How to Size Liquid Ring Vacuum Pumps
Liquid ring vacuum pumps are commonly used to handle “wet” gas mixtures (i.e., mixtures containing condensable vapors). The moisture present in such mixtures can wash away the lubricating oil in traditional “dry” oil lubricated pumps, possibly leading to premature failure.
As the impeller of the vacuum pump rotates, it throws water by centrifugal force to form a liquid ring concentric with the periphery of the casing which does the work of compression. The eccentric mounting of the impeller with respect to the casing results in increased spacing between the impeller blades at the inlet port and decreased spacing towards the outlet port (Figure 1). As gas enters the inlet port, it is trapped between the impeller blades and the liquid ring. Then, as the impeller rotates, the liquid ring compresses the gas and forces it out the outlet port.
The information needed to accurately size a liquid ring vacuum pump includes:
• inlet pressure, usually expressed in mm HgA
• inlet temperature
• mass flow rate, usually expressed in LB/hr and the molecular weight of fluid components
• vapor pressure data for each fluid component
• seal fluid data, if other than water: specific gravity, specific heat, viscosity, thermal conductivity, molecular weight and vapor pressure data
• temperature of the seal fluid or cooling water
• discharge pressure, usually expressed in PSIG