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56 TRG-TRC011-EN
notes
period five
Application Considerations
The second beneficial operating characteristic is that an absorption chiller
works more efficiently and produces more cooling with increased leaving-
chilled-water temperatures.
Applications with two chillers can be either piped in series or in parallel.
Though there are advantages associated with each arrangement, the series
configuration allows a noticeable increase in the overall system efficiency of a
combination gas-and-electric chiller plant. The series arrangement allows the
upstream chiller to cool the water part of the way and uses the downstream
chiller to cool the water the rest of the way to the setpoint. Placing the
absorption chiller in the upstream position allows it to provide a warmer
leaving-chilled-water temperature, 50ºF [10ºC] in this example. This not only
improves the absorption chiller’s efficiency and capacity, but also reduces the
cooling load and energy consumption of the electric chiller.
The series arrangement also has the capability to preferentially load the gas-
burning chiller, allowing the system to maximize the use of lower-cost fuel
during periods of high electrical energy cost. Piping two chillers in series also
means that the entire system-water flow must pass through both chillers.
Exercise care when selecting the chillers to avoid exceeding their maximum
flow rates. Notice that the example series arrangement shown here also takes
advantage of a 16°F [8.9°C] temperature differential across the chillers. This
increased temperature differential allows the water flow rate to be reduced and
results in lower pumping costs.
Overall, the key to successful implementation of a combination gas-and-electric
chiller plant is an intelligent building automation system that optimizes chiller
plant operation relative to electrical and gas utility rate structures.
58°F
58°F
[14.4°C]
[14.4°C]
50°F
50°F
[10°C]
[10°C]
42°F
42°F
[5.6°C]
[5.6°C]
absorption
chiller
absorption
absorption
chiller
chiller
electric
chiller
electric
electric
chiller
chiller
Figure 64