Trane TRG-TRC016-EN Asymmetric Design, notes, System Variations, period three, Different chiller

period three

System Variations

notes

Asymmetric Design

▲ Different chiller
capacities		10,000
▲ Different chiller	hours	8,000
efficiencies
					lag
	operating
		4,000
		6,000			chiller
	annual
		2,000			lead
					chiller
		0
			50% / 50%	60% / 40%

chiller split

Figure 80

Asymmetric Design

Many system design engineers seem to default to using chillers of equal capacity in a multiple-chiller system. There are benefits to using chillers of varying capacities to more favorably match the system loads. Remember that when a chiller is started, so is the associated ancillary equipment (pumps and cooling tower). In general, the smaller the chiller, the smaller the ancillary equipment. Operating the least number of chillers, and the smallest chiller possible, to meet the system load minimizes system energy consumption.

Figure 80 examines the use of a “60/40 split,” that is, one chiller (the “lead” chiller) is sized for 40 percent of the total system capacity and the second chiller (the “lag” chiller) for 60 percent. Notice that the number of hours of chiller (and ancillary equipment) operation is reduced by 15 percent, by changing from two chillers of equal capacity to one chiller at 40 percent capacity and the second chiller at 60 percent. This is because up to 60 percent load, only one chiller is operating. Below 40 percent load, only the lead chiller is operating; between

40 and 60 percent load, only the lag chiller is operating. In the same system with chillers of equal capacity, both chillers and their ancillary equipment are operating when the load is 50 percent or greater.

72 TRG-TRC016-EN