a screw vacuum pump has shafts and rotors secured to the shafts . each rotor has a central hollow chamber which receives the shaft or contains in built - in components rotating along with the rotor to define a relatively thin annular slot segment for cross - flow circulation of coolant . the annular slot segment is disposed between the built - in components and an inner wall of the rotor &# 39 ; s hollow chamber and between a suction side and a delivery side of the rotor . separate feed and discharge lines circulate the coolant through the annular slot segment in such a way that the coolant remains cavitation - free and bubble - free .