What Is a Liquid Ring Pump?

Paul Scott

A liquid ring pump is a positive displacement pump used to compress or to draw vacuums in gasses. These pumps are relatively simple, consisting of a single moving element — a vaned impeller — which rotates in an off-center position within an enclosed casing. The casing is partially filled with a liquid, typically water, which covers a number of the impeller vanes. As the impeller rotates, it forces a ring of fluid to move along the outside of the casing, effectively isolating and sealing the spaces between the individual vanes. The unique, off-center design of the chamber and impeller allows the vacuum created between the vanes to draw gas into the chamber through an inlet port, and force it out of an outlet port.

Man with a drill
Man with a drill

The liquid ring pump is a wonderfully simple device that relies on a single moving part to compress gasses or to induce vacuums in a range of industrial processes. In contrast to other types of gas pumps and compressors, the liquid ring pump features an impeller as its only active, moving part. This element consists of a central hub mounted on a shaft driven by an electric motor. Radiating at regular intervals from this hub are a number of curved blades or vanes. The entire assembly is mounted within a closed chamber that forms the pump body.

The shaft and impeller are mounted in an off-center orientation within this chamber leaving the impeller vanes passing close to the one side of the chamber with a large open area at the other. During operation, this open area is partially filled with a fluid submerging the lower portion of the impeller. When the pump is started, the rotating impeller drives the fluid to form a ring around its outer edge by centrifugal force. This fluid ring effectively forms a seal around the impeller, isolating the spaces, or cells, between the individual vanes.

Due to the off-set position of the impeller and the fluid level in the partially-filled pump body, the size of these cells increases as the cell moves away from the water, and decreases again as it moves around and back into the water. During the upward half of the cycle away from the fluid, the increasing space in the cell creates a vacuum that draws gas through an inlet port cut into the back face of the chamber. As the gas-filled cell moves through its rotational cycle and begins to dip down towards the fluid again, the cell size decreases, compressing the gas. Prior to re-entering the fluid, this compressed gas is forced out of an outlet port cut into the opposite side of the pump body back.

This simple and efficient action produces a constant stream of non-pulsating compressed gas with a bare minimum of moving parts. Water is the most commonly used sealing agent in the liquid ring pump, although various solvents and oils may also be used. The liquid ring pump is used extensively as a compressor or as a vacuum pump in a number of applications, such as medical suction devices, chemical reactors, evaporative coolers, and freeze dryers.

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