The definition of a dry vacuum pump 
is a pump that will not use any fluids to create a vacuum or contact the process gas and can also discharge to atmosphere. ... Timing gears with oil reservoirs and close clearances between your rotors and casing are additional 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 medium to become pumped between your cylinder and the screw chambers and transports it to the gas discharge. ... It also results in a lower high temperature load of the compressed gas.