wet
　　Include oil immersion type and oil bath type two. Oil-immersed filter is through an oil-immersed filter element, the impurities in the air are separated, the filter element material has metal silk fabric, also has foam material. Oil bath type is to suck in dusty air into the oil tank and remove most of the dust, and then in the air with oil mist upward flow through a filter element by wire winding for further filtration, oil droplets and the dust stopped together back to the oil tank.