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44 Chiller System Design and Control SYS-APM001-EN
System Configurations
Series Chillers
If chillers are piped in series, as in Figure 27, the mixing problem disappears
and the starving coils problem (when one of the pumps in a parallel
arrangement is not running) is resolved. Series flow presents a new set of
temperature and flow control challenges.
Figure 27. Series chillers
The flow rate through each chiller is the entire system flow, that is, double the
individual flow rate of two parallel chillers. This means that the chiller
evaporator must accommodate the doubled water quantity. This may be
accommodated by using fewer water passes in the evaporator, which may
result in decreased chiller efficiency.
However, this efficiency loss due to fewer passes is more than offset by the
increased efficiency of the upstream chiller, now operating at a warmer
temperature.
Pressure losses are additive when the chillers are piped in series. This
increases total system pressure drop, thereby using more pump energy. On
the other hand, series chillers work particularly well in low-flow systems,
where the system temperature difference is greater than 14°F [7.8°C],
resulting in less pressure drop.
Low-flow systems were discussed in detail in “Selecting flow rates” on
page 30. Series chillers are also suited to variable flow systems, where the
operating pressure drop is reduced. Variable flow is discussed beginning on
page 55.
Temperature control can be executed in several ways. Figure 27 shows a
strategy where the controller on each chiller is set at the system design
setpoint. Either chiller can be used to meet the system demand for up to 50
System T greater than 14°F
One reason series chilled-water systems
should be designed using at least a 14
degree T is because a lower T ignores
the opportunity for lower flow rates and
reduced pump energy. For small, packaged
chillers, a reason to have at least a 14
degree T is to avoid exceeding the chiller’s
maximum flow rate. The system will more
likely experience evaporator flow limit
maximums if the T is too low.
Chiller 2
setpoint = 42°F [5.5°C]
Chiller 1
setpoint = 42°F [5.5°C]
56°F [13.3°C]
Loads
49°F [9.4°C]
42°F [5.5°C]