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40 TRG-TRC016-EN
notes
period two
Chilled-Water System Design
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].
chillers piped in series
Staggered Set Points
52°F
52°F
[11.1°C]
[11.1°C]
42°F
42°F
[5.6°C]
[5.6°C]
set point =
set point =
48°F
48°F
[8.9°C]
[8.9°C]
set point =
set point =
42°F
42°F
[5.6°C]
[5.6°C]
48°F
48°F
[8.9°C]
[8.9°C]
Figure 46