The pre-filter is mainly used to filter dust particles above 5 μm, as the primary filtration of central air conditioning and centralized air supply system, plays the role of protecting the after end filter. The pre-filter usually has three styles: panel type, pleated type and pocket type, the frame material can be cardboard, aluminum, galvanized, etc., the filter material can be non-woven fabric, nylon mesh, activated carbon filter material, metal mesh, etc., and protective net can be double-side sprayed iron wire mesh and double-side galvanized wire mesh, filtration grade G1, G2, G3, G4,etc.