This is of a dry vacuum pump is a pump that will not use any 
fluids to make a vacuum or contact the procedure gas and can 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 opposite directions. This traps the medium 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.