Media made from metals have already been mentioned, in Section 2C, in the form of sheets and tubes made from metal fibres and powders, usually sintered together to retain their structure. By far the greatest amount of filter media made from
metal, however, is in the form either of woven wire or perforated sheets, which are covered in this part of Section 2, with the common features of high strength, and corrosion and abrasion resistance conveyed by the basic material of construction.
The other common feature, of great importance to their use for filtration, is that these materials can be made with apertures of precise sizes, which also comes from their being made of metal.
It should be noted, however, that the edge filters described in Section 2E are also mainly made of metal and made with precise apertures. It should also be noted that every type of filter medium described in this part as made from metal can also be made of plastic materials, with almost as much strength and almost as much precision in aperture size. Corrosion resistance may actually be higher with plastic wire screens or sheets than for some metals.
There are two major applications for which these metallic media are primarily used: the separation of solid particles by size, and the coarse screening of gas or liquid flows ahead of some finer processing stage. In the coarser, macrofiltration
processes these are very versatile materials: all of the dry classification (sieving, sifting) operations are covered here, as are almost all of the applications of filters for straining and coarse filtration, both of which rely upon the precise size and shape of the apertures in the mesh or sheet.
There are three broad classes of media covered under the above heading: woven wire meshes, sheets perforated with a variety of holes, and elements made up from preformed materials such as rods or bars. Some overlap obviously exists between the woven meshes of this sub-section and the woven monofilament materials of Section 2C.
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