A liquid-ring pump is a rotating positive-displacement gas pump, with liquid under centrifugal force acting as a seal. Description of operation Liquid-ring pumps are typically used as vacuum pumps, but can also be used as gas compressors. The function of a liquid-ring pump is similar to a rotary vane pump, with the difference being that the vanes are a rigid 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 (often 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 spaces 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. A gas (often air) is drawn into the pump through an inlet port in the side 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 exits through the discharge port in the side of the casing. The compressed gas at the discharge of pump contains a small amount of the working fluid, which is usually removed in a vapor–liquid separator. History 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, Connecticut, US. Around the same time in Austria, Patent 69274 was granted to Siemens-Schuckertwerke for a similar liquid-ring vacuum pump. Applications These simple, but highly reliable pumps have a variety of industrial applications. They are used to maintain condenser vacuum on large steam-turbine generator sets by removing incondensable gasses, where vacuum levels are typically 30–50 mbar. They are used on paper machines to dewater the pulp slurry and to extract water from press felts. Another application is the vacuum