Compressor oil is a lubricant used to prevent wear, minimize corrosion of metals and improve compression efficiency. Oils vary by thickness or viscosity, the types of additives used, and whether they are natural or synthetic oils. They can circulate with the gases being compressed, such as in air conditioning systems, or be separate, as in oxygen compressors.
Wear resistance is important, because many compressors operate continuously and can wear out quickly without adequate lubrication. Chemicals called wear additives reduce the metal contact of moving parts. The correct viscosity, or thickness, of the compressor oil is important because the oil acts as a sealant, and reduces the amount of gas that leaks around the compressor pistons, called blow-by.
Synthetic compressor oil is made from artificial lubricants rather than natural petroleum oils. Different molecules are mixed to provide wear protection, proper viscosity and high-temperature stability. Compressors for oxygen and other gases that can cause combustion may use synthetic oil because mixtures can be used that do not burn in the presence of oxygen.
Many systems, such as used for air conditioning and refrigeration cooling, circulate oil and refrigerant in the system as a mixture. The compressor oil provides lubrication while it is inside the compressor, but is not stored there. Moving lubricant through a system can provide cooling for the compressor, because the hot oil will be cooled in the heat exchangers before returning. These systems can also be less expensive to produce, because extra equipment is not needed to keep the oil separated.
Oil-free systems are needed for locations where the presence of compressor oil can cause a fire or contaminate the gas supply. These systems are used for compressing pure oxygen, and are also used for breathing air that is for safety equipment or underwater diving. Sensitive laboratory instruments may also need oil-free air supplies to prevent equipment damage.
Maintaining oil levels in some systems is essential to long service life. Most refrigeration systems are sealed, and only require oil maintenance if a leak occurs. Larger refrigeration systems and commercial gas compressors may use an oil supply tank, where compressor oil is kept in storage until needed by the system. This allows replacement oil to be added without having to shut down the compressor.
Starting in the late 20th century, synthetic oils became more popular for some compressor systems. This was largely due to the ability to create or formulate specific oil characteristics for industrial, medical and consumer uses. Synthetic oils were sometimes more expensive, but better oil stability allowed them to remain in compressors for longer periods before changing was required. This could result in lower costs from the oil itself and lower maintenance labor costs.