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42 Chiller System Design and Control SYS-APM001-EN
System Configurations
Multiple chilled-water systems are more common than single chilled-water
systems for the same reason that most commercial airplanes have more than
one engine—the balance of reliability and cost. The most typical system
configuration, by far, has two chillers. Since system loads can vary throughout
a wide spectrum, multiple chilled-water systems can often operate with one
chiller. During these periods, if the system is designed properly, the energy
required to operate a second chiller and its auxiliaries can be conserved.
This section examines:
• Constant flow systems
• Systems in which flow is constant through chillers, but variable through the
rest of the system
• Systems in which flow varies throughout the entire system—including the
chillers
Parallel Chillers
Figure 25 shows a system with two chillers piped in parallel, using a single
chilled-water pump.
Figure 25. Parallel chillers with a single, common chiller pump
With constant flow loads, water flows in both chillers continually, whether the
chiller is operating or not. Clearly, this can disrupt the supply chilled-water
temperature when only one chiller is operating. The temperatures indicated in
Figure 25 show how the supply water temperature rises when one chiller is
cycled off in response to a part-load condition. This may result in inadequate
dehumidification capabilities or the inability to satisfy specific loads.
Chiller 2 (Off)
54°F [12.2°C]
54°F [12.2°C]
Chiller 1 (On)
49°F [9.4°C]
44°F [6.6°C]
Constant
Flow
Pump