Trane SYS-APM001-EN Preferential loading - series arrangement, Chilled-Water System Variations

Chilled-Water System Variations

Sidestream system control

The flexibility of sidestream applications is increased by the fact that the devices are used to pre-cool return water, not to produce the system chilled- water temperature. This means that they may be loaded by a different signal. In the case of a plate-and-frame heat exchanger, as long as return water is being cooled, there is an advantage to using it. A heat-recovery chiller can be loaded to produce just the amount of hot water necessary using the condenser-water leaving temperature as a signal. If preferential loading is used with an absorption chiller, it may be loaded simply by decreasing its leaving-water temperature.

Sidestream configurations may also be used to preferentially load chillers or a heat exchanger used in a variable-primary-flow system, or to isolate chillers that are incapable of the same flow variations as the rest of the system.

Preferential loading – series arrangement

As previously discussed in “Series Chillers” on page 44, preferential loading is very simple when chillers are placed in series. If both the upstream and downstream chillers are given the system leaving water temperature setpoint, the upstream chiller is preferentially loaded and the downstream chiller operates whenever the upstream chiller can no longer achieve setpoint. If the downstream chiller is given the system setpoint, and the upstream chiller is given a warmer setpoint, then the downstream chiller loads first. Another method for preferential loading uses compressor RLA to determine when to bring on the next chiller.

Figure 47. Preferential loading - series arrangement
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

76 Chiller System Design and Control SYS-APM001-EN