This is 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 
the rotors and casing are extra attributes that define a dried out 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 your cylinder and the screw chambers and transports it to the gas discharge. … It also results in a lower temperature load of the compressed gas.