The definition of a dry vacuum pump is a pump that does not use any fluids to create a vacuum or contact the procedure gas and may also discharge to 
atmosphere. … Timing gears with essential oil reservoirs and close clearances between the rotors and casing are extra attributes that make up a dry vacuum pump.
Principle of Operation
Dried out screw vacuum pumps operate with two screw rotors rotating in opposite directions. This traps the medium to become pumped between your cylinder and the screw chambers and transports it to the gas discharge. … It also results in a lesser warmth load of the compressed gas.