“The basic function of a Vacuum Pump?”
A Nash pump operates with a positive displacement using water as a compressant.
The rotor keeps a ring of water rotating in the body. Water advances into and recedes from the rotor chambers, because the rotor axis is offset from the body axis.
This is what sets up the piston action. As the schematic diagram shows, water almost fills then partly empties each rotor chamber during each revolution. The stationary cones inside the rotor have ported openings for separate vacuum inlet and atmospheric discharge flows.