Wherever located, an air stream is established that draws the dust from its source. The main objective of an extraction plant are high collection efficiency of the filter fabric and the maintenance of an acceptable resistance through the filter material. If the resistance rises above a pre-defined design level, the extraction air volume will decrease and the desired extraction performance will diminish.

At the heart of correct filter design, assuming the selection of the right material for the particular dust in question, is the filtration velocity through the fabric material. It is the air volume divided by the filtration fabric area which equals filtration velocity. For example, a wood waste extraction plant with an air extraction volume of 5000 cu. ft min will typically have a filter with 833 square ft. of filter material thus resulting in a filtration velocity of 6 ft. per minute. It is essential that the correct velocity is applied, as too high a velocity will result in cloth blinding, permeation of the dust to the clean air side of the filter and drop in plant performance.

A layer of dust cake will build up on the filter fabric, which will improve filtration efficiency, but the 'cake' has to be kept to an optimum thickness to maintain filter design resistance. Excessive build up will increase the residue over the filters, which will act as a damper on the system and reduce suction performance.
 
 

The fabric cleaning maybe carried out by mechanical agitation of the filters or by more efficient reverse air pulse cleaning, which shocks the dust from the fabric. See diagrams which show the basic operational principles of the two types of filter.