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 can also discharge to atmosphere. … Timing gears 
with essential oil reservoirs and close clearances between your rotors and housing are additional attributes that define a dry vacuum pump.
Principle of Operation
Dry screw vacuum pumps operate with two screw rotors rotating in reverse directions. This traps the moderate to become pumped between the cylinder and the screw chambers and transports it to the gas discharge. … It also results in a lower temperature load of the compressed gas.