Trane TRG-TRC016-EN manual Staggered Set Points

period two

Chilled-Water System Design

notes

chillers piped in series

Staggered Set Points

set point =48°F[8.9°C] set point =42°F[5.6°C]

42°F

48°F[5.6°C] [8.9°C]

52°F

[11.1°C]

Figure 46

An alternative method of controlling chillers in series involves staggering the set points of the two chillers. This results in the downstream chiller operating first and being preferentially loaded. Any portion of the load that the downstream chiller cannot meet is handled by the upstream chiller.

The example in Figure 46 shows the system operating at about 80 percent of design cooling load. As we mentioned earlier, with three-way valves at the coils, the temperature of the water returning to the chillers decreases at part load. At 80-percent load, the return-water temperature is 52°F [11.1°C], instead of the 54°F [12.2°C] at 100-percent load. The upstream chiller is partially loaded, cooling the water to the 48°F [8.9°C] set point, while the downstream chiller remains fully loaded, cooling the water the rest of the way to 42°F [5.6°C].

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