The bag filters compose a wide category of filters which includes the classes of M5 to F9. A bag filter consists of a number of bag fabricated by densely weaved polyester fibers. The bags are joined within a frame of galvanized metal sheet. Due to the densely weaved polyester fibers, the bag filters can constrain air floating particles, therefore can be used as terminal filters in residential or industrial facilities, but also as pre-filters to absolute filters when high percentage of particulate restriction is required (hospitals, medical industry, etc.). They are distributed mainly in standard dimensions in order to be attached on the air ducts with the relative dimensions.