Page 290 - HVAC Pump Handbook

Rishel_CH10.qxd 21/4/06 6:20 PM Page 287

Basics of Pump Application for HVAC Systems

Basics of Pump Application for HVAC Systems 287

10.6 Number of Pumps

Most HVAC water systems have a broad range of operation due to the
variation in heating, cooling, and ventilating loads. As was discussed
in Chap. 8, it is imperative that the minimum load as well as the
maximum load on a HVAC water system be computed before any
additional evaluation be made of that system.
Most HVAC systems also have more than one pump per water sys-
tem due to (1) the broad range of loads and (2) to provide some redun-
dancy in case of pump failure. Thus, HVAC systems can range from
one pump on small systems with a specific flow range and little need
for standby pump capability up to a number of pumps operating in
parallel to accommodate load range and reliability.
It is obvious that the water system designer must make decisions
early in the system design to ensure that the desired levels of effi-
ciency and reliability are achieved for a water system. Let us review
the two factors that affect the number of pumps operating in parallel.
“Fear of pump failure” has affected the determination of the number of
pumps when it should not be considered. Pumps have a reliability com-
parable with that of other HVAC equipment. Seldom are two fans
installed in parallel because of fear of failure. The consequences of sys-
tem failure should be determined to develop the redundancy needed on
every water system. Hospitals and institutions and critical facilities such
as research laboratories and computer centers obviously should have
standby pump capability to ensure that there is continuous service.
The question on critical installations is how much standby capacity
should be provided? The two most frequent selections are two 100
percent capacity or three 50 percent capacity pumps. The three 50
percent capacity pumps are usually the best selection because they
offer three chances to total failure and the pumps and motors are
smaller. For example, assume a chilled water system for a hospital
has a maximum requirement of 2500 gal/min at 100 ft of head and a
system head curve as shown in Fig. 10.10a. This curve shows a con-
stant friction head of 20 ft that is maintained across all the cooling
coils and control valves. Two pumps at 100 percent capacity would
require two 75-hp motors, while three pumps at 50 percent capacity
each would require three 40-hp motors. Figure 10.10b provides the
pump head-flow curve for the pumps of the two-pump system, while
Fig. 10.10c does so for the three-pump system. The total horsepower
for the two pumps would be 150 hp compared with 120 hp for the
three pumps. If space is available for the three pumps, it would appear
that the three pumps would be more acceptable for this application; a
final decision should be made after a wire-to-water evaluation is
made. This will be discussed later in this chapter.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2006 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.