The definition of a dried out vacuum pump is a pump that does not use any fluids to create a vacuum or Dry Vacuum Pump contact the procedure gas and will also discharge to atmosphere. … Timing gears with oil reservoirs and close clearances between the rotors and housing are additional attributes that define a dried out vacuum pump.
Principle of Operation
Dried out screw vacuum pumps operate with two screw rotors rotating in opposite directions. This traps the medium to be pumped between the cylinder and the screw chambers and transports it to the gas discharge. … It also results in a lesser 
high temperature load of the compressed gas.