Ordinarily, the air compressor oil filter is a coarse filter installed to the inlet of the oil pump, thereby avoiding the impurities' entering into the pump. This kind of filter is simple in structure. It has low resistance but large oil flow. The high-flow filer is fixed onto the oil return pipe of the hydraulic system, for filtering the metal particles, plastic impurities, etc. A chief use of this type filter is to maintain the returned oil cleanness inside the oil tank. Duplex filter features simple structure and convenient use. Apart from the bypass valve, it is also equipped with blocking or pollution warning device, so as to ensure the system safety.