Single and multistage liquid-ring vacuum pumps from HyVac Products Inc., Phoenixville, Pa. (hyvac.com), generate vacuums of up to 29 in. of Hg (25 Torr).
The pump, which can be bronze or stainless steel, has a shrouded rotor turning freely but offset in circular eccentric casing. There is no metal-tometal contact between the rotor and casing. Centrifugal force pushes liquid to the outside of the rotor, forming a seal across the outside edge of the rotor sections.
Rotor sections get increasingly smaller then larger because the rotor is offset from the center of the casing. Gas trapped in the first and largest section gets compressed as the rotor turns, helping create a vacuum. After each revolution, the compressed gas and accompanying liquid are discharged through the exhaust port.
During pumping, the gas is in contact with the sealing liquid and compression is nearly isothermal. And when handling saturated vapor- gas mixtures the liquid ring acts as a condenser, greatly increasing pump capacity.