Rural Water - Quarter 3, 2015 - (Page 26)
A Great Strategy for
Energy Savings and
BY lOnnIe ruSSell, CertIFIeD energY Manager, SOuth CarOlIna rural water aSSOCIatIOn
During the course of
energy audits on water and wastewater
systems, I frequently recommend installing
Variable Frequency Drives (VFDs).
Sometimes there is resistance to this, but
often it is based on horror stories of the
VFDs of yesteryear. I hope, in this article,
to help alleviate those fears and show that
VFDs are an excellent business choice for
more reasons than just energy efficiency.
Throttle with a VFD, not a
valve or damper
If a pump is pumping more water than
is wanted, typically a valve is partially
closed to limit the flow. A good analogy
to this is driving your car at 3,000 rpm
and controlling speed with the brake.
VFDs control the speed and torque of
an AC induction motor by controlling the
frequency and voltage supplied to the
motor (without a VFD, full voltage and
frequency are always applied to the motor).
This would be the same as using the
accelerator to control the speed of the
car verses the brake. This is much more
efficient! How much more efficient?
A natural assumption is that if the
speed of a motor driven pump is reduced
by 50%, the power used is also reduced
by 50%. This direct relationship is true
for the flow (50% reduction in motor
speed reduces the flow 50%) but not
for power. The relationship of speed
to power is a cubic one (power of 3).
This means that if speed (and flow)
is reduced by 50%, power would be
reduced by 87.5%. This is theoretical,
but actual savings usually are to
the 2.2 power (instead of 3), so the
reduction would be 78%. That is still
pretty impressive. If this math escapes
you or you hate exponents, do not
fear, the chart below makes it easy
Pump Affinity Laws
Wheel Velocity Constant, Wheel Diameter Changing
Pressure, Flow, Power
Relative Wheel Diameter
thIrD Quarter 2015
Table of Contents for the Digital Edition of Rural Water - Quarter 3, 2015
From the President
Innovations for Every Utility
What’s Ahead for Water Technology?
Is Your Utility Prepared to Fight Misinformation on Social Media?
Variable Frequency Drives
Available EPA Tools and Resources for Small and Rural Utilities
Rural Water Assists Mount Zion with Water Loss, Finances
Throwing My Loop
Index to Advertisers/ Advertisers.com
From the CEO
Rural Water - Quarter 3, 2015