Page 309 - HVAC Pump Handbook

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Basics of Pump Application for HVAC Systems

306 The HVAC World

condition to start or stop pumps, the kW input control starts or stops
a pump when these actions indicate a possibility of saving energy. For
example, in Table 10.9 and Fig. 10.16, the data indicates that the sec-
ond pump should be started when the system flow reaches 1300
gal/min and not wait until 1800 gal/min flow where one pump can no
longer sustain the desired water system conditions. At 1800 gal/min,
the kW input with one pump running is 33.5 kW and is 26.3 kW with
two pumps running. This is a 21 percent savings in energy.
The software of the pump controller can include this control proce-
dure to ensure that the most efficient number of pumps is running in a
multiple-pump installation. The points of addition and subtraction for
pumps are easily adjustable in contemporary pump controllers. One of
the advantages of this control is the ability of the operators to check the
control procedure to ensure that the pump addition and subtraction
points are correct. This is as follows and assumes that the desired oper-
ating pressure is maintained as pumps are added and subtracted.
1. The total kW input should drop when a pump is added. If it increases
instead, the pump should not have been added, and the set point for
pump addition should be increased until the kW input drops or does
not change when a pump is added.
2. The total kW input should drop when a pump is subtracted. Like
the pump addition, if the kW input increases when a pump is sub-
tracted, the point of subtraction should be lowered until the kW
input drops or does not change when subtracting a pump.

This is a very simple procedure even for installations with a multi-
ple of large pumps. The operators become familiar with what the kW
input should be at particular flows in the water system and can tell
when adverse conditions appear.

10.11 Pump Control

Since variable-speed pumps have entered the HVAC market, there
are two aspects to pump control: (1) pump start-stop procedures and
(2) pump speed control. These procedures must be developed for both
constant- and variable-speed pumping systems.
Pump start-stop procedures consist of a number of control techniques:
1. With system activation or shutdown
2. By system demands such as flow or pressure
3. By system energy evaluation such as wire-to-water efficiency or
kW input

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