The oil-free screw compressor element
No oil implies 
that there is no essential oil for sealing the rotors and for cooling the compressed air flow, elements and rotors. … The element can be cooled by cooling water that flows Oil-free Screw Air Compressor through particular pockets in the component casing.
In a lubricated screw compressor, the lubricant comes in direct in touch with the compressor inlet air and compressed air. … The essential oil contains anti-oxidants to attempt to reduce the break down of the lubrication properties due to internal conditions.