Sedifilt - Syntech Fibres (Pvt)

Continuous filament media revolutionize string wound cartridges

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Courtesy of Sedifilt - Syntech Fibres (Pvt)

Depth filter cartridges made from a cotton string wound media and a metal core were first introduced in the market in the mid-1930s. By the late 1970s cartridges with a polypropylene (PP) core and yarn had become popular because they had a very wide range of chemical resistance, and therefore could be used in a wide variety of applications. Initially, the string media was a roving (a bulky intermediate stage before the final textile yarn forming process). It was later replaced by ‘friction spun’ yarn that is similar in appearance to a roving, but is relatively bulkier, giving improved dirt holding capacity and reduced resistance to the flow of liquids.

Media Migration
String wound cartridges made from roving or friction spun yarns are, however, prone to media migration. These yarns comprise short chopped fibres, typically about 75 mm in length, which make them inherently susceptible to media migration because many of the short fibres on the yarn’s surface are not fully locked-in. The normal textile process (comprising fibre bale opening, carding, drawing, spinning) by which these yarns are made, can break some of the fibres into even shorter lengths, and further add to the media migration problem. Figure 1 shows roving and friction spun yarn media used in standard string wound cartridges. The loose ends of the cut fibres can be seen protruding from the surface of the yarns. Fibres that are not fully locked-in can become loose with the flow of liquid and/or through an increasing pressure differential.

Chemical Leaching
Cartridges wound from PP roving or friction spun yarn media suffer from another major problem, namely chemical leaching. In the manufacturing of these media, ‘spin finish’ is applied on the surface of the fibres. Spin finish contains a variety of chemicals including lubricants, surfactants, anti-static agents, antioxidants, emulsifiers, bactericides, etc. The amounts of these chemicals can vary from 0.5% to as much as 2% by weight of the media. Unless the filter is pre-washed, these chemicals can begin to leach out in the initial period of use, and can often be observed as foaming in the filtrate. The leaching of these chemicals can be detrimental to downstream processes and can pose possible health problems when used for drinking water filtration applications.

Other Shortcomings
Roving and friction spun yarn media have a compact round cross-section. Cartridges wound from these yarns do not form a stable structure, and when subjected to conditions of flow and pressure fluctuations the media is prone to shifting, which gives rise to a ‘tunneling’ effect and particle unloading. Figure 2 shows a standard cartridge wound from friction spun media. An unstable structure also creates a problem in achieving consistent micron ratings. Fine and coarser filters are made by winding the yarn closer together or with a gap between, respectively. As the gap between the yarns is widened, the compact round yarns tend to roll to one side or the other giving inconsistent results. Moreover, round yarns typically form a diamond pattern having gaps between adjacent yarns, as well as between layers. The liquid to be filtered obviously takes the least resistant path, which in this case is between the yarn gaps rather than through the whole media.

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