©2024 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 66, no. 5, May 2024.
About the Author
Dave Moser, P.E., is a principal engineer at Cascade Energy, Inc.
Variable frequency drives (VFDs) are an energy-efficient method of matching system output to the load. Energy engineers often estimate the power draw of a VFD-driven system using the full load (100% speed) power draw and the estimated or actual operating speed. This article offers guidance on the equations to use when developing those calculations and aims to clarify the differences between the speed-to-power and flow-to-power exponent values.
Let’s say you’re an energy engineer that has found a VFD-driven fan operating at 100% speed during an energy audit due to a faulty duct static pressure sensor. Control system trend data is available for VFD speed, but not VFD power (a common scenario). The maintenance staff replaces the sensor, and fan speeds drop to ~60% speed on average. How do you calculate the fan energy savings associated with that change, with only fan nameplate horsepower (HP) and VFD speed trend data? This article addresses how to answer this question.
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