Page 153 - HVAC Pump Handbook

Rishel_CH06.qxd 20/4/06 6:06 PM Page 150

HVAC Pump Performance

150 HVAC Pumps and Their Performance

Parallel operation enables the water system designer to select a
number of pumps that will produce efficient operation from minimum
to maximum system flow. Parallel operation of pumps requires great
care to ensure that the number of pumps running is the most efficient
for a particular flow and head condition. When operating pumps in
parallel, the total capacity and head of the pumps must be that re-
quired by the uniform system head curve or any point within the sys-
tem head area. (See Chap. 9 on “Configuration of a Water System” for
a description of uniform system head curves and system head areas.)
The great mistake that is often made when operating pumps in par-
allel is to measure the capacity of one existing pump at a certain head,
install another equal pump, and find out that the two pumps do not
produce twice the capacity. Pump operation cannot be predicted with-
out computing the system head curve or system head area of the water
system. This is pointed out here because of the danger of trying to run
pumps in parallel without evaluating the head of the water system.
This will be covered, in detail, in Chap. 10 on pump application.
Figure 6.9 is a composite of head-flow curves for three pumps each
with a capacity of 1700 gal/min at 60 ft head operating at a speed of

90
1780 RPM
80

70
One pumps pumps
Two
Three
System and pump head—FT. 50 5,100 GPM
pump
60
at 60 FT.
40

30

20 System head curve

10

0
0 1000 2000 3000 4000 5000
System flow—GPM
Figure 6.9 Head-flow curves for three equal pumps operating in parallel.

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