Part I: Variable speed drive energy savings

David W. Spitzer

Installing variable speed drives can yield significant electrical energy savings. Every few years, I am asked to review a paper citing the energy savings that can be obtained in pumping applications where the author uses the Affinity Laws to incorrectly calculate electrical energy savings when applying a variable speed drive. However, misapplication of the Affinity Laws can result in over-estimates of the energy savings (sometimes grossly) associated with operating the pump with a variable speed drive. Let me explain.

The Affinity Laws for centrifugal pumps state that the pump’s:

• Flowrate is proportional to the pump motor speed
• Pressure generated is proportional to the square of the pump motor speed
• Motor power consumption (kW) is proportional to the cube of the pump motor speed

The operation of centrifugal equipment with variable speed drives can provide significant energy savings that can often justify the investment for both new and retrofit applications. Consideration should be given to applying them to the majority of pumps that are used in continuous operations. However, the Affinity Laws can seem relatively simple and straightforward — maybe too straightforward. For example, consider a piping system where a motor/pump produces 100 percent flow at a pressure of 10 meters of water column. It would seem logical to calculate the energy savings at 50 percent of flow as follows:

• 50 percent of flow means that the motor/pump operates at 50 percent of speed (per the first Affinity Law)
• Power consumption is (0.50)³ or 12.5 percent of the power at 100 percent flow (per the third Affinity Law)
• There is an 87.5 percent energy savings when operating the motor/pump with a variable speed drive at 50 percent of flow

WRONG!!! Give it some thought until next month.

David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes, LLC offering engineering, seminars, strategic, marketing consulting, distribution consulting and expert witness services for manufacturing and automation companies. He has more than 35 years of experience and has written over 10 books and 250 articles about flow measurement, instrumentation and process control.

Mr. Spitzer can be reached at 845-623-1830 or spitzerandboyes.com. Click on the "Products" tab to find his "Consumer Guides" to various flow and level measurement technologies.