Liquid ring pump

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.

The earliest liquid-ring pumps date from 1903, when a patent was granted in Germany to Siemens-Schuckert. US Patent 1,091,529, for liquid-ring vacuum pumps and compressors, was granted to Lewis H. Nash in 1914. They were manufactured by the Nash Engineering Company in Norwalk, CT. Around the same time, in Austria, Patent 69274 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.

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