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Fabrics make up the largest component of filter media materials. They are made from fibres or filaments of natural or synthetic materials, and are characterized by being relatively soft or floppy, lacking the rigidity of dry paper, such that they
would normally need some kind of support before they can be used as a filter medium.

The fibres or filaments can be made up into a fabric as they are, by means of some kind of dry-laying process, to produce a felt or similar material. Such ‘ noninterlaced ’ fabrics are generally referred to as ‘nonwoven’, and they are covered later in this section.

If the fibres or filaments are first spun into a continuous yarn, then the resultant yarn can be woven or knitted into a fabric, and such ‘ interlaced ’ materials are the woven fabrics covered here. If the material used in the weaving process is a single
filament of wire or plastic, then the resultant material may be counted as a fabric, but is more often called a mesh, and is also covered separately later.

Textile fibres come from many sources, both natural and synthetic. The natural materials come either from vegetable sources: cotton, flax (linen), jute, and wood cellulose, or from animals: silk, wool, fur, hair. The synthetic materials are produced either from natural resources such as glass, ceramics, carbon, metals or reconstituted cellulose, or they are totally synthetic, being extruded from thermoplastic polymers.

The naturally fibrous materials all have fibres that are extremely long by comparison with their diameters, except in the case of wood cellulose, where the manufacturing process produces fibres whose lengths are measured only in millimetres.
These wood cellulose fibres are too short to spin into a yarn, and are only usable in the wet-laying processes that produce paper and related materials.

The remainder of the natural fibres have lengths measured in centimetres, and can be over 30 cm long in the case of wool, while silk can be produced as a single filament. The synthetic materials are actually produced as continuous filaments,
which can then be cut or broken into fibres of any length.

Natural fibres have a diameter dictated by their source, and this is usually less than a millimetre. The synthetic fibres and filaments are mainly formed by some kind of extrusion process from the molten state, with a diameter to match that of
the spinning nozzle through which they are extruded. Their diameters can thus exist in a wide range, from much greater than those of natural products, to considerably finer sizes.

The length and diameter of a natural fibre may be increased by converting the material into a yarn, although yarns may also be made up of filaments. Because of their much greater length, filaments may just be bundled together to make a yarn,
although the bundles are usually twisted to give a reasonably constant diameter. The shorter, staple, fibres have to be twisted quite tightly, after being spun to line them up, in order to give adequate strength to the resultant yarn.

Yarns made from filaments are usually thin, smooth and of a lustrous appearance. Staple yarns are usually thicker, more fibrous (hairy) in appearance, and with little or no lustre.

Yarns can also be made up from tapes of various kinds. In the case of filter media, these tapes would probably be fibrillated, or made of other perforated material.

Woven fabrics are then made up from single filaments, or multifilament yarns, or from twisted staple yarn. The last of these is normally used as a single strand, but two or more spun strands may be combined into ply yarns, where the strands are
twisted together, usually (but not necessarily) in the opposite sense from the twist in each strand.

Fabrics can be considered as a direct, and physically stronger, alternative to papers and are employed in a similar manner for pleated elements, etc. Fabric elements were originally the most commonly used type of filter medium for fine filtration and are generally comparable with modern paper media as regards to the performance achievable. Until the appearance of treated paper elements, they were regarded as a superior type, although the two are now strictly competitive for similar duties. Some typical filtration performance curves for these materials are shown in Figure 2.3 . Treated papers are now the more common because of lower cost, but fabric elements are capable of withstanding higher working pressures with similar geometry. However, fabric elements have a lower specific resistance than paper elements and, being thicker, can also carry a heavier load of contaminant per unit area. This latter advantage is normally offset by the fact that, for the same overall size, the surface area of the fabric element is reduced because of its greater thickness.

For a similar design of element, fabric would probably be preferred for larger sizes of filter, or where a degree of true absorption is required as well as mechanical

screening. The description ‘fabric’ is rather loose, compared with the term ‘ paper, ’ which is quite specific. Fabrics can comprise a whole range of materials (as in Table 2.4 ), woven and nonwoven, whose properties may be further modified by impregnation with synthetic resin or similar treatment. Equally the term ‘ cloth ’ is often used to describe fabric media, natural or synthetic, and even a woven wire cloth.

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