening the valve after a period of time whereby to expel any separated material from the chamber.

A third aspect of the 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 fluid inlet for receiving dirty fluid and a fluid outlet for emitting cleaned fluid; a tube positioned within the chamber; a magnet positioned within the tube; and a wiper which surrounds the tube; wherein the wiper is inclined with respect to the longitudinal axis of the tube.

The first, second and third aspects of the invention can be combined with one S another, as can any of the preferred features of the different aspects of the invention.

Further aspects of the invention provide methods of purging material using the 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 1; Figures 3A -3F show a sequence of operations for using a second embodiment of the separator; Figures 4A - 4F show a sequence of operations for stripping material from a tube used in the separators of Figures 1 and 3; Figure S shows a third embodiment of the magnetic separator; Figures 6A 6I show a sequence of operations for using the separator of Figure 5; and, Figures 7A - 7F shows a sequence of operations of using a fourth embodiment of the magnetic separator.

Figure 1 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 20 is directed perpendicularly or tangentially to the longitudinal axis 15 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 S stainless tube and the magnets can be neodymium magnets.

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 ] O axis 15 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. The angle between the axis of the magnets/pole pieces and the ejection bulkhead 40 is determined by the aspect ratio, which is: diameter of magnet ( pole piece) length of magnet ( pole piece) Depending on the aspect ratio, the bulkhead can be inclined between O and 90 to the longitudinal axis of the tube 60. 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.

Without correct cloaking of the powerful permanent magnets, debris will be retained on the bulkhead 40 and nearby pipe work. If this is allowed to occur then this material will immediately contaminate the clean fluid supply at the end of the purge cycle, as described below. Shielding is accomplished by placing a material with a permeability much greater than one between the field source and the outlet. Such material must be highly permeable to prevent passage of magnetic fields. Shielding materials commonly used have a permeability of 300 to over 500,000, depending on flux density. For a static magnetic field, as when the magnets are located in section 12 of tube 60, the shield effectiveness is directly proportional to shield thickness because the shield's reluctance to magnetic flux is inversely proportional to its thickness. The degree of shielding is achieved, for a given total thickness, by dividing it into two or more concentric shields separated by at least the thickness of the material. In this case, a medium permeability material is used for one layer and a high permeability material for the other layer. The lower permeability material is located closest to the field source, i.e. the permanent magnets in array 50. Thus, the medium permeability laminae act as a buffer that sufficiently diverts the magnetic field to enable the lower reluctance (higher permeability) material to attain the required attenuation. As the external field is strong enough to cause the medium permeability material to approach saturation, an additional diverting shield of low permeability high flux carrying capability is required. This then caters in abstract for the next generation of "rare earth" magnets or even the application of high temperature (ambient) superconductor magnets when they may become available.

Figure 2 shows the magnet array in more detail. The array 50 is movable within the tube 60. The array 50 comprises a plurality of annular magnets 51 with pole pieces 52 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.

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.

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, as shown in Figure 1, or a horizontal orientation with an outlet directly below the bulkhead 40, as shown in Figure 3A, 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. To illustrate these alternatives, and to show how the separator is used, reference will now be made to Figures 3A-3F. For ease of explanation, butterfly valves are shown in each of the ports that connect to the inlet 20 and outlet 30, rather than a single valve which moves between ports.

Figure 3A shows the separator in a normal state for separating material from a fluid flow. A flow of contaminated fluid is introduced to inlet 20 at system pressure via port 25. The volume of the housing 10 is such that the flow velocity is dropped to a minimum, thus prolonging the time that the contaminated fluid remains in the chamber 13. Magnet array 50 is positioned in the flow path. 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 and port 35.

During this state, a thick bed of magnetic and paramagnetic particles accumulates on the outer wall of tube 60 and these also entrap nonmagnetic 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. Figures 3B to 3F show the sequence of steps required to clean or purge the tube of accumulated material. Firstly, at step Figure 3B, compressed air is fed to the chamber 13 via port 37 and valve 38. This air displaces the fluid in the chamber 13 until all fluid has been displaced through the port 25. 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.

At Figure 3C valve 25 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 3C to that shown in Figure 3D. In moving between these positions, the magnet array passes through the bulkhead 40 to the 'cloaked' position in portion 12 of tube 60.

S 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.

