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TRG-TRC016-EN 39
period two
Chilled-Water System Design
notes
Temperature control in a series system can be accomplished in several ways,
depending on the desired operating sequence. The first method, shown in
Figure 45, has both set points adjusted to the desired system supply-water
temperature. Assuming equally-sized chillers, either chiller can meet the load
below 50 percent. Above 50-percent load, both chillers operate and the
upstream chiller is preferentially loaded. This means that the upstream chiller is
operated at full capacity and any portion of the load that remains is handled by
the downstream chiller.
This strategy may be desirable in systems that benefit from preferentially
loading the upstream chiller. Examples include:
n Using a heat-recovery chiller in the upstream position. Because the chiller is
at full capacity whenever the system load exceeds 50 percent, the amount of
heat available for recovery is maximized.
n Using an absorption chiller in the upstream position. An absorption chiller
works more efficiently, and has a higher cooling capacity, with higher
leaving-chilled-water temperatures. The absorption chiller in the upstream
position provides a warmer leaving-chilled-water temperature at design
conditions, 48°F [8.9°C] in this example. This arrangement preferentially
loads the gas-burning absorption chiller, allowing the system to maximize
the use of a lower-cost fuel during periods of high electrical-energy cost.
Alternatively, equal loading of the two chillers in series can be accomplished
using a chiller-plant control system to monitor system load and balance chiller
loading. The set point for the downstream chiller is set equal to the desired
system supply-water temperature, and the set point for the upstream chiller is
then dynamically reset to maintain equal loading on both chillers. The control
system must be stable enough to prevent control hunting or chiller cycling
during periods of changing load.
chillers piped in series
Equal Set Points
54°F
54°F
[12.2°C]
[12.2°C]
42°F
42°F
[5.6°C]
[5.6°C]
set point =
set point =
42°F
42°F
[5.6°C]
[5.6°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 45