Filtration fabric selection

The key parameters in fabric selection have been described (Hardman, 1994) as the thermal and chemical conditions, filtration requirements, equipment considerations and cost. 

Industrial filtration systems may be of many types. The most common type is the baghouse shown in Fig. 29-3. The filter bags are fabricated from woven material, with the material and weave selected to fit the specific application. Cotton and synthetic fabrics are used for relatively low temperatures, and glass cloth fabrics can be used for elevated temperatures, up to 290 C. 

The choice of the filter medium is often the most important consideration to ensure efficient operation of a filter. Its function is generally to act as a support for the filter cake, while the initial layers of cake provide the actual filter. The filter medium should be selected primarily on the basis of its ability to retain solids without binding. It should be mechanically strong and corrosion resistant, and should offer as little resistance as possible to the flow of the filtrate. The media are made from widely different materials such as cotton, wool, linen, nylon, jute, silk, glass fiber, porous carbon, metals, rayon and other synthetics, and miscellaneous materials like porous rubber. Cotton fabrics are most commonly used because they are available in a wide variety of weaves, and are cheap. 

In the case of nonwoven fabrics, a reduction in pore size is achieved by compressing the fibres into a more dense structure (loads up to 300decaNm" may be necessary) and, by selection of the appropriate conditions, a more durable (Fig. 3.31) surface can also be obtained through partial fusion of the surface fibres. With woven fabrics, on the other hand, some deformation of the yams may be necessary to achieve the optimum filtration properties. This is particularly graphic in the case of fabrics woven from monofilament yams, as shown in Fig. 3.32 and Fig. 3.33. 

Microporous membranes are often used in many processes to remove impurities or contaminants through size-selective filtration. The breath figures method also finds application in this field, specially the approaches that facilitate the easy transfer to other supports. Another prerequisite is the formation of through pores that penetrate from the top of the layer to the bottom and the use of ice support favors this fact. For example, highly uniform membranes of PS-h-PDMAEMA have been prepared with pores on the micrometer scale for size-selective separation. The films were prepared by casting at an air-ice interface and easily transferred onto other supports [219]. Miktoarm star copolymers with proper water wettability and mechanical stability have been used to fabricate separation membranes also using ice substrate [131]. Moreover, the breath figures approach has been employed to build polymer membranes on structured substrates in order to obtain hierarchically structured microsieves [208]. 

Selection of Filter Material. The basic requirement for filter fabrics is that they should provide cleanup at the maximum filtration velocity with the lowest possible hydraulic resistance. Moreover, the fabric must be resistant to the effects of high temperatures and aggressive media it must be mechanically strong and must have an adequate dust-holding capacity, i.e., it must retain on its surface a certain quantity of adherent dust in the form of a layer that can be removed readily by mechanical action. 

Fabric Filters The most common filtration devices are fabric filters. There are many kinds of fabrics. Type of contaminant and temperature of the air are but two factors that affect selection. Some filters are in the form of tubes or stockings others have an envelope or pleated form. Air moves through the fabric bags and dust collects inside them. The more material that collects, the greater the efficiency, the smaller the particles collected and the higher the pressure drop across the filter. 

Fabrication of cocontinuous PS/PLA blends with a gradient porous structure (GPS) for selective applications (e.g., graded membranes for enhanced filtration and graded porous beams for structural applications) was studied by Yao et al. [40]. They melt blended PLA (4032D, = 330 kDa)... 

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