The stripping operation of the bulkhead 40 is shown in Figures 4A-4F. As the magnet pole pieces pass through the bulkhead, stripping of debris 65 commences at a point 62 on the magnet tube 60 away from the exit chute. Debris are dragged around the magnet tube by the action of the moving pole piece until all particles are on the side of the outlet 30. This is repeated for the next magnetlpole piece and so on. Since only a small percentage of the pole piece is being stripped at any given time forces are effectively single partial pole stripping load rather than full multiple pole. This minimises mechanical load on the magnet assembly. This action ensures that all debris is properly located on the outlet 30 side of the magnetic tube(s) 60 ready for ejection. This angle of the bulkhead 40 also offers minimum resistance to debris release. Finally, the action of each magnet and pole piece passing through the bulkhead is to transport debris down the exit ramp. It should be noted that in Figures 4A-4F particles are shown in clusters rather than an amorphous mass, to better illustrate in graphic form their movement. Although the bulkhead is shown here as a planar form, it may be curved, such as a complex curve, to aid movement of debris.

Returning to Figure 3D, the horizontal orientation of the tube 60 and the positioning of inlet 20 directly below the bulkhead 40 allows stripped material to fall away from the tube and towards the dry waste port 27. Once magnet array 50 has reached the cloaked position within part 12 of tube 60 valve 28 opens to connect inlet to waste outlet port 27. 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 shielding of the bulkhead 40 and the portion of tube 60 within part 12 of the housing ensures that waste is not attracted by the magnetic field exerted by magnet array 50.

In the step shown in Figure BE 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.

Immediately opening the clean exit valve 36 at this stage will result in the volume of air within chamber 13, at atmospheric pressure, being discharged into the clean outlet line 35, which may be undesirable under certain circumstances. Thus, in an optional step, a fluid float valve is fitted at the compressed air inlet 37. The float valve opens as the chamber pressure drops to zero. As the fluid supply valve 26 opens, contaminated fluid cleansed by the magnets fills the upper chamber until full. Liquid pressure closes the air relief valve 38, leaving chamber 13 full of fluid. Fluid out valve 36 then opens and fluid flow is delivered without entrained air. The unit is now in the normal separating mode as initially described in Figure 3A. The duration of the purging cycle will vary according to the volume of chamber 13, air pressure and liquid pressure, but a total duration of around 10 seconds is envisaged.

The embodiment just described has a single tube 60 and is suitable for small scale applications. Figure 5 shows a further embodiment of the separator. The main differences are that the housing includes multiple tubes 160, arranged in a circular array 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. A frusto-conical, or otherwise tapered, bulkhead 140 encircles each tube 160, extending between the wall of the housing 110 and the entry to the outlet 130. The bulkhead 140 is inclined towards the outlet conduit 130. Shielding 114, 116 between the storage portion of tubes 160 and the outlet 130 is sufficient to prevent debris from being attracted towards the magnets, and 'sticking' to the inner wall of outlet conduit 130. The shielding may only surround the outlet conduit 130, as shown in Figure 5. Alternatively, individual tubes 160 can each be provided with shielding. Alternatively, a combination of these techniques may be used. Figure 5 shows two layers of material: a layer of medium permeability material 114 and a layer of high permeability material 116, with the lower permeability material 114 being located closest to the field source, i.e. the permanent magnets in array 150. The bulkhead 140 also includes shielding material.

A further difference in this embodiment is the provision of a diffuser plate 170 which extends across the housing 110. The diffuser plate 170 has a slightly smaller diameter than the housing 110 or incorporates apertures adjacent tubes 160. The diffuser plate 170 serves to guide incoming fluid from the central inlet 120 to the region ofthe tubes 160 so that the fluid flow is subject to the magnetic field exerted by magnets 150. The magnets 150 are constrained from moving beyond the diffuser plate 170, thus preventing accumulation of debris in a hard to purge position. The tubes 160 are shown arranged in a circular array, but may be arranged in any other appropriate arrangement. Figures 6A- 6I show the sequence of steps for operating the separator.

