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32 TRG-TRC016-EN
notes
period two
Chilled-Water System Design
Properly designed, operated, and maintained, any of these three methods can
result in good space comfort control. However, they have different effects on
the chilled-water system.
The use of three-way valves or face-and-bypass dampers results in variable
return-water temperature and relatively constant chilled-water flow through
the entire system. The use of two-way valves results in constant return-water
temperature and variable water flow through the entire system. Before
choosing one of these control methods, it is necessary to determine the
effect that it will have on the other parts of the chilled-water system.
In the past, the water flow rate through the chiller evaporator was to remain
as constant as possible. The vast majority of chilled-water systems employ
pumping schemes that maintain a constant flow rate of water through each
chiller evaporator. Even in the most-carefully-designed chilled-water systems,
however, the flow through the chillers will still vary slightly due to system
Load-Terminal Control Options
▲ Three-way modulating valve
◆ Constant water flow
◆ Variable system return-water temperature
▲ Two-way modulating valve
◆ Variable water flow (pump energy savings)
◆ Constant system return-water temperature
▲ Face-and-bypass dampers
◆ Constant water flow
◆ Variable system return-water temperature
◆ Enhanced dehumidification capability with “wild” coils
Figure 37
Chiller Evaporator Flow
▲ Constant flow is most
common
▲ Variable flow is
possible
◆ Can reduce energy
consumption
◆ Use only with
advanced chiller and
system controls
evaporator
evaporator
Figure 38