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In Iowa, fabric filters, commonly called baghouses, are widely used industrial strength "vacuum cleaners." They remove particulate matter found in smoke, vapors, dust or mists.
The filters remove particles from exhaust gases, leaving the particles on the filter while the cleaner air passes through. Collected particulates form a "dust cake" on the filter that is routinely cleaned off by a blast of air in the opposite direction or by mechanical shaking. The dust cake falls into a hopper for disposal or reuse in the industrial process.
Filter bags hang in a sturdy house. Sometimes the house is insulated when cleaning hot gases to prevent corrosive moisture or acid mists from condensing and harming the equipment. Sometimes dozens, even hundreds of cylindrical filters, each eight to 40 feet long may hang in a series of houses at one location.
Filters are made of woven cotton, wool or synthetic materials. Some synthetic materials can withstand high temperatures or are resistant to chemical reaction. Each baghouse must meet the needs of the particular industry process. Gas temperature, moisture content and chemical reactivity decide what filter material is used.
When an industry applies for an air quality permit to install a baghouse, DNR air quality engineers must determine if the baghouse has enough filters and surface area to remove an adequate amount of pollutants before issuing the permit. The DNR also reviews the design to ensure the proposed fan size is adequate to pull or push air through the series of filters. This is determined by the "pressure drop" or the measurement of air resistance across the filters. This can be learned from similar baghouses or calculated using mathematical equations and laws of physics.
A baghouse with a high-pressure drop needs a larger fan or more energy to move dirty air through the filters. Proper design, reflected in a good permit, helps ensure the right equipment is installed to remove enough pollutants. This can reduce the number of operating problems when using the baghouse and prevent possible air pollution violations. DNR staff also reviews the bag cleaning method, hopper design, and other factors.
Effectively removes large percentages of particulate matter.
Bag wear or failure, holes in the house can reduce the 'vacuum' and overall efficiency. Collected dry materials must be carefully handled to prevent release into the air.