How a Liquid Ring Vacuum Pump Works

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A liquid ring vacuum pump has an impeller with blades attached to a center hub, located in a cylindrical body, but off-set from the center.

In Diagram 1A on the right you can see the blades near the top of the pump are closer to the outside wall than at the side of the pump. They are furthest away at the bottom of the pump; you can see this more clearly in Diagram 1B.

The impeller sits between two end plates (port plates) which have shaped holes cut into them called ports.

The pump requires a liquid (also called the sealant) to create vacuum as follows. Prior to starting the pump, it should be partially filled with the liquid sealant (typically to the center of the body, although this can be plus or minus a couple of inches). The liquid can be water (making it a water ring pump), oil or a solvent, depending upon the application. As you can see in the Diagrams, when the pump is at rest, some of the impeller blades are immersed in the sealant liquid.

When the pump starts, the impeller slings the liquid sealant by centrifugal force, to the outside walls of the body, forming a ring of liquid at the outside walls of the body. Because the impeller is off-set from the body, some of the blades are fully immersed in liquid, and some are almost out of the liquid. The area of void space without liquid, is sealed off between the liquid (and hence the term "sealant") and between the impeller blades, called an "impeller cell". As we follow one impeller cell from the top of the pump, counter-clockwise, you can see the liquid recedes from the center hub, acting as a liquid piston to create a larger cell. This is the suction of the pump, drawing in air, gases, or vapors thru the "inlet port" at the sides of the impeller. After impeller cell passes the inlet port and travels toward the discharge port, the sealant liquid is forced back toward the center hub of the impeller, creating the compression step. As the impeller cell passes the discharge port, the compression is at its highest, and the gases, along with some of the liquid sealant are exhausted thru the discharge port to atmosphere. Although the diagrams show a very smooth ring of liquid, in actuality, the liquid sealant is highly turbulent, which is why some of the liquid sealant is discharged with the gases.

 Single-Stage vs Two-Stage Liquid Ring Pumps

  Inside a Liquid Ring Vacuum Pump

Diagram 1A of a Liquid Ring Vacuum Pump
Diagram 1A

Diagram 1B of a Liquid Ring Vacuum Pump
Diagram 1B

The above describes the cycle of a single-stage pump; so in one revolution, we have suction (pulling a vacuum), and compression (back to atmosphere). Some manufacturers' single stage pumps can operate to just 26"HgV, while others can operate to 28"HgV and others to 29"HgV. A two-stage pump is just two of these operating in series, so the discharge of the 1st stage goes into the suction port of the 2nd stage. Two-stage pumps have better efficiency at higher vacuum levels (higher than 23"HgV) than a single stage pump and are designed to operate at vacuum levels higher than 20"HgV.

A two-stage pump is also a much better choice if you are handling solvents at higher vacuum levels (higher than 23"HgV). This has to do with the effect of sealant temperature rise being spread across two stages and its relationship to the sealant vapor pressure in liquid ring vacuum pumps.


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