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Magnetic filters are specialized straining systems for the removal of iron and other ferro-magnetic particles from liquid suspensions and flows of solid particles. They are in effect simple magnets or magnetic assemblies that, when suitably located in a fluid system, can attract and retain ferrous metal, nickel and cobalt particles that may be present in that system, and also composite particles in which a ferromagnetic material is entrained. Their main uses are for the trapping and retention of ferrous metal machining or wear products in lubrication systems and hydraulic systems (particularly when running-in a new system), removal of ferrous particles from ceramic slip in the pottery industry, removal of ferrous particles from process feed lines and pneumatic conveyors, and the separation and retention of swarf from machine tool coolants.

Elements employed in such cases are invariably permanent magnets. Until the appearance of high energy permanently magnetic materials, the efficiency of magnetic filters was somewhat limited. With modern alloys offering a remanence in excess of 10,000 gauss and a BHmax of the order of 4  106 gauss-oersteds, the efficiency of a permanent magnet can be extremely high.

In its simplest form, a magnetic filter may be in the form of a plug replacing the conventional drain plug in a crank case, as in Figure 3.79 . Ferrous metal particles flowing into the magnetic field generated by the plug are attracted to the plug, where they adhere and remain trapped. The plug can then be cleaned by scraping when it is removed, for example at each oil change. Plugs of this type are particularly useful for trapping initial wear products generated during the running-in period of internal combustion engines, gearboxes, gear pumps and similar machines. A more efficient form of magnetic drain
plug, instead of relying purely on magnetic attraction, traps the ferrous contaminants between a number of magnetized rings or magnets encircling the plug core.

For other applications the magnetic element can be designed to suit the flow conditions involved. Basically any such assembly of magnets should be designed so that fluid is caused to flow over or through those parts of the elements at which the magnetic field is strongest, and preferably this flow should be free from turbulence.

It is also desirable, whenever possible, to arrange that the majority or all of the particles are retained outside the mainstream flow, so that accumulation of contaminant cannot impede the flow. For maximum efficiency it is further desirable that the direction of flow of the fluid is the same as that of the magnetic field. The particles are then more readily diverted to the magnetic elements and withdrawn from the main flow. The normal arrangement is a series of magnets of cylindrical or quadrant shape retained in position by a non-ferrous metal cage or cylinder, and clamped between two mild steel pole pieces at the top and bottom of the assembly. The external field passes between the pole pieces via a mild steel cage, which has a series of gaps, across which a strong flux is maintained. These gaps provide a concentration of the field and thus ensure a strong field gradient for effective removal of ferrous contaminants. A magnetic assembly of this type is shown in Figure 3.80

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