This is of a dry vacuum pump is a pump that will not use any fluids 
to make a vacuum or contact the process gas and may also discharge to atmosphere. … Timing gears with essential oil reservoirs and close clearances between the rotors and casing are additional attributes that define a dried out vacuum pump.
Principle of Operation
Dry screw vacuum pumps operate with two screw rotors rotating in opposing directions. This traps the medium to be pumped between the cylinder and the screw chambers and transports it to the gas discharge. … It also results in a lower heat load of the compressed gas.