Page 148 - HVAC Pump Handbook

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HVAC Pump Performance

HVAC Pump Performance 145

6.2.5 Variable-speed pump head-flow curves
Since speed variation is an added factor, the variable-speed pump’s
head-flow curves must be provided for at least one diameter of im-
peller. If another impeller diameter is desired, a second set of curves
should be provided for that diameter. The speed variation is usually
shown from around 45 to 100 percent speed, or typically from 800 to
1750 rev/min. Below 45 percent speed, pump performance becomes a
variable, and the pump manufacturers do not like to certify pump
performance at such low speeds for most HVAC pumps. Figure 6.1b
describes a typical set of pump curves for a large centrifugal pump
operating at a maximum speed of 1750 rev/min. This pump has a spe-
cific speed of 1860. It must be remembered that this group of curves is
for one impeller diameter, while Fig. 6.1a is for another pump at con-
stant speed and various impeller diameters.
Due to the many variable-speed pumps that are now used in the
HVAC field, it is important that the HVAC water system designer
become familiar with what happens when the speed of a pump is var-
ied from minimum to maximum speed. Figure 6.1b is one of the most
important figures in this book. Of particular importance in this figure
is the curve describing the parabolic path of the best efficiency point
as the pump changes speed. This curve will help the water system
designer understand how the efficiency of a pump varies as the speed
changes. The variations in efficiency at different points on the pump
head-flow curves are also provided in this figure as lines of constant
efficiency. Lastly, this figure is a graphic representation of the pump
affinity laws that are so important.

6.2.6 Best efficiency curves
In variable-speed pumping, the most important pump curve is the
best efficiency curve; it was first seen in Fig. 6.1b. This is the parabol-
ic curve that the best efficiency point follows, as the speed of the
pump is reduced. As will be seen later, it is the basic rule of variable
speed pump control that the pump operate as closely to this curve as
is possible. Programming of multiple pumps operating in parallel is
based upon keeping the pumps in operation close to this curve.
Other best efficiency curves are seen in Fig. 6.7 and 6.13. Figure 6.7
describes the best efficiency curves for four small but popular end-
suction pumps.
There will be a number of pump operations in this book that place
great emphasis on the best efficiency curve. Radial thrust discussions
will emphasize that this thrust is the lowest at the best efficiency

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