Page 280 - HVAC Pump Handbook

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Basics of Pump Application for HVAC Systems

Basics of Pump Application for HVAC Systems 277

10.2.2 Selecting variable-speed pumps
Selection of a variable-speed pump is more complicated than that for a
constant-speed pump because the efficiencies of the motor and variable-
speed drive must be evaluated along with the efficiency of the pump
itself. The point of selection is less critical because the pump controls
should not allow the pump to “carry out,” as was the case with the
constant-speed pump in Fig. 10.2.
The variable-speed pump should be selected slightly to the right of
the best efficiency point where possible. Figure 10.1c describes this
point. If this were a constant-speed pump, the pump would be picked
at or just to the left of point A. As a variable-speed pump, it should be
selected at point B or to the right of the best efficiency point. As the
pump speed is reduced to minimum speed point C, the pump passes
closer to the parabolic curve for best efficiency, from maximum to
minimum speed. This demonstrates graphically what may need to be
done with a computer program to make the best possible selection.
Several pump selections should be made at various points, letting the
computer make the wire-to-water efficiency run at each point. Compu-
tation of wire-to-water efficiency will be described in detail later in
this chapter. The “carryout” condition described above for the constant-
speed pump is eliminated by the variable-speed pump and its controls,
so it can be selected to the right of the best efficiency point without
fear of improper pump operation.

10.3 Pump Speed
Traditionally, pumps have been selected for the HVAC industry at
1750 or 1150 rev/min, and in large pumps, speeds of 720 and 850
rev/min are not uncommon. The advent of the variable-frequency
drive has brought a new dimension to pump selection that is often
overlooked. Pumps now can be selected at speeds other than these
standard induction motor speeds due to the flexibility of the speed
output of the variable-speed drive.
Pump speeds as high as 5 to 10 percent greater than electric motor
induction speed can be used to increase the overall performance of a
pump. Most variable-speed drive manufacturers allow this speed
increase on their drives. The nameplate rating in amperes must not
be exceeded, and the available torque for the drive at such maximum
speeds must be greater than that required by the pump at any point
on its head-flow curve. Likewise, the electric motor must be selected
for the maximum horsepower required by the pump at the maximum
speed. This enables the water system designer to secure more effi-
cient selection of variable-speed pumps.

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