Fabric filters cleaning

A wide variety of woven and felted fabrics are used in fabric filters. Clean felted fabrics are more efficient dust collectors than woven fabrics, but woven materials are capable of giving equal filtration efficiency after a dust layer accumulates on the surface. When a new woven fabric is placed in service, visible penetration of dust may occur until buildup of the cake or dust layer. This normally takes from a few hours to a few days for industrial applications, depending on dust loadings and the nature of the particles. 

FIGURE 53.3 Fabric filters (a) large, multicompartment fabric filter cleaned by mechanical shaking. (Courtesy of Wheelabrator-Shint.) (b) Envelope-type fabric filter cleaned by reverse flow. (Courtesy of Shinto Dust Collector.)... 

Cleanable Granular-Bed Filters The principal objective in the development of cleanable granular-bed filters is to produce a device that can operate at temperatures above the range that can be tolerated with fabric filters. In some of the devices, the granules are circulated continuously through the unit, then are cleaned of the collected dust and returned to the filter bed. In others, the granular bed remains in place but is periodically taken out of service and cleaned by some means, such as backflushing with air. 

Fabric filters are limited by physical size and bag-life considerations. Some sacrifices in efficiency might be tolerated if higher air-cloth ratios could be achieved without reducing bag life (improved pulse-jet systems). Improvements in fabric filtration may also be possible by enhancing electrostatic effects that may contribute to rapid formation of a filter cake after cleaning. 

Typical new equipment design efficiencies are between 99 and 99.9%. Older existing equipment have a range of actual operating efficiencies of 95 to 99.9%. Several factors determine fabric filter collection efficiency. These include gas filtration velocity, particle characteristics, fabric characteristics, and cleaning mechanism. In general, collection efficiency increases with increasing filtration velocity and particle size. 

Fabric filters have had a long history in performing effectively in a wide variety of applications. Common applications using pulse-jet cleaning systems include ... 

Cost estimates are presented below for pulse jet cleaned fabric filters. The eosts are expressed in fourth quarter 1998 dollars. The cost estimates assume a conventional design under typical operating eonditions and do not inelude auxiliary equipment... 

ICAO 1999. Institute of Clean Air Companies internet web page www.icac.com. Control Technology Information Fabric Filters, page last updated January 11, 1999. STAPPAIALAPCO, 1996. State and Territorial Air Pollution Program Administrators and Association of Local Air Pollution... 

Combined Cyclonic and Fabric Filtration There are a number of arrangements where separate cyclones and fabric filters are combined into a single design to enhance gas cleaning. The side figure provides an example. 

The most common equipment for cleaning recirculated air from particles is fabric filters, mechanical collectors, electrostatic precipitators, and cleaners and wet collectors.For cleaning of recirculated air from gases, absorbers and adsorbers such as activated carbon, sometimes with impregnation for specific gases, and impregnated alumina are most common. The performance of different air cleaning equipment is described in many textbooks and handbooks. 

Air cleaning (dust collection) can be cost effective for LVHV systems handling valuable dusts. Care must be taken when handling potentially toxic dusts from air cleaners. Regular, routine reconditioning of fabric filters (e.g., by automatic shaking or pneumatic pulsing) is impottant. This can be accomplished on a set maintenance schedule or as a function of pressure drop across the fabric filter. It is not recommended to recirculate airflow back to the workplace because of the low air volume and potential hazards in the event of filter failures. 

This type of bag-cleaning method is a fundamental characteristic of this type of collector. Terminology in the fabric filter field is not totally consistent or comprehensive. Table 13.2 presents acceptable definitions for common fabric filter terminology. 

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