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The sintered (thermally bonded) materials will normally be made into tubular shapes, by moulding or isostatic pressing, before sintering, and these shapes will be self-supporting, so there is no need for a core (unless required in the forming
process). These shapes can then be considered as candles rather than cartridges, a single element in its housing probably being called a cartridge, and a multi-element housing called a candle filter.

Porous plastic elements are sintered by special methods from high molecular weight thermopolymer powder. The pore size and filtration characteristics are controlled by careful selection of cryogenically ground powder to give a particle retention
in the range from 5 to 200m. The elements are manufactured in specially designed individual moulds and most shapes are possible. Porous plastic elements are used for compressed air filtration and for general use in environments where temperatures do not exceed 80°C. An array of porous plastic elements is shown in Figure 3.53 .

Sintered metal filters provide the possibility of closer control of pore size, shape and uniformity than can be achieved with plastics, and the resulting matrix is much stronger, more rigid and more resistant to heat. Pore sizes may range from submicrometre dimensions up to as much as 1mm. Thus, theoretically at least, virtually any cut-off down to the finest ultra-fine filtering requirements can be provided by a sintered metal powder element. In practice, however, this range of application is modified by the increasing resistance to flow with diminishing pore size, which naturally tends to decrease porosity.

Porosity, or the proportion of voids to overall matrix volume, can be controlled over a wide range with sintered metal elements. Whilst increasing porosity decreases resistance to through-flow, the strength of the matrix decreases rapidly. For reasonable mechanical strength, it may be necessary to restrict porosity, by adopting a suitable compromise based on strength/permeability requirements. In the case of filter elements, porosity may range up to 70%, or possibly higher
for low pressure drop elements. The particular advantage of sintered metal filter elem ents, apart from the fine cut-off that can be provided, is their high strength and rigidity compared with non-metallic media, which makes them particularly attractive for high pressure applications.

Sintered powder metal filters fall broadly into two categories: those produced by sintering loose powder in a mould, and those produced by compaction. Spherical particles are preferred for either, because they pack more uniformly and thus provide more uniform pore sizes. Spherical particles are relatively easy to produce and classify by spray atomization and sieving. The coarser grades of sintered metal filters are produced from particles with a particle diameter of about 1mm. Since
pore size is usually about 15% of the particle diameter, this would yield a pore size in the material of 150um. Such sintered metal filters are expensive to produce and thus not competitive at this level with wire mesh. Manufacturing costs of sintered metal filters are rather less with smaller particle diameters, whereas the cost of woven wire mesh increases with decreasing pore size, while mesh strength and rigidity are reduced.

Sintered metal filters become increasingly more competitive from pore sizes of about 100um downwards. At the finer end, however, spherical metal particles of the order of 5 to 10m diameter are expensive to produce and costly to classify
although they have been produced down to 1um in size. Some typical porous metal elements are shown in Figure 3.54.

For most general purposes sintered bronze filters are suitable. For particularly arduous duties involving very high pressures, high temperatures or corrosive fluids the filter elements may be sintered from stainless steel, Monel, pure nickel, Hastelloy, titanium or even tungsten. Bronze and cupro-nickel sinter readily at low temperatures and thus a whole variety of shapes can be produced directly from metal powder in stainless steel or carbon moulds. The mould is passed through a furnace with a protective atmosphere to sinter the powder. Pressed or machined shapes may require subsequent treatment to open up surface pores. The elements can be machined to closer tolerances than can be produced by direct moulding, but   machining should only be used for non-effective areas of the filter element such as the finishing of shoulders for registration purposes. Although these conventional methods of moulding are still widely used, the introduction of iso-static pressing has meant that a much wider variety of shapes and sizes can be produced.

Porous stainless steel elements may be employed where high strength, greaterresistance to temperature and high resistance to corrosion are required. Porous stainless steel elements are commonly produced in the form of plates or discs used directly for filter element construction. The material commonly employed is a stainless steel equivalent to BS304S15, but with a maximum carbon content of 0.05%. Where the solid metal is subject to attack, corrosion of the porous metal will be greater because of the greater surface area exposed. The fine filtration provided by sintered metal elements, together with the controlled porosity ensuring a true absolute rating, makes them an attractive choice for high duty, high temperature applications.

The metal cartridges described above are based on metal powders, but it should be noted that sintered metal cartridges are also available made from metal fibre and wire mesh. Both materials are sintered to give the necessary integrity of the medium (and, in the case of wire mesh, to maintain the precision of the apertures formed in the mesh when it was woven). Wire mesh elements are usually pleated and used for the thin media tasks described earlier, while sintered mesh cartridges operate by depth filtration. Wire meshes can be laminated and then sintered together to make a thick medium with precise variations in pore size through the medium. The sintered fibre elements typically have higher flow rates for a given pressure differential, and a higher dirt-holding capacity than powdered metal elements with the same rating. Sintered fibre materials can be made thin enough to pleat, thus providing excellent filtration as a cartridge. As with powdered metal elements, the fibre and mesh cartridges, when fully loaded with contaminants, can easily be reclaimed by backwashing, ultrasonics or chemical cleaning.
Ceramic powders can be sintered into a wide variety of porous shapes for use as filter elements. In the form of porous pottery, ceramics were one of the earliest materials used for filtration. Porous ceramic filters for use as cartridges are generally in the form of a plain cylinder with a thick wall, the thickness of which provides the depth of filter medium for retention of the solids in a filtration process. As far as tubular elements are concerned, these are either plain cylinders (i.e. open at both ends) or flanged candles (i.e. candles with a flange on the open end for fixing in the cartridge housing or to the tube plate of a candle filter).

Sintered ceramic element sizes vary widely, with one-piece elements available in lengths in excess of 1m. Operating temperatures of 900–1000°C are practicable, and with a more refractory bond ceramic candles can be used up to 1600°C. Pore sizes range broadly from 100m to 1 mm, with average porosities of 35 to 45%. In terms of chemical resistance, ceramics are used in the 1–9 pH range (it should be noted that the chemical resistance of a ceramic medium is dependent on the operating conditions and should be carefully checked for each particular application). The versatility of porous ceramic media makes them particularly effective for a wide range of filtration applications.

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