Liquid Ring - How it Works
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 Figure 1 on 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, it should be 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.
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. 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 through 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 through the discharge port to atmosphere. Although the diagrams show a very smooth ring of liquid, in actuality, the liquid sealant is highly turbulent. Resulting in some of the liquid sealant being discharged with the gases.
Single-Stage vs Two-Stage Liquid Ring Pumps
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 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.
Solvent Sealed Liquid Ring Pumps
Liquid Ring Vacuum Pumps can use any liquid compatible with the process as the sealant liquid provided it has the appropriate vapor pressure properties. Although the most common sealant is water, and the pump is sometimes called a Water Ring Vacuum Pump, almost any liquid can be used. The second most common sealant is oil (see our section on Oil Sealed Liquid Ring Systems). The ability to use any liquid, allows the liquid ring vacuum pump to be ideally suited for solvent recovery. If, for example, in a vacuum dryer application the solvent being dried is toluene, then we can use toluene as the sealant fluid and recovery 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, as this directly affects the capacity and end vacuum of the vacuum pump. You can get beyond the vapor pressure limitation utilizing additional stages of compression such as ejectors and boosters. In fact, we have 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.