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 is usually cooled by cooling water that flows through Oil-free Screw Air Compressor china special pockets in the element casing.
In a lubricated screw compressor, the lubricant comes in direct in touch with the compressor inlet air and compressed air. … The oil contains anti-oxidants to attempt to reduce the breakdown of the lubrication properties due to internal conditions.