Fabric filters materials used

Because of their inherently high efficiency on dusts in all particle-size ranges, fabric filters have been used for collection of fine dusts and fumes for over 100 years. The greatest limitation on filter application has been imposed by the temperature limits of available fabric materials. The upper limit for natural fibers is about 90°C (200°F). The major new developments in filter technology that have been made since 1945 have followed the development of fabrics made from glass and synthetic fibers, which has extended the temperature limits to about 230 to 260°C (450 to 500°F). The capabihties of available fibers to resist high temperatures are still among the most severe limitations on the possible applications of fabric filters. 

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. 

Semidry Scrubbers The advantage of semidry scrubbers is in that they remove contaminants by way of a solid waste that is easier to dispose of (less expensive). Initially, the scrubbing medium is wet (such as a lime or soda ash slurry). Then a spray dryer is used to atomize the slurry into the gas which evaporates the water in the droplets. As this takes place, the acid in the gas neutralizes the alkali material and forms a fine white solid. Most of the white solids are removed at the bottom of the scrubber while some are carried into the gas stream and have to be removed by a filter or electrostatic precipitator (discussed later). Although semidry systems cost 5-15% more than wet systems, when combined with a fabric filter, they can achieve 90-95% efficiencies. Dry scrubbers are sometimes used in a very similar fashion, but without the help of gas-liquid-solid mass transfer, these systems use much higher amounts of the solid alkali materials. 

The vast increase in the application of membranes has expanded our knowledge of fabrication of various types of membrane, such as organic and inorganic membranes. The inorganic membrane is frequently called a ceramic membrane. To fulfil the need of the market, ceramic membranes represent a distinct class of inorganic membrane. There are a few important parameters involved in ceramic membrane materials, in terms of porous structure, chemical composition and shape of the filter in use. In this research, zirconia-coated y-alumina membranes have been developed using the sol-gel technique. 

PTFE, known under the trade names Teflon and Fluon, is resistant to all chemicals, except molten alkalies and fluorine, and can be used at temperatures up to 250°C. It is a relatively weak material, but its mechanical strength can be improved by the addition of fillers (glass and carbon fibres). It is expensive and difficult to fabricate. PTFE is used extensively for gaskets and gland packings. As a coating, it is used to confer non-stick properties to surfaces, such as filter plates. It can also be used as a liner for vessels. 

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. 

See reference 36 for discussion of various types of dust collection equipment. Fabric filters will be used in order to recover all the fines in a dry state. These should recover 99% of the material larger than 0.2 microns.36 A continuous-envelope fabric filter will be used, since this seems to have the lowest annual costs. For this filter ft3 of surface area is required for about each 2 ft3/min of air. 

Fabric filters filters made from fabric materials and used for removing particulate matter from gas streams (see Baghouse). 

Filter Mediums are materials used for filtering, such as filter paper, filter pulps, cotton fabrics, cotton ducks, cotton twills, cotton chain, cotton batting, wool cloths... 

Example 7.8. Steinhart et al. [288] used the formation of a precursor film to fabricate nanotubes of polymers. For this purpose a porous alumina filter with cylindrical holes of defined size is brought into contact with a polymer melt (Fig. 7.17). This is done at a temperature, where the polymer is liquid, which is significantly above room temperature. The precursor film of the polymer wets the walls of the pores within, typically a few seconds. Before the pores are filled completely the filter is removed from the melt, the sample is cooled to room temperature, and the filter material is dissolved in potassium hydroxide. 

Filters can be grouped on the basis of different characteristics. For example, they can be grouped on the basis of the design capacity of the filters according to the particle concentration or loading [Svarovsky, 1981] or they can be grouped on the basis of the materials of the filters, e.g., fabric or nonfabric [Cooper and Freeman, 1982], Most filters in use are bag filters, which are fabric. The common fabric materials include cotton, polyester, wool, asbestos, glass, acrylic, polytetrafluoroethylene (Teflon), poly (m-phenylene isophthalate) (Nomex), polycaprolactam (Nylon), and polypropylene. 

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