This is of a dry vacuum pump is a pump that will not use any fluids to create a vacuum or contact the procedure gas and will also discharge to atmosphere. … 
Timing gears with essential oil reservoirs and close clearances between your rotors and housing are extra attributes that define a dried out vacuum pump.
Principle of Operation
Dried out screw vacuum pumps operate with two screw rotors rotating in reverse directions. This traps the medium to end up being pumped between your cylinder and the screw chambers and transports it to the gas discharge. … In addition, it results in a lesser heat load of the compressed gas.