period four
Chiller-Plant Control
notes
load indicators
Temperature
supply-water temperature
return-water 
temperature
chiller-plant controller
Figure 91
Today’s chiller controls can very accurately control the chiller’s leaving-water temperatures over a wide range of loads. This is especially true of centrifugal and helical-rotary chillers. This fact allows constant-flow chilled-water systems, similar to the system shown in Figure 91, to use the system supply- and return- water temperatures to determine system load.
By sensing a rise in the temperature of the water leaving the chiller plant, the control system can determine when the operating chillers can no longer maintain the desired temperature. Often, the supply-water temperature is allowed to drift a predetermined amount before an additional chiller is turned on, to ensure that there is enough load to keep an additional chiller operating.
Deciding when it is appropriate to turn a chiller off is more complex. The control system may monitor the system ∆T, that is, return-water temperature minus supply-water temperature. This information, along with the capacities of the operating chillers, allows the control system to determine when a chiller can be turned off. To help stabilize system operation, the control system should use logic to prevent load transients from causing unwarranted chiller cycling.
In constant-flow systems that are suffering from “low ∆T syndrome” (airside systems that return water to the plant at lower temperatures than desired), some of the load terminals may starve for flow before the capacity of the operating chiller is exceeded. To preserve system efficiency, this situation is best dealt with by solving the airside problem. Typical causes of low ∆T syndrome include: a poorly-balanced flow system, dirty filters or coils, poorly performing air-handler controls, incorrect coil control valves, or undersized air handlers.
