The oil-free screw compressor element
No oil means that there is no oil for sealing the rotors and for cooling the compressed surroundings, components and rotors. … The element is usually cooled by cooling water that flows through particular 
pockets in the Oil-free Screw Air Compressor component casing.
In a lubricated screw compressor, the lubricant comes in direct in contact with the compressor inlet air and compressed air. … The oil contains anti-oxidants to try to reduce the break down of the lubrication properties because of internal conditions.