The definition of a dry vacuum pump is a pump that will 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 the rotors and housing 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 end up being pumped between the cylinder and the screw chambers and transports it to the gas discharge. … In addition, it results in a lesser temperature load of the compressed gas.
