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The previous part of this Section covered processes that dealt with filtration in a minor way, but which were not primarily intended for the separation of particles from fluids. The majority of the Section is now concerned with those materials
intended for filtration and will describe them in sufficient detail to explain how they are used and their main advantages. This first topic of the group of filtration media deals with those materials that are based on fibres of various kinds, short or long, spun into a yarn or laid down individually to create a random mass.

These fibres may come from natural sources such as wool or cotton, or the basic cellulose of wood, or they may be synthetic, produced by the extrusion of a molten polymer, which means that they are probably produced initially as a continuous
filament and then broken up to form the required length of fibre. This appears to imply that such materials are organic in origin, but modern manufacturing techniques enable almost any inorganic material to be produced as fine fibres: carbon,
glass, metals and ceramics, and so as a yarn or pad of randomly oriented fibres.
The properties of the finished fibrous material as a filter medium are then very much related to the properties of the fibres or filaments themselves, the most 2important correlation being that of size: the finer the fibre from which the material is made, the finer the particle that will be trapped by the resultant filter medium. The selection of a fine fibre from which to make a particular filter medium must be made in the realization that the finer fibres will produce a less strong material. The fibre choice then becomes an optimization exercise among degree of filtration (cut-point and efficiency), pressure drop (i.e. energy consumption in operation) and mechanical strength. A weak material, but one with the required filtration performance, can be strengthened by supporting it on a stronger substrate – but at an increase in cost per unit of filtration area.

It is not unusual that a filter medium is over-specified, and consequently costs more than it need do. As seen in Section 1A, some filter media are extremely expensive, and there would appear to be little point in purchasing a costly or technologically
advanced medium, when there is a well tried, tested and less expensive alternative available.

Nevertheless, it is also true that the demands being made upon filters in terms of finer degrees of filtration are growing, and this will lead to a continual lowering of filtration cut-points and increasing filtration efficiencies, with as little increase in
first cost or operating costs as possible.

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