Liquid ring
From Wikipedia, the free encyclopedia
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 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
[edit] Description of operation
The liquid ring pump compresses gases by means of an vaned impeller rotating within and eccentric to a cylindrical casing. Liquid (usually water) is fed into the pump and by centrifugal action, forms a moving cylindrical ring against the inside of the casing. This liquid ring creates a series of seals the space between the impeller vanes to form compression chambers. The eccentricity between the impeller rotating axis and the casing geometric axis results in a cyclic variation of the volume enclosed by the vanes and the ring.
Gas (or air) drawn into the pump via an inlet port in the end of the casing, is trapped in the space formed by the impeller vanes and the liquid ring. The impeller rotation compressed the gas, which is then discharged through another port in the end of the casing.
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 via heat exchanger or cooling tower, then returned to the pump casing. In other systems, the discharged hot liquid (usually water) is treated as a waste stream. In this case, fresh, cool water is used to make up the loss. Environmental considerations are making such once through systems are increasingly rare.
[edit] Types and Applications
Liquid ring systems can be single or multi-stage. Typically a multi-stage 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 vacuum generated 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), assuming air is being pumped and the ring-liquid is water at 15ºC (60F) or less.
These simple, but highly reliable pumps have a variety of industrial applications. One typical industrial 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.
