Fabric ( Bag ) Filters
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Fabric filters are widely used form of high efficiency ( . 99% ) dust collectors, even for sub-micron size dust. The basic principle of fabric filtration is to select a fabric media, which is permeable to gas but retains the dust up to 0.3 micron size. When the dust-laden gas is allowed to pass through an unit area of fabric, initially the fabric separates the particles developing a dust cake on the fabric surface. Then the dust dominates the filter mechanism. Different types of filter media with different chemical composition are developed and available in the market to suit the different gas temperatures and composition ( Table – B1).
As the dust cake thickness increases, the resistance to flow increases, hence it needs periodic cleaning. The most common methods of cleaning are mechanical / manual shaking and reverse air flow cleaning. Fabric filters are designed on the basis of an optimum filtration velocity which is defined as the maximum acceptable gas velocity flowing through a unit area of fabric, technically known as “Air to Cloth Ratio, M3/M2/hr). At this optimum gas velocity across the fabric an optimum pressure drop of 120 -150 mmWc is maintained. Based on the air to cloth ratio, the fabric filters are classified as;
• Low ratio Fabric Filters, Air to Cloth Ratio in the range 30 – 50 M3/M2/hr
• High Ratio Fabric Filters, Air to Cloth Ratio in the range 80 -130 M3/M2/hr
Low Ratio Fabric Filters
Most commonly, woven cloth is used as the fabric media for this type of fabric filters. The dust form a cake on the surface of the fabric and simple cleaning mechanism like vibration, shaking and air flow in the reverse direction ( reverse air flow ) are employed. For handling given volume of gas, large filtration area is required. To manage the air flow through the filter media, the required fabric area is stitched into number of cylindrical bags (sleeves) . The bag size is in the range of 150 – 200 mm diameter and 8 – 10 M long.
The bags are arranged on base plate vertically up ward in the metal housing and are closed and kept under tension at the top (Fig – B6). The dust-laden gas enters into the hopper and then in to all the sleeves, leaving the coarse particles in the hopper by force of gravity.
The remaining dust in the gas forms a cake on the inner surface of the bag and the clean gas leaves the bag from outer surface to the housing outlet, either by forced draft or by induced draft. To clean the bags periodically, it is essential to close the filtration process and dislodge the dust cake either by shaking or by reverse air flow. The cleaning process of the bags takes about 3 to 5 minutes time. Normally the bag houses are fabricated into number of compartment, so that one compartment can be taken out of operation for cleaning while other compartments are in filtration process. This type of cleaning is know as “Off-line Cleaning”.
High Ratio Fabric Filters
For high ratio fabric filters, non-woven cloth is used as the fabric media. The filter media has a base fabric and the fabric fibers are needled into the base. It forms the third dimension, thickness, to the fabric. The dust separation takes place, not only on the surface of the fabric but also over the fibers, inside the fabric. For cleaning the deposited dust, high pressure compressed air at 5 – 7 atm is required. For effective cleaning, the bag size is in the range of 100 – 150 mm diameter and 3 M long.
The bags are arranged on base plate vertically down ward in the metal housing and are supported by cages (Fig – B7). The dust-laden gas enters into the casing, leaving the coarse particles in the hopper by force of gravity. The remaining dust in the gas forms a cake on the outer surface of the bag and the clean gas leaves the bag from inner surface to the housing outlet, either by forced draft or by induced draft. To clean the bags periodically, high pressure compressed air at 5-7 atm is introduced at the open end of the bag on the top. The release of the compressed air is only for a fraction of a second with the help of solenoid valves and a diaphragm control. The dislodged dust cake from the surface of the bag, collected by the hopper. Since the cleaning of bags is achieved by a high pressure compressed air, these fabric filters are also known as “Pulse-jet bag filters” As the cleaning operation takes place only for a fraction of a second, no bag is necessarily taken out of filtration process. Hence the bag houses are fabricated in a single compartment. This type of cleaning is know as “On-line Cleaning”.
• Excellent high efficiencies for particles less than 1 microns
• Clean air often can be re-circulated
• Large volumes can be handled
• Dry dust collection and disposal or reuse
• Occupies large floor area
• Medium operation and maintenance problems
• Temperature, pressure and moisture content limitations- Pre treatment is required
• Handles limited inlet dust concentrations; Pre-collector is required
• Relatively high capital investment, operation and maintenance cost
• Required trained operators
• Not suitable for sticky and flammable dusts
• Involves imported fabric media