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TRG-TRC016-EN 37
period two
Chilled-Water System Design
notes
Figure 43 shows an example of the pump–system curve relationship. When
both pumps are operating, the system receives 100 percent of design flow.
When only one pump is operating, the intersection of the pump’s performance
curve with the system curve results in about 65 percent of design flow.
This configuration also presents problems to chiller operation. The starting or
stopping of a pump for one chiller affects the flow through the other chiller.
Using this same example, if one chiller is operating and a second chiller and
pump are started, the total water flow in the system does not double. The
system and pump performance curves will “rebalance,” resulting in an increase
in system flow of only 35 percent of total flow. The new total flow rate,
however, is now divided equally between the two chillers. This results in a rapid
reduction in water flow through the original operating chiller, from 65 percent of
total system flow to 50 percent. This rapid decrease in flow often results in a
loss of temperature control and may cause the chiller to shut off on a safety.
In order to overcome this problem, the chiller-plant control system should
anticipate the starting of additional pumps and unload operating chillers prior
to the start of an additional chiller. Again, this configuration is sometimes
acceptable for two-chiller systems, but is not often used in larger systems
because the part-load system flow problems are further multiplied.
head pressure
head pressure
percent flow
percent flow
2 pumps
2 pumps
1 pump
1 pump
system
system
curve
curve
100%
100%
65%
65%
chillers piped in parallel
Dedicated Pumps
Figure 43