Nonwoven, Random Fibre Media

The problem of correlating the permeability of filter media with the bade dimendons of the mat ials con odng the septum has received atterdon p avies, 1952]. Badcaify, the problem is to relate B, as defined in Equation 4.4 to variables such as fibre diameter, weave constmetion, etc. Whilst the permeability of random system is not the princ al interest here, it is inq)ortant to record some work rqported in this area, in view of the growing application of nonwovens in the area of pressure filtratum. 

Perhaps the best-known equation for describing the required relationsli in packed beds of particles or fibres is the Kozeny-Carman equation in terms of the porosity of e and the specific snrftice Sv of the medium  

The so-called Kozeny Constant, K has been drown to be a variable [Johnson,1986, p.l56] riririch is dependent on the porodty of the deposit rapid increases in K when 8 0.7 have been reported. Fibrous structures of wool, cotton, rayon, glass and steel wool have received attention and semi-en irical equations of the form  

Fibrous iilters are often used in clarification processes, e.g. in hydraulic systems, in the electronics industry to ensure the absolute clarity of rinse water or in the beverage industry prior to bottling. 

Filter media efficiency tests are made using dilute suspensions of standard particles. A wide variety of test powders is available, in fine and coarse grades. Industrial users will tend to use a test mixture typical of the products of interest, e.g. yeast in the browing industry. In the gaseous field, both solid (sub-micron sodium chlorid crystals) and liquid particles (di-octyl phthalate dispersion) are used. 

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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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