Dust Control Handbook for Industrial Minerals Mining and Processing

times. When considering the depth of loading onto felted fabrics, the filter bag is actually actingas a support for the dust cake. Dust is allowed to penetrate into the fabric to form a layer of duston the bag. Thus, the dust creates a "porous" cake with much smaller air passages, allowing thedust to filter itself. Without this cake, the filter would bleed through and never obtain acceptableoutlet emissions. Therefore, in the design and operation of a baghouse using felted media, it isimportant to use appropriate air to cloth ratios, cleaning cycles, and cleaning pressures. Theoptimum collector operation maintains a constant pressure drop of 3–6 inches wg and minimizescleaning cycles and compressed air usage. This does not mean that a system running higher thana 6-inch pressure drop is not operating satisfactorily. Some systems can operate effectively athigher pressure drops. The system is operating correctly as long as the pressure drop is constantand the required air volume is being achieved.Surface loaded media do not require a dust cake to achieve optimum performance. As the nameimplies, the dust is collected on the very surface of the fabric. Some surface loaded media usemembranes composed of a very thin layer (1 mil) of PTFE (polytetrafluoroethylene) materialthat is laminated to a substrate. This substrate material is a conventional felt. The laminate isapplied in a special process that controls the size of the pore openings; thereby the collectionefficiency is predictable. These pore openings are so small that they will only allow extremelysmall submicron particles to pass, making these fabrics extremely efficient.Dust collectors using felted media typically run higher pressure drops than those operating withsurface-loaded media because the filter or dust cake adds resistance to the system. As previouslymentioned, the pressure drop tends to range from 3–6 inches wg, while surface-loaded mediaoperating without a filter cake typically run in the 2–4 inches wg range.The standard fabrics used in shaker collectors are woven cotton and polyester sateens andlightweight felts. Pulse jet and reverse air collectors use felts ranging in weight from 14 to asheavy as 22 ounces. Woven fabrics are not used in the last two types of collectors because thecleaning system is much more aggressive than shaking. Pulsing woven fabrics opens up theweave and causes leakage and undesirable emission levels. Cartridge filters typically usecellulose/synthetic blends and spun-bonded polyester media.When matching fabric choice to dust collection needs, the basic criteria are temperature, inletloading, particle size distribution, gas composition, abrasion, static charge, release, andefficiency requirements. The following are some basic guidelines for each criterion.1. Temperature. Inlet gas temperature should be matched with a fabric capable ofcontinuous operation at or above the maximum temperature the system might experience.2. Inlet loading. Inlet grain loading (normally expressed in grains/dry standard cubic feet)is especially important when the loading is very light. Light loadings can make itdifficult for the media to operate with sufficient dust cake.3. Particle size distribution. Finer particle size distributions often require using higherefficiency media.4. Gas composition. The designer must make sure that if corrosive compounds orhydrocarbons are present in the airstream, then the media selected will be able towithstand that environment.Fundamentals of Dust Collection Systems 47