A bag filter or fabric filter is an air pollution control device and dust collector that removes particulates out of air or gas released from commercial processes or combustion for electricity generation. Power plants, steel mills, pharmaceutical producers, food manufacturers, chemical producers and other industrial companies often use bag filter to control emission of air pollutants. Bag filter came into widespread use in the late 1970s after the invention of high-temperature fabrics (for use in the filter media) capable of withstanding temperatures over 350 °F (177 °C). Unlike electrostatic precipitators, where performance may vary significantly depending on process and electrical conditions, functioning baghouses typically have a particulate collection efficiency of 99% or better, even when particle size is very small. Operation Most bag filter or fabric filter use long, cylindrical filter bags (or tubes) made of woven or felted fabric as a filter medium. For applications where there is relatively low dust loading and gas temperatures are 250 °F (121 °C) or less, pleated, nonwoven cartridges are sometimes used as filtering media instead of bags. Dust-laden gas or air enters the dust collector through hoppers and is directed into the bag filter compartment. The gas is drawn through the filter sleeve, either on the inside or the outside depending on cleaning method, and a layer of dust accumulates on the filter media surface until air can no longer move through it. When a sufficient pressure drop (ΔP) occurs, the cleaning process begins. Cleaning can take place while the bag filter is online (filtering) or is offline (in isolation). When the compartment is clean, normal filtering resumes. Bag filters are very efficient particulate collectors because of the dust cake formed on the surface of the bags. The fabric provides a surface on which dust collects through the following four mechanisms: Inertial collection – Dust particles strike the fibers placed perpendicular to the gas-flow direction instead of changing direction with the gas stream. Interception – Particles that do not cross the fluid streamlines come in contact with fibers because of the fiber size. Brownian movement – Sub micro meter particles are diffused, increasing the probability of contact between the particles and collecting surfaces. Electrostatic forces – The presence of an electrostatic charge on the particles and the filter can increase dust capture. A combination of these mechanisms results in formation of the dust cake on the filter which eventually increases the resistance to gas flow. The filter must be cleaned periodically.
Types Bag filters are classified by the cleaning method used. The three most common types of bag filters are mechanical shakers, reverse gas, and pulse jet.
