Fabric filters, also commonly referred to as bag houses, are used in many industrial applications. They operate similar to a vacuum cleaner. The Dust-laden gases pass through fabric bags where the particulates are trapped on the fabric surface.
After enough dust layer is built up on the filters (which indicated by buildup in pressure across the fabric) it is then periodically removed by blowing high pressure (compressed) air back through the fabric, reversing the flow of gases, or shaking the fabric.
Dust from the fabric then falls to a collection hopper from which it is removed periodically. The layer of dust accumulating on the bag is usually referred to as the dust cake. Fabric filters, collect particles with sizes ranging from sub micron to several hundred microns in diameter at efficiencies generally in the range of 99 or 99.9 percent.
The particle laden flue gas leaves the boiler and heat recovery unit and enters the filter inlet plenum which in turn distributes the gas to each of the fabric compartments for cleaning. An outlet plenum collects the cleaned flue gas from each compartment and directs it toward the induced draft fan and then to stack. Inlet and outlet dampers are provided which allow isolation of each compartment for bag cleaning and maintenance.
Each compartment has a hopper for inlet gas flow as well as for particulate collection and removal. The individual bags are closed at one end and connected to a tube sheet at the other end from which the gas passes through the bag assembly.
Collection of the particulate on the bag fabric is very important as it increases the filter efficiency. The major forces leading to this collection include impingement by direct contact and dust cake sieving, minor forces which assist in the collection are diffusion, electrostatic forces, Vander WaaIs forces and gravity.
The dustcake is formed by the accumulation of particulate on the bags over an operating period. Once formed, the dustcake itself provides most of the filtration, however impingement collection is most effective on the larger particles and the sieving process collects all particle sizes, and a dustcake maximize overall collection. As the dustcake builds the flue gas pressure drop increases across the fabric filter, so at this point the bags must be cleaned.
This occurs after a predetermined operating period or when the pressure drop reaches a set point. Each compartment is then sequentially cleaned to remove the excess dustcake and to reduce the pressure drop. A residual dust coating can be used to enhance further collection in pulse jet fabric filters.
The bags are tightly packed into one or more filter compartments with one compartment normally off-line for cleaning. Most bag houses contain as many as ten or more compartments with several hundred bags per compartment.
Fabrics used in bag houses can be made of a number of different materials such as paper, cotton, Nomex, polyester, fiberglass, Teflon, and even spun stainless steels. The filter bag selection depends on the corrosive properties of gases, dust loading, particle size and importantly temperature of dust laden gases. The area of filter bag is decided by air to cloth ratio which determines pressure drop and collection efficiency.
Fabric filters are used where high efficiency Particle collection is required. Limitations are imposed by gas characteristics such as temperature, corrosiveness and particle characteristics (primarily stickiness) that affect the fabric or its operation and that cannot be economically accommodated. Also for very large volumes, the operating cost is very high since the pressure drop is high.