FEATURE
Fabric filters abate air emissions Frank R. Culhane Wheelabrator-Frye Inc., Pittsburgh, Pa. 15222
Lower in energy requirements and cost in certain applications, they are preferred over other air pollution control devices The highly efficient fabric filter is one of three air pollution control devices which was, and is, broadly and successfully applied throughout industry. Design and application engineers have incorporated engineering features into the fabric filter, enabling it to be successfully utilized in areas routinely considered the realm of the electrostatic precipitator and scrubber. I t is a device which has grown from a method of product recovery in the late 1800's to a sophisticated machine widely applied today to meet a variety of air pollution control problems. Tracing its development Stack losses--smoke, dust and/or fume-consisting of either semifinished or finished product values created the proper economic climate for the adaptation and development of the baghouse as a means of effectively separating finely divided solids from the carrier gas stream. The nonferrous metallurgical and carbon black industries are cases in point. The carbon black industry went through a 5-year period in the late 40's and early 50's during which considerable pilot plant and prototype baghouse work established the necessary application and design parameters required for successful operation of fabric filters in a very severe and harsh environment. After this period, the baghouse became the exclusive means of gas cleaning throughout the carbon black industry, achieving virtually 100% recovery of fine particulate carbon black from stack gases. Much earlier, at the turn of the century, the same economic reasoning and justification by the nonferrous discipline of the metallurgical industry led to the adaptation and development of the baghouse for recovery of stack metal losses. The magnitude of these losses range between 5 and 6 % of the furnace feed for most nonferrous pyrometallurgical processes. Large tubular bags, fabricated of wool, mounted in a hopper, connected to a flue, and enclosed in a brick house, best recovered these stack values. As community pressure increased the demand to control air pollution to maintain the improve the quality of life, the function of the baghouse was changed from a device used for product recovery to an-effective means of stack gas cleaning. Commercially, the bsghouse became known as a fabric filter or fabric collector.
Figure 1 Woven-type lowenergysystems
Most established manufacturers of industrial gascleaning equipment have added fabric filters to their existing line of equipment, providing a greater degree of equipment selectivity, and offering the optimum and best means of solving a specific emission problem. Today the process, plant, and consulting engineers have a broad choice between a commerical baghouse. the high-energy scrubber, and the electrostatic precipitator in determining the most effective and best means of solid particulate control. The prime growth and development of the fabric filter has followed a corresponding technical growth of synthetic fibers. The early and traditionaL barriers to baghouse development have been operating limitations of temperature, fiber resistance to chemical attack, and fiber dimensional stability and sustained tensile and flex strength. These limitations have dictated the design of the system. Initially, the only available fibers-cotton and woollimited operating temperatures to either 180 or 220°F, although their stability formed ideal grid systems. These systems support and hold a sufficient quantity of the gasseparated particulate as a filter cake, preventing bleeding during filtration and puffing after periodic cleaning. The first man-made fiber-rayon-became available in the twenties. This enabled experimenting with a. WOOl-raYOn combination which'provided greater tensile strength and increased the abrasion resistance qualities of the wool. Nylon was introduced in the thirties, and offered greater abrasion resistance than either cotton or wool but did not increase the operating temperature significantly. These fibers added the filament fiber form to the fabric, thereby increasing the ease of cake removal during periods of cloth cleaning. The greatest stride in fiber development came in the late 40's with the advent of acrylic, polyester, and mineral fibers. In 1946, the upper temperature operating limits were increased from 220' to 275'F through the application of acrylic fibers. This increase of 55' represented a major application breakthrough for the baghouse, ailowing the use of spray towers for thermal control, with resulting dew points in the 150-165°F range. In 1953, through improved finishing of fiber glass fabric, the upper operating temperature limit was further increased from 275'to 55OOF. Research and development continue on fiber improvement with an emphasis toward the increase of operating temperature limits beyond 550'F and the reduction of thermal control requirements. However, regardless of higher operating temperatures of the baghouse. the design engineer must consider the trade-off between the savings of gas cooling and the increase in gas-cleaning cost because of the increase in gas volume. if, on the other hand, gas cooling is carried to an extreme to reduce the gas volume, increased chemical attack is experienced as the acid dew point is approached.
