Continuous fibre, filter media

Where the filter medium is in the form of a needle felt (as found in pulse-jet collectors), some manufacturers have offset the deficiencies found in some of the above fibres by producing composite structures, e.g. polyar-amid, polyimide or PPS fibres on the surface of the felt with a PTFE scrim or base-cloth providing a framework of support. Although under certain physicochemical conditions the surface fibres become degraded, it has been found that the skeleton of support provided by the PTFE base-cloth is often sufficient to enable satisfactory performance to continue. 

Polyester fibres, for example, will degrade when exposed to strong bases and prolonged hydrolysing conditions, and polyamide fibres will not tolerate continuous exposure to strong acids. Polypropylene is widely used as a filter medium because it is relatively inert to both acids and bases, but is attacked in oxidising environments such as when chlorine or heavy metal salts are present. PTFE (polytetrafluoroethene) fibres are resistant to most chemicals, but their use is often prohibited by the high cost. 

The properties of a fabric, especially as regards its behaviour as a filter medium, depend very much on the way in which the yams are woven together. Many properties, however, are intrinsic in the nature of the basic fibre or filament, and of the way in which it is made up into a yam. There are three basic types of yam in wide use for filter media monofilament, which is a single continuous filament of synthetic material (or silk) multifilament, which comprises a bundle of identical continuous filaments that may or may not be twisted together and staple, which... 

Flow through the clean medium will be determined by the geometric characteristics imposed on the medium by the weaver, in fitrming various patterns (plain, twill, sateen, etc.) fi om basic yams. In woven cloth, the latter are either sohd monofilaments, or are multifilamanets (which can be further subdivided into continuous or staple-fibre constmetions, depending on the type of filament used). In some cases, the sur ce of the medium may be modified to improve its ability to release the filter cake, etc. Nonwovens are paper-like, random arrays of fibres which can be obtained in many forms uniform fibres, mixed and conposhe pads, etc. These media, like wovens, can be supplied sur ce modified. 

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