Compressor types can also be defined by lubrication, i.e. whether lubricated or non-lubricated. Non-lubricated compressors are usually referred to as “oil free”. ISO 8573-1 Class Zero (2010) provides for a purity level that ensures no oil is in contact with the compressed air, and typically indicates no oil is used in the compressor, at all. “Oil-less” is another nomenclature used to indicate no oil used in the compressor. Oil free and oil-less are about as good as it gets when it comes to quality of compressed air. (There are treatments available to remove impurities that are ingested through the intake of the compressor, but the compressor does not add any impurities to the air stream.) Both reciprocating and rotary compressors use special materials (TeflonR piston rings or coating of rotors, for instance) and sealing media such as water sealing between rotors. Timing gears are also used to keep rotors separated.
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