Trane SYS-APM001-EN manual Series Chillers, System Configurations, Series chillers

System Configurations

System ΔT greater than 14°F

One reason series chilled-water systems should be designed using at least a 14 degree ΔT is because a lower ΔT ignores the opportunity for lower flow rates and reduced pump energy. For small, packaged chillers, a reason to have at least a 14 degree ΔT is to avoid exceeding the chiller’s maximum flow rate. The system will more likely experience evaporator flow limit maximums if the ΔT is too low.

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
56°F [13.3°C]	49°F [9.4°C]		42°F [5.5°C]
	Chiller 2	Chiller 1
	setpoint = 42°F [5.5°C]
	setpoint = 42°F [5.5°C]

Loads

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

44 Chiller System Design and Control SYS-APM001-EN