ASHRAE Journal - December 2021 - 16

TECHNICAL FEATURE
of 3% to 7%; and the fan aerodynamic losses are in the
range of 25% to 40%.6 Larger losses typically occur when
the fan system operates in part-load condition irrespective
of technology types.
Since the end users and building owners are interested
in overall energy consumption of the system, design
engineers should consider losses of all components in
the HVAC system, including selecting an efficient fan.
For example, if the design engineer selected a fan with
a high FEG rating but has chosen low-efficiency drive
components (motor, controller, etc.), the overall energy
savings of the given system is negatively impacted compared
to selecting efficient components along with an
efficient fan.
Figure 1 to Figure 3 (page 15) show some of the popular
fan systems used in commercial HVAC equipment today,
highlighting where mechanical and electrical losses
occur.
guarantee a lower overall energy consumption of the
given system. This is because FEG only looks at the peak
efficiency of the fan impeller. Peak efficiency is often
confused with the actual fan efficiency across its full
operating loads.7 In other words, the fan efficiency can
vary considerably at different conditions on a fan curve.
The operating efficiency can vary dramatically from the
peak efficiency down to single digits based on the location
of the operating condition on the efficiency curve.7
Fan manufacturers often specify a wide operating range
for a given fan, and engineers could select a fan anywhere
in the operating range. This can result in lower
operating efficiency of the fan.
To address the shortcomings in FEG, the U.S.
Department of Energy (DOE) in 2016 requested the help
of the Air Movement and Control Association (AMCA),
a trade association of fan manufacturers, with support
from other organizations, to help define a new fan
efficiency metric. As a result of this, AMCA developed
the fan energy index (FEI), which accounts for motor,
VSD, and mechanical drive losses and impeller efficiency
all at the design operating point. FEI is defined
as the ratio of the total electrical power consumed by
the selected fan system to that of a reference fan system
at a given design point. The reference fan system
power is a function of the rated airflow and fan pressure.
Those interested in knowing more about FEI may
refer to the AMCA 207 publication, which explains how
16
ASHRAE JOURNAL ashrae.o rg D ECEM BER 2 0 2 1
to calculate electrical input power from traditional shaft
brake horsepower (BHP) ratings, and AMCA 208, which
defines the FEI calculation.8,9
The wire-to-air efficiency is calculated at the system
level and includes all drive component losses. These
losses can be determined either through measurement
or through calculation as in AMCA 207. Wire-to-air
efficiency makes it very convenient for designers to
understand how different components affect energy
consumption. It is also useful to OEM manufacturers for
comparing and differentiating their products with those
of their competitors.
Mathematically the wire-to-air efficiency is defined as,
ηWA
==
Power out
Power in
overall
Airflow()×Pressure static or total
Input Electric power
cfm
Selecting the fan with the highest FEG rating will not ηη ηη ηη (2)
WA== ×××
controller motor
where
ηcontroller
ηmotor
ηfan
transmission
fan
= Controller efficiency, such as VFD, %
= Motor efficiency, %
ηtransmission = Mechanical transmission efficiency (beltdrive,
pulley, bearings, etc.), %
= Fan aerodynamic efficiency, %
ηoverall
Ps
Pt
Pe
= Overall efficiency of the air moving system,
which is equivalent to wire-to-air
efficiency, %
= Static pressure, in. w.c.
= Total pressure, in. w.c.
= Input electrical power, W
Power out of the fan system is simply the aerodynamic
air power, which is the product of the airflow and fan
pressure. AMCA terminology states that total efficiency
refers to using total pressure, and static efficiency refers
to using fan static pressure in the calculation. For the
purpose of this article, static pressure is used in determining
the power out since the velocity pressure in an
air-handling unit is typically negligible.
In the case of direct-drive fan applications, the transmission
efficiency (ηtransmission) is equal to 100%. Most
types of motorized impellers (electronically commutated
motor [ECM]), the motor efficiency (ηmotor) and the controller
efficiency (ηcontroller) are measured or calculated
simultaneously.
()
()Pe
(1)
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ASHRAE Journal - December 2021

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