Page 145 - HVAC Pump Handbook

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HVAC Pump Performance

142 HVAC Pumps and Their Performance

6.2.4 Typical constant-speed pump
head-flow curves
Figure 6.6 through d describes typical pump head-flow curves for a
broad range of HVAC applications. No particular type of centrifugal
pump is represented. These curves are for pumps with good formation
and have best efficiencies that are at the level desired for these water
systems. Almost all HVAC applications should have pumps with the
best efficiency above 80 percent, excepting small pumps or pumps in
low-flow, high-head applications.
2
Velocity head, V /2g of the pump connections, must be considered
when the head-flow curve is developed. On pumps with suction pipes,
the difference between the velocity head for the water flowing out of
the discharge connection and for the water flowing into the pump suc-
tion is included in these head-flow curves. On axial-flow pumps tak-
ing water directly from a sump, there is no inlet velocity head deducted
from the discharge velocity head. There may be an efficiency reduc-
tion for single-impeller performance that does account for some entrance
losses.
Normal pump head-flow curves are provided by manufacturers at
particular induction motor speeds such as 875, 1150, and 1750 min.
The newer, high efficiency, induction motors have higher, full load
speeds such as 885, 1180, and 1775 rev/min. Pump manufacturers are
now providing pump curves at such full speeds. A range of impeller
diameters will be shown on each curve. A pump is considered to be
trimmed to a specific diameter for each particular application. The
range of impeller trimming is shown on a typical head-flow curve,
from minimum to maximum diameter. For example, the pump curve
described in Fig. 6.6d has an unusual impeller diameter range, from
8.2 to 13.0 in diameter. Although this much trimming is allowed, cut-
ting the impeller to such an extent has a great effect on pump
efficiency. This particular pump has a peak efficiency of 87 percent;
cutting the impeller to its minimum diameter would reduce the effi-
ciency to 75 percent. Great care must be taken in trimming pump
impellers; an alternative is to install a variable-speed drive and
operate the pump at a lesser speed. Pumps must never be trimmed
below the minimum diameter shown by the pump manufacturer on
the published head-flow curves.
Note. All manufacturers’ pump head-flow curves are dynamometer
curves at a fixed speed such as 1750 rev/min. Actual field performance
will be different from these curves when the pumps are operated by
induction motors whose speed varies from 1750 rev/min fully loaded to
1800 rev/min unloaded. Manufacturers’ pump curves should not be

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