Page 159 - HVAC Pump Handbook

Rishel_CH06.qxd 20/4/06 6:06 PM Page 156

HVAC Pump Performance

156 HVAC Pumps and Their Performance

For a fixed pump speed:

1. The pump capacity varies directly with the impeller diameter:
Q d
1
1
(6.4)
Q d
2 2
2. The pump head varies as the square of the impeller diameter:
h d 2
1
1
(6.5)
h d 2
2 2
3. The pump brake horsepower required varies as the cube of the
impeller diameter:
bhp d 3
1
1
(6.6)
bhp d 3
2 2
It should be reemphasized that pump impellers should not be
changed without a thorough review of the water system characteristics.

6.4.1 Affinity laws
Unfortunately, the specific speed of the pump may change from mini-
mum allowable impeller diameter to maximum diameter. Therefore,
the affinity laws for impeller change may not be accurate. Figure 6.14
describes an actual set of pump curves for a particular pump with
different impeller diameters and a constant dynamometer speed of
1750 rev/min. The parabolic path of the best efficiency point is drawn
on this curve. The best efficiency point with a full diameter is shown
as point A at 1335 gal/min and 72 ft of head with 88 percent efficiency.
If the affinity laws were to prevail, the equivalent point for a 7 in diam-
eter impeller should be at point B or 960 gal/min at 38 ft. Instead the
actual point for this impeller diameter is at Point C or 900 gal/min at
33 ft. This is about a 6 percent loss in pump duty. One might argue
that this isn’t much considering that the maximum and minimum
allowable impeller diameters were used. However, any anomaly in
these calculations creates a feeling of distrust of the whole program.
As demonstrated in the previous section, as the pump impeller
diameter changes, the specific speed of the pump may change also
so that the pump performance does not follow these affinity laws for
impeller diameter change. (Specific speed has been defined in Chap.
4.) Not all pumps evince this variation in specific speed with impeller
diameter, but there is appreciable evidence of changes in specific

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