Oil-free piston compressors have piston rings made of PTFE or carbon, and alternatively, the piston and cylinder wall can be profiled (toothed) as on labyrinth compressors. Larger machines are equipped with a crosshead and seals on the gudgeon pins, and a ventilated intermediate piece to prevent oil from being transferred from the crankcase and into the compression chamber. Smaller compressors often have a crankcase with bearings that are permanently sealed.
Modern, high-speed, oil-free screw compressors have asymmetric screw profiles, resulting in significantly improved energy efficiency, due to reduced internal leakage. External gears are most often used to synchronize the position of the counter-rotating rotors. As the rotors neither come into contact with each other nor with the compressor housing, no lubrication is required inside the compression chamber. Consequently, the compressed air is completely oil-free. The rotors and housing are manufactured with ultimate precision to minimize leakage from the pressure side to the inlet.
A scroll compressor is a type of (usually) oil-free orbiting displacement compressor, i.e. it compresses a specific amount of air into a continuously decreasing volume. The compressor element consists of a stator spiral fixed in a housing and a motor-driven eccentric, orbiting spiral. The compression cycle is in progress for 2.5 turns, which virtually gives constant and pulsation-free air flow. The process is relatively silent and vibration-free, as the element has hardly any torque variation as compared to a piston compressor,
Diaphragm compressors form another group. Their diaphragm is actuated mechanically or hydraulically. The mechanical diaphragm compressors are used with a small flow and low pressure or as vacuum pumps. Hydraulic diaphragm compressors are used for high pressure applications.