These are broadly the same as the steps shown as Figures 3A-3F. However, the compressed air inlet 124 is at the same end of the chamber 113 as the dirty fluid inlet 123. During the purging process compressed air is admitted into the chamber 113 via inlet 124 and displaces cleaned fluid out of the chamber via port 133. Mateial is collected at the entry to the outlet 130 and is expelled from the outlet 130 and waste port 134 by pressurising and then depressurising chamber 113. Figure 6G shows a non-return valve 125 opening, air within the chamber being displaced (Figure 6H) and the non- return valve closing (Figure 6I).

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.

A common application of magnetic separators is in cleansing the coolant used in machining operations, such as the milling or drilling of metal. Most coolant systems run with system fluid pressure between 2 and 6 Bar. As factory air compressed supplies typically supply 5.5 to 7 Bar, the standard factory air supply can be used without any further equipment. However, certain specialist applications can use a coolant pressure of 20 Bar. As compressed air is being used to displace liquid from the unit prior to purging, the air pressure must be above system fluid pressure.

An air pressure booster can be used to provide compressed air at around 22 Bar. In general, an air pressure booster can be used to provide a compressed air supply which is greater than the system fluid pressure.

The embodiments described above make use of a magnet array which is movable within a tube. This is preferred, as the magnet array can be readily moved within the clean environment inside the tube. An alternative arrangement is now shown in Figure 7A. A cylindrical housing 210 defines a fluid flow chamber 213 with an inlet 220 and outlet 230. A tube 260 is mounted within the chamber and a magnet array 250 is mounted within the tube 260. An annular wiper block 240 is mounted within the chamber, encircling the tube 260 to form a sliding fit with the outer wall of tube 260 and the inner wall of chamber 213. The wiper block has an inclined face.

Unlike previous embodiments, magnet array 250 is stationary and, instead, wiper block 240 is movable along the length of tube. Operation of this arrangement will now be described with reference to Figures 7A-7F. Firstly, as shown in Figure 7C, during normal operation dirty fluid enters via port 225 and inlet 220, flows along the chamber 213 and exits via outlet 230 and port 235. At the start of the purging operation, as shown in Figure 7B, air is applied via port 237 to flush fluid from the chamber 213, drying any accumulated material in the process. As shown in Figure 7C, outlet port 225 is then closed so as to pressurise chamber 213. Valve 228 is opened and the chamber 213 is depressurised. Wiper block 240 moves along the length of tube 260, driven initially by air applied to port 261 and then a combination of air applied via ports 261 and 237. As the wiper block 240 moves along the tube 260 it carries accumulated material with it. Wiper block stops at the position shown in Figure 7D, directly above outlet 227. Wiper block 240 includes shielding material, as previously described. The distance between the end stop position of the wiper block 240 shown in Figure 7D and the end of the magnet array 250 also provides some shielding.

Further compressed air is applied via port 262 across the face of wiper 240, blowing any remaining material towards outlet 227. Wiper block is then returned to its initial position by applying fluid into port 225. Once air has been displaced from port 237, valve 236 opens.

