Bag filtration

Figure 4-65. Bag filtration with mechanical shaking for bag cleaning. Courtesy of Dracco Div. Fuller Co.

Figure 4-66. Bag filtration with continuous reverse air cleaning. Courtesy of W. W. Sly Mfg. Co.

Filter bags for liquids are effective in removing particulate contaminants in the processing of a wide variety of materials. Bag filtration is a pressure-driven operation, wherein the product to be filtered is forced through the filter unit. Such a unit would normally consist of the filter housing, a retaining basket and the filter bag. 

Bag filtration used in electrocoating of metal parts has proved successful for a number of reasons. These include ... 

The carbon black (soot) produced in the partial combustion and electrical discharge processes is of rather small particle si2e and contains substantial amounts of higher (mostly aromatic) hydrocarbons which may render it hydrophobic, sticky, and difficult to remove by filtration. Electrostatic units, combined with water scmbbers, moving coke beds, and bag filters, are used for the removal of soot. The recovery is illustrated by the BASF separation and purification system (23). The bulk of the carbon in the reactor effluent is removed by a water scmbber (quencher). Residual carbon clean-up is by electrostatic filtering in the case of methane feedstock, and by coke particles if the feed is naphtha. Carbon in the quench water is concentrated by flotation, then burned. 

In 1992, as much as two-thkds of all spunlace fabrics were used in medical products other appHcations included wipes, industrial apparel, intedinings, absorbent components, filtration, and coating. Medical product appHcations also accounted for about one-thkd of all wet-laid nonwovens other appHcations included tea bags, meat casings, filter media, battery separators, and wipes. Most bonded-pulp fabrics were used as wipes or absorbent components. 

PPS fiber has excellent chemical resistance. Only strong oxidising agents cause degradation. As expected from inherent resia properties, PPS fiber is flame-resistant and has an autoignition temperature of 590°C as determined ia tests at the Textile Research Institute. PPS fiber is an excellent electrical iasulator it finds application ia hostile environments such as filter bags for filtration of flue gas from coal-fired furnaces, filter media for gas and liquid filtration, electrolysis membranes, protective clothing, and composites. 

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. 

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 next stage may best be described as a primitive hot-filtration process. Two members of the village sit across the front of a simple fire resembling a Dutch oven, holding between them a bag about 30 feet long and about two inches in diameter. The lac inside the bag melts and, through one of the operators twisting the end of the bag, the lac is squeezed out. The lac is then removed from the outside of the bag and collected into a molten lump which is then stretched out... 

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