Nonwoven media are fabricated in the form of belts or sheets from cotton, wool, synthetic and asbestos fibers or their mixtures, as well as from paper mass. They may be used in filters of different designs, for example, in filter presses, filters with horizontal discs and rotary drum vacuum filters for liquid clarification. Most of these applications handle low suspension concentrations; examples are milk, beverages, lacquers and lubricating oils. Individual fibers in nonwoven media are usually connected among them as a result of mechanical treatment. A less common approach is the addition of binding substances. Sometimes the media are protected from both sides by loosely woven cloth. Nonwoven media of various materials and weights, and in several grades of retentiveness per unit weight can be formed, in either absorbent or nonabsorbent material. These filter media retain less dispersed particles (more than 100 /xm) on their surface, or close to it, and more dispersed particles within the depths of the media.
Nonwoven filter media are mostly used for filter medium filtration with pore clogging. Because of the relatively low cost of this medium, it is often replaced after pore clogging. In some cases, nonwoven media are used for cake filtration. In this case, cake removal is so difficult that it must be removed altogether from the filter medium. Nonwoven filter media can be prepared so that pore sizes decrease in the direction from the surface of the filter media contacting suspension to the surface contacting the supporting device. This decreases the hydraulic resistance of filtration and provides retention of relatively large particles of suspension over the outer layer of the nonwoven medium. Nonwoven filter media of synthetic, mechanically pressed fibers are manufactured by puncturing the fiber layer with needles (about 160 punctures/cm2), and subsequent high temperature treatment with liquid which causes fiber contraction. Such filter media are distinguished by sufficient mechanical strength and low hydraulic resistance, as well as uniform fiber distribution. Filter media from fibers connected by a blinder are manufactured by pressing at 70N/cm2and 150°C. These media have sufficient mechanical strength, low porosity and are corrosion resistant. Filter media may be manufactured by lining a very thin layer of heat-resistant metal (e.g., nickel 360) over a fiber surface of inorganic or organic material. Such filter media may withstand temperatures of 200°C and higher. Of the flexible filter media described, the synthetic fabrics are perhaps the most widely relied on in industrial applications. Each filtration process must meet certain requirements in relation to flowrate, clarity of filtrate, moisture of filter cake, cake release and nonbinding characteristics. The ability of a filter fabric to help meet these criteria, and to resist chemical and physical attack depend on such characteristics as fiber type, yarn size, thread count, type of weave, fabric finish and yarn type (monofilament, multifilament or spun). Monofilament yarns consist of a single, continuous filament with a relatively smooth surface. The different sizes are specified as a measurement of the diameter in mils or in micrometers. Multifilament yarns are made from many fine filaments extruded simultaneously. The different sizes are specified by a measurement of weight known as the denier. These yarns are generally used for filter fabrics which require a smooth surface and relatively tight weave. Spun yarns are made from filaments which are chopped in short lengths and then spun or twisted together. Spun yarns are made into filter fabrics with a hairy, dense surface very suitable for filtration of very fine particles. It is necessary to select the type of fiber that will offer the most resistance to breakdown normally caused by chemical, temperature and mechanical conditions of the filter process. Tables 3 and 4 can serve as rough guides to proper media selection. Table 5 provides linear conversion units between mesh size, inches and micrometers.
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