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Uses and limitations The characteristics of most submicron particles to agglomerate have resulted in the ability of filter fabrics to remove 99.9% by weight of submicron solid particles: This high order of fine particulate efficiency is routinely being achieved in separating; carbon, as fine as 320 A, from process reactors silicon dioxide, in 0.02-0.25-p range, from submerged arc furnaces 100% of particles below 5 p and 95% below 2 p , in ferrousfume emissions
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Multi.stage
Figure 2 Felt-type highenergy systems
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istribution baffle
Environmental Science & Technology ~
Figure 3. Primary alummum reduction plant uses fabric fillers
Submicron fume is molecular in size and the ability of the baghouse to remove it is due to the filter cake formed by proportional number of like agglomerated particles. The finer particles cause a tighter filter cake, resulting in greater head loss per unit of gas flow in comparison with a cake formed from coarser dust particles. A critical factor in sizing a baghouse is the resultant pressure drop across the filter fabric. Within reasonable
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cloth resistance, the filtration efficiency for all practical purposes is constant. The selected filtering velocity varies from application to application, depending upon the filtration characteristics of the dust and fume particulate burden. In general, the pressure drop across the cloth will vary directly with the time between cloth-cleaning periods, exponentially with the filtering velocity, and directly with the filterability constant. The filterability of any given quantity of particulate depends on the particle size and distribution, shape, surface properties, and electrostatic forces. Experience and application know-how are the final determining factors regarding sizing of a baghouse for a given application. The combination of the variables affecting the sizing of cloth collectors are many, and it is necessary to work from known results in like applications, and to allow for unknowns when direct experience is not available. For new applications, a 500-2500 cfm pilot unit will provide specific answers to filtration characteristics of the dust and fume in question. Most new baghouse applications have developed through pilot test units. Current work being done on coal-fired boilers in the power plant industry for the collection of fly ash has been preceded by extensive pilot plant work. It is broadly accepted within the industry that the baghouse will not work for gas below the dew point. If, however, other factors eliminate a precipitator or scrubber, and if a high degree of dust and fume recovery is critical, an external source of heat may be indirectly or directly
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Characteristics of haghouse design Cloth-cleaning ctassification
General description
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Filter type Media. . . 'Cloth-cleaning description
Filtration stops Spacing requirements
Low-energy systems
The filter fabric is formed into tubes, opened a t onc end an0 closed at tne otner. 1 hc open cnn is mointed into a cell plate or tnirnb e floor wnicn is located over a troueh. tvDe hoDPer. in a sinde comoartment. Several cbrnpartmenis are manifolded into a single baghouse. The gas flows throJgh a dirty gas :nlet manifold into the hoppers and floars Joward into the bottom of tne tube. Preseparation of coarse particles in the hoppers is thus achieved, beneficially reducing the load to the fabric. The flow pattern is usually from the inside of the bag outward, depositing the dust and fume inside the bag, and enabling inspection and maintenance from the clean-gas side of the baghouse Surface filter Woven fabric Filament fiber fabric is usually cleaned by repressuring. Staple fiber fabric is usually cleaned by mechanical shaking. A combination of both repressuring and shaking is used for difficult-to-remove particulates (Fieure . - 1). Filtration is interrupted during periods of cloth cleaning. achieved by use of compartment dampers Sufficient centerline spacing between tubes is necessary to prevent rubbing and to provide unrestricted gas flow from cloth to compartment outlet duct. For example, 6. in, centers on 5 4 . diameter bags, 9-in. centers on 8-in. diameter bags, and 14-in. centers on 1l1/*-icT.diameter bags are recommended
High-enerpy systems
The filter fabric is formed into tubes, opened a t one end and closed at rhe other. The open end is mounted in a tnimble ceiling with tne c osed end naneine verticallv over a homer, in a sinde i &a c The rra
