The first Liquid Ring Pump dates back to 1903

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.

The princip
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 electrical motor. 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 30-50mBar. They are used on paper machines to dewater the pulp slurry and to extract water from press felts. Another application is the vacuum forming of molded paper pulp products (egg cartons and other packaging).
Other applications include soil remediation, where contaminated ground water is drawn from wells by vacuum. In petroleum refining, vacuum distillation also makes use of liquid ring vacuum pumps to provide the process vacuum. Liquid ring compressors are often used in vapor recovery systems. This type of pump is desirable because it can be
sealed with a fluid compatible with the process gas/vapor that would harm other type of pumps (i.e.: oil sealed) and is relatively inexpensive. Reliability is high because of its simplicity.

Preferred for vacuum trucks
Over the years the liquid ring pump development has given one of the best performances for vacuum pumps, and compared to other types, the liquid ring is the preferred pump choice for the vacuum truck business due to very low noise level, powerful performance, very low maintenance costs and the long life time often for more than 10-12 years.