The definition of a dried out vacuum pump is a pump that does not use any fluids to make a vacuum or contact the process gas and can also discharge to atmosphere. … Timing gears with oil reservoirs and close clearances between the rotors and casing are extra attributes that make up a dry vacuum pump.
Principle 
of Operation
Dry screw vacuum pumps operate with two screw rotors rotating in opposing directions. This traps the moderate to be pumped between the cylinder and the screw chambers and transports it to the gas discharge. … In addition, it results in a lesser warmth load of the compressed gas.