Page 152 - HVAC Pump Handbook

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HVAC Pump Performance

HVAC Pump Performance 149

head-flow curves operating in parallel; the head is the same for one or
two pumps operating, but the capacity is doubled for two-pump opera-
tion. 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 required by the uniform system
head curve or any point within the system head area. (See Chap. 9 for a
description of uniform system head curves and system head areas.)
The great mistake that is often made with 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
without 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.
Considerable care should be exercised in trying to operate pumps
in parallel that have different head-flow curves; this is described in
Fig. 6.8b. There is great danger that the pump with the lower head-
flow curve, pump A in this figure, can be operated at the shutoff
head condition and cause heating in the pump. This will happen
if these pumps are operated together at any less than 62 percent
capacity of one pump. Also, it may be very difficult to program and
operate the pumps together at an adequate wire-to-water efficiency.
Pumps are operated in series when the flow can be handled by one
pump throughout the operating range but the head is so high that a
number of pumps in series is needed. Figure 6.8c describes the fact
that with pumps in series, the head developed by all the pumps is
added together at the same system flow. Turbine-type pumps are ex-
cellent for such high-head, low-flow service. Two or more bowls or
stages are connected in series to provide the required head.
Some large HVAC systems exist in which multistage pumps must
operate in parallel; in this case, the resulting pump curves for two
pumps are a combination of Fig. 6.8a and c with 200 percent capacity
and 200 percent head of one pump impeller.

6.3.1 Pumps operating in parallel
The broad variation of water requirements in HVAC water systems
usually requires more than one pump in parallel to handle all of the
system flow-head requirements. Also, standby capacity may necessitate
the installation of an extra pump. By far, most of these pump applica-
tions consist of multiple pumps operating in parallel with few of them
in series operation.

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