Variable Frequency Drives for a Vacuum Pump System
Editor’s Note: Responding to requests from readers many of whom work with vacuum pumping systems Pumps and Systems is introducing this new column with the January issue. Look for it periodically.
In many process plant applications, using a Variable Frequency Drive (VFD) to control the capacity of a liquid ring vacuum pump (LRVP) is an innovation. Previous system designs consisted of sizing the LRVP to operate efficiently for a certain portion of the process cycle and then using a recycle control valve system to maintain the desired LRVP suction pressure. This usually meant a loss of system efficiency for most of the operation cycle.
A liquid ring pump can be used in vessel evacuation or batch processes where the load volume is high at startup and diminishes as the vessel is emptied and the subsequent vacuum level increases. At the end of the cycle the usual intent is to hold as high a vacuum level as possible for a given period of time. This high vacuum level is a function of the properties of the service liquid, which boils or vaporizes as specific conditions exist. The liquid ring pump handles the least amount of net capacity at this high vacuum. The majority of the capacity at this point is the vaporized service liquid. The LRVP operating at full load rpm is oversized for the vacuum end point. By slowing down the pump, the operator can maintain the desired vacuum, but pumping capacity is decreased, lowering the brake horsepower.