  How
to Make the Best Choice
Pick a size that's just right. There
is broad anecdotal evidence that many fans and motors are
larger than necessary for their intended use.
One investigator concluded after making
field measurements on about 1,000 motors that about half
operated at less than 60 percent of their rated load and
a third operated at less than half their rated load. Probably
the fans whose motors were attached were similarly oversized.
That's bad news for those who pay the Purchasing Advisor
HVAC: Fans energy bills, because fans operate at their highest
efficiency within a relatively small range. Outside of that
range, efficiency drops off dramatically. To pick the appropriate
size, use a fan chart such as that shown in
Figure 1.
For new construction, carefully calculate
the airflow and pressure drop and then add a safety factor.
In a retrofit case, use the chart with data from actual
measurements of flow and pressure to determine the optimum
size, rather than looking for a like replacement. Check
the cost-effectiveness of high-efficiency options.
Axial fans are the most efficient, but
consider backward curved fans where centrifugal design must
be used. To evaluate the cost-effectiveness of high-efficiency
fans, estimate the time spent in full- and part-load operation
and calculate the potential savings as shown in Table
1.
In addition, consider how variable-frequency
drives (VFDs) might figure into the equation, especially
if there are significant number of operating hours spent
at part load. A VFD provides significant benefit during
part-load operation for airfoil and fixed-pitch axial fans,
but less benefit when applied to a forward-curved fan (horsepower
requirements for forward-curved fans drop off more steeply
with reduced airflow than for other fan designs, so the
VFD provides less of an efficiency improvement).
A VFD should not be employed on a variable-
or adjustable pitch axial fan. Such fans are designed to
operate at a constant speed, and varying fan speed can cause
it to operate at a resonant frequency, causing excessive
vibration that can actually make the fan blades break free
from the hub (potentially causing substantial damage to
surrounding equipment). Pay attention to entrance and exit
conditions. The conditions at the entrance and exit to a
fan greatly influence fan system efficiency
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