Page 150 - HVAC Pump Handbook

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HVAC Pump Performance

HVAC Pump Performance 147

point, it is considered to be steep; it is considered flat if this rise is less
than 25 percent. For example, if the design condition of a pump is
500 gal/min at 100 ft of head and the shutoff (no-flow) condition is
120 ft, it would be considered a flat-curved pump. If the shutoff condi-
tion were 135 ft, the pump would be considered steep-curved. The
pump curves in Fig. 6.8 are considered to be flat. The shape of the
pump curve was of great concern when constant volume-water sys-
tems were popular. It was desired to have relatively steep pump curves
for these operations so that a minor change in system head would not
make a great change in system flow. Steep curves were preferred, and
flat-curved pumps were avoided. Small, constant-speed pumping sys-
tems with three-way control valves on the heating or cooling coils
must be concerned with the shape of the pump curve. A flat-curved
pump may create instability in the operation of the three-way valves.
With the advent of variable-volume water systems utilizing variable-
speed pumps and system differential pressure controls, there is very lit-
tle need to be concerned about the shape of the pump curve. Peak effi-
ciency is what is sought, not the shape of the curve. In fact, flat-curved
pumps are desired. Pumps with flat-curved head-flow characteristics
should be sought for variable-speed pumping applications, provided
there is no loss in pump efficiency. There is less speed reduction with a
flat-curved pump than with a steep-curved pump. With less speed
reduction, the wire-to-shaft efficiency of the variable-speed drive and
motor is greater because the speed reduction is less.

6.3 Series/Parallel Operation of
Centrifugal Pumps
The broad variation in water requirements in HVAC systems usually
requires more than one pump to handle all the system flows from mini-
mum to maximum capacities. Likewise, condenser water service in some
tall buildings requires pump heads that cannot be handled economically
by a single-stage pump. The first condition requires parallel operation of
more than one pump, and the second condition requires more than one
impeller operating in series. By far most HVAC applications consist of
multiple pumps in parallel with few of them in series operation. Figure
6.8 describes the various resulting head-flow curves when operating
pumps in parallel and in series.
Parallel operation enables the water system designer to select a num-
ber of pumps that will produce efficient operation from minimum to
maximum system flow. Operating pumps of equal size and head in par-
allel results in the capacities of the pumps being multiplied together, as
shown in Fig. 6.8a. This figure describes two pumps with such equal

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