period two
Chilled-Water System Design
notes
chillers piped in series
Equal Set Points
set point =42°F[5.6°C] | set point =42°F[5.6°C] |
42°F
48°F[5.6°C] [8.9°C]
54°F 
[12.2°C]
Figure 45
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:
nUsing 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.
nUsing 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.
