This is of a dry vacuum pump is a pump that does not use any fluids to make a vacuum or contact the procedure gas and may also discharge to atmosphere. … Timing gears with oil reservoirs and close clearances between your rotors and casing are additional attributes that define a dry vacuum pump.
Principle of Operation
Dry 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 lower warmth load of the compressed gas.