Trane TRG-TRC016-EN manual notes, Chilled-WaterSystem Design, period two, Dedicated Pumps

period two

Chilled-Water System Design

notes

chillers piped in parallel

Dedicated Pumps

	2pumps
headpressure	1pump
	system
	curve
	65%	100%

percent flow Figure 43

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.

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