The principles of bag house operation may be straightforward, but the design of a baghouse is complicated. Baghouse designers must carefully evaluate volumetric gas flow and velocity, static pressure drop, and gas-to-cloth ratio, in actual cubic feet per minute per square foot of filtration area (acfm/ft 2), to obtain optimum removal of flue gas particulates—from submicron to several hundred microns in diameter—with collection efficiency in excess of 99.9%. Fuel combustion products, operating temperature, dust loading, physical characteristics of the particulates, and the dew point of any condensable in the gas stream also play a vital role in the choice of the fabric material and bag design, particularly in coal-fired plants using coal that produces flue gas with high dust content.
Particulate capture occurs best on the surface of the filter media of fabric filter units, as the gas passes through the filter fabric and the clean gas is vented to atmosphere. Exhaust gas is drawn through the filter media forming a dust layer on the surface of the fabric media. When the prescribed pressure drop is reached, the filter bags are cleaned, either online or offline. Particulates captured on the surface of the filter fabric are typically separated by a short upstream burst of high-pressure air to remove the dust particulates from the filter media surface. The particulates are subsequently collected in a hopper found at the bottom of the bag house enclosure for disposal.
Whenever the dust collection occurs through the entire section of the filter media, bag houses experience premature pressure drop and an increase in emissions. It is therefore paramount to keep the particulate capture on the surface of the filter media (we call it surface-oriented filtration).
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