Page 160 - HVAC Pump Handbook

Rishel_CH06.qxd 20/4/06 6:06 PM Page 157

HVAC Pump Performance

HVAC Pump Performance 157

Figure 6.14 Aberration on affinity laws pertaining to pump impeller diameter.

speed with impeller diameter change as shown in Fig. 6.14. Often
it is found only after the pump has been tested at various impeller
diameters. This makes it difficult to use impeller change in computer
programs for predicting pump performance without a careful evalua-
tion of the actual pump curves.
The next question is: why would anyone want to chop away the
best part of an impeller? The efficiency of this pump (Fig. 6.14) is a
maximum of 88 percent at full diameter compared to around 71 per-
cent at minimum diameter. Obviously, if the desired design condition
is 900 gal/min at 38 ft, there is some manufacturer that has a pump
that would give a higher efficiency than 71 percent at this condition.
Suppose, however, that the design condition is 1100 gal/min at 55 ft?
This would require the pump to have its impeller cut to some diame-
ter near 8.5 in. Why not slow the pump to reach the desired head-
flow point but equip the pump with a full size impeller to get the
maximum efficiency? This does require torque calculations to ensure
that the motor can handle the pump at the design condition and
reduced speed. This is not difficult when in the hands of experienced
hydraulics engineers who are knowledgeable about pump and motor
performance. The best selection may be an impeller diameter less than
the full diameter but still greater than 8.5 in. The engineer conducting

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