Dust collection fabric types

Fabric-filter systems, commonly called bag-filter or bag-house systems, are dust-collection systems in which dustladen air is passed through a bag-type filter. The bag collects the dust in layers on its surface and the dust layer itself effectively becomes the filter medium. Because the bag s pores are usually much larger than those of the dust-particle layer that forms, the initial efficiency is very low. However, it improves once an adequate dust-layer forms. Therefore, the potential for dust penetration of the filter media is extremely low except during the initial period after startup, bag change, or during the fabric-cleaning, or blow-down, cycle. 

The frequency of cleaning depends on the nature and concentration of the dust. Typical cleaning intervals vary from about 2 to 15 minutes. However, the cleaning action of the pulse is so effective that the dust layer may be completely removed from the surface of the fabric. Consequently, the fabric itself must serve as the principal filter media for a substantial part of the filtration cycle, which decreases cleaning efficiency. Because of this, woven fabrics are unsuitable for use in these devices and felt-type fabrics are used instead. With felt filters, although the bulk of the dust is still removed, an adequate level of dust collection is provided by the fabric until the dust layer reforms. 

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. 

This chapter will provide the reader with a general introduction to the more common types of solid-gas (dust collection) and solid-liquid filtration mechanisms. The raw materials, polymers, fibres, and different types of fabric construction employed in filter media manufacture and some typical fabric finishing processes are discussed in detail. A brief look at filter market developments will also be considered. 

At the entering end of the drum, at the higher end, the dust collection system is fitted. Nowadays, there are basically two types of dust collection systems - the cyclone type and the backhouse type (use of fabric filters) the wet collection system used in the past is almost obsolete. 

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. 

Because of the wide diversity of thermal and chemical conditions that prevail in dust collectors, the correct choice of fibre type is the most fundamental of all considerations when designing or selecting the filter fabric. A selection of the more common fibre types that are used in dust collection and their properties is shown in Table 7.2. 

The aim of the laboratory experiments is to determine the influence of an upstream iorrization rrrrit on the dust collecting efficiency of a fabric filter, plate-type precipitator and scrubber irsing the ECOPOL iorrization injector. 

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