Liquid ring style vacuum pumps
An internal passage joins the openings from the pump inlet to an inlet port in the cone. There's also a passage from the cone discharge to the discharge connection on the head. Some NASH pumps have a port plate configuration rather than conical, but the principle is the same. This diagram demonstrates what the rotor and body do while the pump is in operation. The spinning of blue liquid forms a ring due to centrifugal force.
Because the rotor axis and body axis are offset from each other, the liquid ring is not concentric with the rotor. Air or gas traverses the internal passage to the cone inlet port. As the white dots indicate, the gas is drawn into the rotor chambers by the receding liquid ring, similar to the suction stroke of a piston in a cylinder. The liquid ring does the job of pistons, while the rotor chambers play the part of cylinders.
As each chamber rotates past the inlet port, the chamber carries a volume of air or gas around with it. The white dots are confined between the cone and the ring of rotating liquid. The gas is compressed as the liquid ring converges with the cone. This represents the compression of air or gas from vacuum up to atmospheric pressure, or from atmospheric up to positive pressure in a liquid ring compressor. A liquid-ring pump is a rotating positive-displacement pump. They are typically used as a vacuum pump , but can also be used as a gas compressor.
The function of a liquid-ring pump is similar to a rotary vane pump , with the difference being that the vanes are an integral part of the rotor and churn a rotating ring of liquid to form the compression-chamber seal.
They are an inherently low-friction design, with the rotor being the only moving part. Sliding friction is limited to the shaft seals.
Liquid-ring pumps are typically powered by an induction motor. The liquid-ring pump compresses gas by rotating a vaned impeller located eccentrically within a cylindrical casing. Liquid usually water is fed into the pump and, by centrifugal acceleration, forms a moving cylindrical ring against the inside of the casing.
This liquid ring creates a series of seals in the space between the impeller vanes, which form compression chambers. The eccentricity between the impeller's axis of rotation and the casing geometric axis results in a cyclic variation of the volume enclosed by the vanes and the ring.
Gas, often air, is drawn into the pump through an inlet port in the end of the casing. The gas is trapped in the compression chambers formed by the impeller vanes and the liquid ring. The reduction in volume caused by the impeller rotation compresses the gas, which reports to the discharge port in the end of the casing.
Compressed gas on discharge of pump contains certain amount of working liquid which is usually removed in vapor—liquid separator. The earliest liquid-ring pumps date from , when a patent was granted in Germany to Siemens-Schuckert.
US Patent 1,,, for liquid-ring vacuum pumps and compressors, was granted to Lewis H. Around the same time, in Austria, Patent was granted to Siemens-Schuckertwerke for a similar liquid-ring vacuum pump. Liquid-ring systems can be single- or multistage. Typically a multistage pump will have up to two compression stages on a common shaft. In vacuum service, the attainable pressure reduction is limited by the vapour pressure of the ring-liquid.
As the generated vacuum approaches the vapour pressure of the ring-liquid, the increasing volume of vapor released from the ring-liquid diminishes the remaining vacuum capacity. The efficiency of the system declines as a result. Single-stage vacuum pumps typically produce vacuum to 35 Torr mm Hg or 47 millibars 4. Some ring-liquid is also entrained with the discharge stream.
This liquid is separated from the gas stream by other equipment external to the pump. In some systems, the discharged ring-liquid is cooled by a heat exchanger or cooling tower , then returned to the pump casing. In some recirculating systems, contaminants from the gas become trapped in the ring-liquid, depending on system configuration. These contaminants become concentrated as the liquid continues to recirculate, eventually causes damage and reduced life to the pump.