ASHRAE Journal - May 2024 - 40
TECHNICAL FEATURE
maximum energy efficiency (minimum VFD input
power). Based on our experience, the default setting
for many VFDs is " constant " or " linear. " Operating a
centrifugal fan or pump VFD at " constant " or " linear "
torque will result in a speed-to-power exponent less
than 2.7.
Data from a VFD manufacturer shows an exponent
of 2.2 is more appropriate for VFDs serving centrifugal
loads with their torque setting set to " constant " or
" linear " (Figure 7). Changing that setting to " variable " or
" squared " can be a quick, easy way to reduce energy use.
First confirm with the manual and/or VFD manufacturer
that the setting change is appropriate for the situation.
Conclusion
The most accurate method of determining VFD input
power draw at a given operating condition is the VFD
screen power readout or a power meter on the line
side of the VFD. Often this is not available, though, at
baseline and proposed speeds. Using the affinity laws
and estimates of baseline and proposed speed is a good
method when VFD screen power readouts or power
meter data are not available.
A speed-to-power exponent of 2.7 is appropriate
for use with centrifugal and axial fans and pumps,
using Equation 2. This exponent does not vary based
on the shape of the system curve (minimum static
pressure level). The flow-to-power exponent will vary by
minimum static pressure, but not the speed-to-power
exponent. The 2.7 speed-to-power exponent can be
applied with confidence in power draw calculations for
centrifugal and axial fans and pumps.
Resources
1. DOE. 2012. " Adjustable Speed Drive Part-Load Efficiency, Motor
Systems Tip Sheet #11. " U.S. Department of Energy.
2. Integral Group, 2013. " Energy Efficiency Baselines for Data
Centers, " pp. 54-56. Integral Group.
3. Maxwell, J. 2005. " How to Avoid Overestimating Variable Speed
Drive Savings. " Proceedings of the Twenty-Seventh Industrial Energy
Technology Conference.
Bibliography
Wray, C. 2018. " Fan Systems: Part-Load Energy Performance &
TAB/Commissioning Opportunities. " ASHRAE Annual Conference.
ashrae.org/228
ASHRAE Publishes First Zero Energy and
Zero Carbon Building Evaluation Standard
ASHRAE Standard 228 sets requirements for evaluating whether a building or group
of buildings meets a defi nition of " zero net energy " or a defi nition of " zero net carbon. "
40
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ASHRAE Journal - May 2024
Table of Contents for the Digital Edition of ASHRAE Journal - May 2024
Contents
ASHRAE Journal - May 2024 - Intro
ASHRAE Journal - May 2024 - Cover1
ASHRAE Journal - May 2024 - Cover2
ASHRAE Journal - May 2024 - 1
ASHRAE Journal - May 2024 - Contents
ASHRAE Journal - May 2024 - 3
ASHRAE Journal - May 2024 - 4
ASHRAE Journal - May 2024 - 5
ASHRAE Journal - May 2024 - 6
ASHRAE Journal - May 2024 - 7
ASHRAE Journal - May 2024 - 8
ASHRAE Journal - May 2024 - 9
ASHRAE Journal - May 2024 - 10
ASHRAE Journal - May 2024 - 11
ASHRAE Journal - May 2024 - 12
ASHRAE Journal - May 2024 - 13
ASHRAE Journal - May 2024 - 14
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ASHRAE Journal - May 2024 - 20
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ASHRAE Journal - May 2024 - 27
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ASHRAE Journal - May 2024 - 29
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ASHRAE Journal - May 2024 - 34
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ASHRAE Journal - May 2024 - 40
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ASHRAE Journal - May 2024 - 42
ASHRAE Journal - May 2024 - 43
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ASHRAE Journal - May 2024 - Cover3
ASHRAE Journal - May 2024 - Cover4
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