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TRG-TRC016-EN 95
period four
Chiller-Plant Control
notes
Constant-flow chilled-water systems frequently require individual chiller set-
point control. Its purpose is to help maintain the system supply-water
temperature by compensating for the bypass of return water through
non-operating chillers.
The chiller-plant control system adjusts the individual set points for the
operating chiller to “overcool” the water before it mixes with the higher-
temperature water that bypasses through the non-operating chiller. The result
is that the chilled water supplied to the system is as close as possible to the
desired temperature. There are limits to the amount of overcooling. Depending
on the design of the chilled-water system, one of two situations may exist.
Either the chiller may not have been selected to produce cold-enough water,
or the temperature required may be below the freezing point of the water being
cooled. In either case, the control system must be intelligent enough to limit
overcooling in order to prevent damage to the chiller.
Additionally, the control system must know when to turn another chiller on to
meet the system chilled-water-temperature set point. Turning an additional
chiller on may be required to meet the system demand for flow, even though
the operating chiller may not be fully loaded.
constant-volume pumping system
Chilled-Water Set Point Control
54°F
54°F
[12.2°C]
[12.2°C]
37°F
37°F
[2.8°C]
[2.8°C]
54°F
54°F
[12.2°C]
[12.2°C]
off
off
on
on
45.5°F
45.5°F
[7.5°C]
[7.5°C]
lowering operating
lowering operating
chiller set point
chiller set point
helps regain control
helps regain control
Figure 103