Page 238 - HVAC Pump Handbook

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The Use of Water in HVAC Systems

The Use of Water in HVAC Systems 235

reason to use any balance valves on them. There are thousands of
efficient HVAC water systems in operation that have no balance
valves whatsoever. Small constant-volume water systems can be
balanced using manual-balance valves.
Three-way coil control valves. These valves are a holdover from the
days of constant-volume water systems. They should be eliminated
when these systems are converted to variable-volume water sys-
tems. There are special and useful applications for three-way valves,
such as in diverting water on cooling towers, changing water flows
in chilled water and ice storage systems, and for blending water
temperatures.
Multiple-duty valves. As shown in Chap. 5, these valves are also
holdovers from the days of constant-volume systems. They are great
energy wasters and should be removed or opened fully when found on
variable-volume systems.
Crossover bridges. Here again, we have a device from the con-
stant-speed days that is unnecessary today and should be avoided
due to its energy waste and the increased pressure that it adds
to the water system. Figure 8.8 describes a typical installation
of a crossover bridge to a variable primary pumping system, and
Fig. 8.9 is its hydraulic gradient.
Figure 8.9 demonstrates the great energy waste of 96 ft across the
return valve. Also, since the main system pump and the building
pump are in series, the overall system pressure is increased to 227 ft
or 98 psig. Both of these unsatisfactory conditions can be eliminated
by changing the building pump to a booster pump which will be
described later in this chapter. Fig. 8.10 shows the hydraulic gradient
for Fig. 8.9.
Figure 8.11 describes the old method of connecting a four-building
system to a central chilled water plant while Fig. 8.12 shows the
simplicity of a contemporary variable primary system. Forty check
valves with bypass piping, four crossover bridges with return valves,
ten multiple-duty valves, four building pumps, and three secondary
pumps are eliminated. If the buildings are far apart, distributed
pumping with small circulators in the central chilled water plant
may be more efficient, Fig. 8.13. These circulators are started when
the system flow drops to the minimum allowable flow for the
chillers.
Throughout this manual will be examples of how to reduce pump
head by eliminating these four types of mechanical devices that waste
pump head.

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