Oil-injected screw compressors – positive displacement rotary screw compressors are constructed of one or more screw shaped helical rotors that are matched, meshed, and work together, which draw air into the compressor housing into the coils of the interlocking rotors. As outside air flows along the axis of the screw rotors through the system, it is increasingly compressed and discharged at the desired pressure. Similarly to piston compressors – the air is compressed using the displacement method (i.e. reducing the volume of the sucked air portion of air). Oil-injected compressors combine oil with pressurized air flowing through the system, then filter it before discharge and return the oil to the oil separator tank for further use. Regular maintenance of these types of compressors includes routine replacement of oil, filters and air/oil separator. These are available in single stage and two stage types.