Page 266 - HVAC Pump Handbook

Rishel_09.qxd 20/4/06 6:34 PM Page 263

Configuring an HVAC Water System

Configuring an HVAC Water System 263

9.6.1 Three types of systems
There are three basic system arrangements in HVAC piping: (1) direct
return systems for loop piping, (2) reverse return systems for loop pip-
ing, and (3) open piping systems for open tanks such as cooling towers
and energy-storage systems. Figure 9.9a and b describes direct and
reverse return piping, and Fig. 9.10a describes an open piping system
utilizing a cooling tower.
At one time, reverse return piping was always used to balance the
friction to all terminal units, particularly when they were equipped with
three-way temperature-control valves. Today, with the great emphasis
on energy savings, three-way temperature-control valves have very few
logical applications. Reverse return piping can be useful on buildings
where it may be the most economical arrangement. This is shown in
Fig. 9.11, where the supply pipe can go up one side of a building and
down the other. One advantage for reverse return piping is the reduc-
tion in maximum pressure drop across the temperature-control valves.
Figure 9.9c is a pressure-gradient diagram for a reverse return system
that demonstrates this fact. This diagram demonstrates why reverse
return piping should be used on constant-volume systems. With the
same pressure drop across each cooling coil, its control valve, and pip-
ing, balancing the system is much easier than with direct return. This
diagram also demonstrates why reverse return piping is no longer
needed with contemporary control valves on variable-volume systems.
Direct return piping is the most economical arrangement for most
contemporary buildings utilizing the energy savings of variable-volume
systems. It requires a minimum of piping and usually has less pipe
friction than an equivalent reverse return system. The pressure-
gradient diagram for a direct return system is shown in Fig. 9.6.
As seen in this diagram, the full pump head can be exerted across
the temperature-control valves. This is not a problem for the
designer of contemporary control valves as long as the maximum
head that can be imposed across the control valves is recognized.
The maximum head is not imposed on the valves except during
full-load conditions.
Open piping systems usually are encountered with cooling towers
or open energy-storage tanks; Figure 9.10a describes a typical open
system with a cooling tower serving the condenser of a chiller. The
pressure-gradient diagram for this system is shown in Fig. 9.10b.
Pressure-gradient diagrams are seldom needed for elementary cooling
tower applications because the operating pressures are relatively low.
On some complex cooling tower applications and process cooling oper-
ations, they are valuable for determining pump head and system
arrangement.

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.