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.
Nearly all mobile compressors are diesel engine powered oil-injected screw compressors. Very small as well as very large mobile compressors are sometimes offered with electric motors. Oilfree mobile compressors are produced only by the world’s leading manufacturers, for service work in the process industry, utilities and offshore industry. and a low total weight. Operation in climates with medium to high humidity, in particular when the load/unload profile contains load cycles at high pressure or long periods of unload operation, will cause some of the atmosphere water content to condense inside the compressor oil circuit. This has a negative impact on the lubricated compressor components as well as on the oil itself. Having just 1% of water in the oil will reduce the useful life of the bearings by 40%. The most modern mobile compressors can be equipped with an electronic oil temperature control system to protect the compressor’s operating life.