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36 TRG-TRC016-EN
notes
period two
Chilled-Water System Design
If separate, dedicated pumps are used with each chiller, a pump-and-chiller pair
can be turned on and off together as the cooling load varies. This solves the
temperature mixing problem that occurred in the previous, single-pump
configuration, but it presents a new problem in a system that uses a constant-
flow method of terminal control.
Below 50-percent load, only one chiller and one pump are operating. The total
water flow in the system decreases significantly, typically 60 to 70 percent of
full system flow. Ideally, at this part-load flow rate, all of the coils will receive
less water, regardless of their actual need. Typically, however, some coils
receive full water flow and others receive little or no water. In either case,
heavily-loaded coils will usually be “starved” for flow. Examples of spaces
with constant heavy loads that may suffer include computer rooms, conference
rooms, photocopy rooms, and rooms with high solar loads.
54°F
54°F
[12.2°C]
[12.2°C]
42°F
42°F
[5.6°C]
[5.6°C]
off
off
on
on
60% to 70%
60% to 70%
of system flow
of system flow
coil starved for flow
coil starved for flow
chillers piped in parallel
Dedicated Pumps
Figure 42