Needle-punched filters

Typical textile fibers used, for example, in a needle-punched filter fabric, are a blend of 3.3- and 6.6-dtex (3- and 6-denier) polyester staple. These fibers are - 5 cm long, have diameters ranging from 18 to 25 pm, mass-per-unit-length or linear density values ranging from - 350 to 650 mg per 1000 m, and length-to-width ratios in the order of 1000 to 1. 

Pollutant Loading Typical inlet concentrations to cartridge collectors are 1 to 23 grams per cubic meter (g/m ) (0.5 to 10 grains per cubic foot (gr/ft )). Cartridge filters, which utilize synthetic, nonwoven media such as needle punched felts fabricated of polyester or Nomex, are able to handle inlet concentrations up to 57 g/m (25 gr/ft ). 

In nonwoven geotextiles, the fibers are much thinner but far more numerous. The various types are needle-punched, resin-bond, and melt-bond. All contain a labyrinth of randomly oriented fibers that cross one another so that there is no direct line of flow. The fabric must have enough open space to allow liquid to pass through, while simultaneously retaining any upstream movement of particles. The needle-punched nonwoven type is very commonly used as a filter material. 

One of the most successful industrial applications of polymeric catalytic membranes is the Remedia Catalytic Filter System to destroy toxic gaseous dioxins and furans from stationary industrial combustion sources by converting them into water, CO2, and HCl. The system consists of an expanded polytetrafluoroethylene (PTFE) microporous membrane, needle-punched into a scrim with a catalytically active PTFE felt. The catalyst is a V2O5 on a Ti02 support. The microporous membrane captures the dust but allows gases to pass to the catalyst where they are converted at temperatures as high as 260° C. 

Needled felt media (Fig. 10.48b) consists of intertwined short fibers, pressed together, and mechanically fixed with a needle punch machine. The efficiency of such filter media varies with its density, composition, and relative thickness. Needled felt media is strong and durable, but, for maximum filtering efficiency requires the build-up of a dust layer (see below). 

The pad filter (often called a cassette) is a special case of the range of panel filters, all of which are made in standard sizes, to fit air-conditioning installations. The pad, as its name implies, is a thick flat sheet of fibrous filter medium, made either by wet-laying (as in paper) or by dry-laying (as in a felt). Felt pads are the most common, and they can be either as-laid, which would normally be the case for natural fibres such as cotton or wool, or needle-punched, for synthetic fibres. 

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