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50 TRG-TRC016-EN
notes
period two
Chilled-Water System Design
In very large systems, a primary-secondary-tertiary pumping configuration is
sometimes used. The primary pumps circulate water through the chillers. The
secondary distribution pumps circulate the water around the distribution loop.
The individual load terminals are decoupled from the distribution loop and each
load terminal has a dedicated tertiary pump.
A “load terminal” in this case may be an individual cooling coil or an entire
building. In systems that use tertiary pumping, the load terminal must be
controlled so that only the quantity of water required is drawn from the
distribution loop. Water must not be allowed to flow into the return piping until
it has experienced the proper temperature rise. The two-way valve modulates
to maintain the design return-water temperature. A constant-volume tertiary
pump circulates water through the load terminal.
Some tertiary pumping systems use a small bleed line to ensure that water will
be immediately available when the tertiary pump is started, and to provide an
accurate control signal when the two-way valve is closed. It also keeps the
distribution pump from “dead-heading,” or trying to pump when all of the
two-way valves are closed. If a bleed line is used, it should be of a much smaller
diameter than the rest of the piping.
Tertiary Pumping
b
y
p
a
s
s
p
i
p
e
b
y
p
a
s
s
p
i
p
e
distribution
distribution
pump
pump
tertiary
tertiary
pump
pump
two
two
-
-
way
way
valve
valve
bleed line
bleed line
Figure 56