Liquid Ring Vacuum Pumps
Theory of Operation
A liquid ring vacuum pump has an impeller with blades attached to a center hub. The blades are located in the cylindrical body, but are off-set from the center.
In Figure 1 to the right, you can see the blades near the top of the pump are closer to the outside wall than at the sides and bottom of the pump. You can see this more clearly in Figure 2.
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 is partially filled with the liquid sealant. The liquid can be water (making it a water ring pump), oil or a solvent, depending upon the application.
One Cycle of a Single-Stage Liquid Ring Pump
When the pump first starts, centrifugal force slings the liquid sealant from the impellers to the outside walls of the body. This forms a ring of liquid on the outside of the body. The offset design causes some blades to be fully immersed while some are almost out of the liquid. The area of void space seals off between the liquid (hence the name “sealant”) and the impeller blades. This area is an “impeller cell.”
Following one impeller cell from the top of the pump, counter-clockwise, you can see the liquid recedes from the center hub. The sealant acts as a liquid piston to create a larger cell. This is the suction of the pump because it draws in air, gases, or vapors through the “inlet port” at the sides of the impeller. The impeller cell passes the inlet port and travels toward the discharge port which forces the sealant liquid toward the center hub of the impeller. This creates the compression step.
As the impeller cell passes the discharge port, the compression is at its highest because the impeller cell is at its lowest volume. The discharge port exhausts the gases, along with some of the liquid sealant, to atmospheric pressure. Although the diagrams show a very smooth ring of liquid, in actuality, the liquid sealant is highly turbulent. This results in some of the liquid sealant being discharged with the gases.
Single-Stage vs Two-Stage
The above describes the cycle of a single-stage pump. 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. Some can even pull down to 29″HgV. A two-stage pump design means there are two single-stage pumps operating in series. The discharge of the first stage goes into the suction port of the second stage. Two-stage pumps have better efficiency at higher vacuum levels (higher than 23″HgV) than a single-stage pump and 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.
Using Solvents to Seal a Pump
Liquid ring pumps can use any compatible liquid with the process as the sealant liquid, provided it has the appropriate vapor pressure properties. The most common sealant is water; however, liquid ring pumps can use almost any solvent. The second most common sealant is oil (see our section on Oil Sealed Liquid Ring Systems). The liquid ring vacuum pump is ideal for solvent recovery because of the ability to use any liquid. For example, if a vacuum dryer application is drying toluene, then we can use toluene as the sealant fluid. We can recover up to 99% of the solvent for reuse. In fact, Wintek has designed and supplied solvent sealed liquid ring vacuum pumps sealed with:
- Isopropyl Alcohol (IPA)
- Isopropyl Alcohol (IPA)
The vapor pressure of the sealant is very important in the sizing of the vacuum system because this directly affects the capacity and end vacuum of the vacuum pump. Use additional stages of compression, such as ejectors and boosters to get beyond the vapor pressure limitation of the sealant. In fact, Wintek has supplied methanol sealed systems operating at 5 torr, and water sealed systems operating at 0.03 torr.
Be sure to require all potential suppliers to provide a full performance curve of the vacuum pump with the solvent being used (not just a water curve). This is especially important if you are considering a Boosted Liquid Ring Vacuum System.