Power Consumption of a Liquid Ring Vacuum Pump
June 18th, 2015 | Posted by Andy Smiltneek
This article describes how to predict the power consumption of the liquid ring vacuum pump. This is the first step in sizing a vacuum pump drive. The vacuum pump can also be the largest consumer of energy in your process and a significant number in your cost analysis.
Approximately two-thirds of the energy that goes into a liquid ring vacuum pump is used to compress the gas going through it. The other one-third is lost in moving the water and in internal leakage. The measure of efficiency is volume delivered versus power expended (Cubic Feet per Minute vs Horsepower or CFM/HP in imperial units, Cubic Meters per Hour versus kilowatts in metric units). The best vacuum pumps will deliver 28 CFM/HP. These are the very large cone ported liquid ring pumps, which will be more completely discussed in a subsequent article.
Manufacturers of vacuum pumps supply pump curves. The pump curves show for a given speed of the pump the volume of air at the vacuum level indicated that will move through the pump and the power needed to drive the pump at that specific vacuum level. The pump curves are experimentally determined by the manufacturer in tests governed by the HEI standards. The best manufacturers validate these curves with hundreds of tests. The new pump is guaranteed to be within ± 5% of these curves.
The curves supplied for the pumps are for operation of the pump with air pulled into the inlet through one or many orifices thus restricting the flow and creating the vacuum. The standard is air at a temperature of 60 degrees Fahrenheit, 50% relative humidity, 29.92“Hg atmospheric pressure, with 60 degree cooling water and no backpressure on the pump. Any deviation from these conditions will cause the pump capacity and the power required by the pump to change. The pump performance will be corrected as well, and will be certified at standard conditions in the factory certification test. Correction factors must be applied to the conditions your pump will be running to convert to the standard conditions so you can chose the right pump at the right speed.
In general, higher seal water temperature will result in lower pump capacity and higher horsepower. Higher incoming air temperature with associated higher humidity can result in lower horsepower and higher capacity. Operation at any elevation above sea level will decrease the pump horsepower but also decrease the motor rating and the thermal horsepower rating of the gearbox. Restriction of the pump discharge will increase the power needed by the pump. Condensing of water vapor as the pressure drops can greatly enhance the volume the pump handles. So don’t forget to compensate for these factors.
To determine the power, you first determine the vacuum level you need and then determine the gas volume flow. Next, correct the conditions to standard. Then, go to the curve and find the speed of the vacuum pump to achieve the flow that you want at the vacuum level desired. Finally, read from the power curve the power needed at this operating point. This power is the power needed at the shaft of the vacuum pump. In subsequent articles we will describe sizing the drive, but for this discussion you can assume that you will have about 10% drive loss if the drive is sized to meet requirements and about 13% if the drive is oversized. Now, simply take the power number and add the drive loss to it and you will have an estimate of the power consumed.
Why oversize the drive? Given the high cost of energy and the high cost of running a plant and the much higher cost of having the plant shut down, sizing of the drive becomes a compromise of capital cost, drive efficiency, factor of safety, and prediction of future vacuum needs. Within the factor of safety is the uncertainty of the volume and vacuum level needed in normal running conditions, the backpressure on the pump, and the power needed to get through the transitional phases of startup and process upsets.
The two biggest challenges in this prediction are determining the gas flow needed and correcting that gas flow to standard conditions. Somarakis has been through this process many times. So, for a more convenient way to determine power consumption, call us and we will request all of the variables that we need and predict the power consumption for you. We might be able to suggest ways to save energy as well.
As a final note, be aware that the pump design affects the efficiency and that difference in efficiency is reflected in the pump curve. A flat sided pump can use as much as 25% more power than a cone ported pump for the same volume and vacuum. My next article will be on sizing the drive, and the one after that will discuss the flat sided and cone ported pumps.