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 fluid inlet for receiving dirty fluid and a fluid outlet for emitting cleaned fluid; a tube positioned within the chamber; a magnet positioned within the tube; and a wiper which surrounds the tube; wherein the wiper and the magnet are arranged to permit relative movement between them during a purging operation whereby to remove material from the tube, and wherein the relative movement is such that removed material is collected adjacent the fluid inlet or fluid outlet.
2. A magnetic separator according to claim I wherein the wiper is directed towards the fluid inlet or fluid outlet.
3. A magnetic separator according to claim 1 or 2 wherein the wiper is inclined with respect to the longitudinal axis of the tube.
4. A magnetic separator according to any one of the preceding claims wherein the magnet is movable along the tube and the wiper is positioned adjacent the fluid inlet or fluid outlet.
5. A magnetic separator according to claim 4 wherein the tube comprises a first portion which lies within the chamber and a storage portion which lies outside of the chamber, the wiper being positioned at the intersection of the first and storage portions.
6. A magnetic separator according to claim 5 wherein the wiper forms a bulkhead between the fluid flow chamber and a part of the housing accommodating the storage portion of the tube.
7. A magnetic separator according to claim 6 wherein the bulkhead comprises a fluid-tight seal.
8. A magnetic separator according to any one of claims 4 to 7 wherein the magnet is movable along the tube in response to differential air pressure across the magnet.
9. A magnetic separator according to any one of claims I to 3 wherein the wiper is movable along the outside of the tube to a position adjacent the fluid inlet or fluid I O outlet.
10. A magnetic separator according to claim 9 wherein the wiper is movable to a position which lies beyond the magnet.
11. A magnetic separator according to any one of the preceding claims wherein the wiper comprises shielding means for shielding material from the magnetic field of the magnet.
12. A magnetic separator according to any one of the preceding claims wherein the magnet is a linear array of magnets, spaced along the longitudinal axis of the tube.
13. A magnetic separator according to any one of the preceding claims wherein the separator further comprises a controller which is arranged to close a valve at the inlet or outlet of the chamber and to apply compressed air to the chamber whereby to pressurise the chamber and to open the valve after a period of time whereby to expel separated material from the chamber.
14. A magnetic separator according to claim 13 wherein the controller is further arranged to cause relative movement between the wiper and the magnet while the chamber is pressurised whereby to remove material from the tube.
15. A magnetic separator according to claim 13 or 14 wherein the controller is further arranged to admit compressed air into the chamber for a period which is sufficient to empty the chamber of fluid, before closing the valve.
16. A magnetic separator according to any one of the preceding claims wherein there is a plurality of tubes positioned within the chamber, each having a magnet positioned within them.
17. A magnetic separator according to claim 16 wherein the plurality of tubes are arranged in a circular array and the outlet is aligned with a portion of the tubes and shielding material is positioned between the first part of the outlet and storage portions of the tubes.
18. A method of purging collected material from a magnetic separator which comprises a housing which defines a fluid flow chamber, the chamber having a fluid inlet for receiving dirty fluid and a fluid outlet for emitting cleaned fluid, a tube positioned within the chamber, a magnet positioned within the tube and a wiper which surrounds the tube, the method comprising: causing relative movement between the wiper and the magnet to remove material from the tube so that removed material is collected adjacent the fluid inlet or fluid outlet.
19. A magnetic separator for separating material from a fluid flow comprising: a housing which defines a fluid flow chamber, the chamber having a fluid inlet for receiving dirty fluid and a fluid outlet for emitting cleaned fluid; a valve at the inlet or outlet; and a controller for controlling operation of the separator, wherein the controller is arranged to perform a purging operation comprising closing the valve, admitting compressed air into the chamber whereby to pressurise the chamber and opening the valve after a period of time whereby to expel any separated material from the chamber.
20. A magnetic separator according to claim 19 which further comprises a tube positioned within the chamber, a magnet positioned within, and movable along, the tube, wherein the controller is arranged to move the magnet along the tube to a position which lies outside of the chamber before opening the valve.
21. A magnetic separator according to claim 20 further comprising a wiper which surrounds the tube, and wherein the controller is further arranged to move the magnet past the wiper while the chamber is pressurised whereby to remove separated material from the outside of the tube.
22. A magnetic separator according to any one of claims 19 to 21 wherein the controller is further arranged to admit compressed air into the chamber for a period which is sufficient to empty the chamber of fluid, before closing the valve.
23. A method of purging collected material from a magnetic separator which comprises: a housing which defines a fluid flow chamber, the chamber having a fluid inlet for receiving dirty fluid and a fluid outlet for emitting cleaned fluid and a valve at the fluid inlet or outlet; the method comprising: closing the valve and admitting compressed air into the chamber whereby to pressurise the chamber, and opening the valve after a period of time whereby to expel any separated material from the chamber.
24. A method according to claim 23 wherein the separator further comprises a tube positioned within the chamber, a magnet positioned within the tube and a wiper which encircles the tube, the method further comprising moving the magnet past the wiper while the chamber is pressurised whereby to remove separated material from the outside of the tube.
25. A method according to claim 23 or 24 further comprising admitting compressed air into the chamber for a period which is sufficient to empty the chamber of fluid, before closing the valve.
26. A magnetic separator for separating material from a fluid flow comprising: a housing which defines a fluid flow chamber, the chamber having a fluid inlet for receiving dirty fluid and a fluid outlet for emitting cleaned fluid; a tube positioned within the chamber; a magnet positioned within the tube; and a wiper which surrounds the tube; wherein the wiper is inclined with respect to the longitudinal axis of the tube.
27. A magnetic separator or a method of purging a magnetic separator substantially as described herein with reference to and as shown in the accompanying drawings.