period four
Chiller-Plant Control
notes
load indicators
Capacity
supply-water temperature
flow meter
return-water 
temperature
chiller-plant controller
Figure 93
Another method of monitoring system cooling load is to measure the system water flow rate and temperatures directly, and then calculate the load. Although it would appear that direct measurement of the actual system load would be an excellent way to determine when to turn chillers on and off, this method has several drawbacks. It requires the use of flow meters with high accuracy and high turndown capacities. Although flow meters have become more accurate and less expensive, they require special installation conditions for reliable accuracy—conditions seldom achievable in real installations. Also, the equipment typically requires regular calibration. For these reasons, direct measurement of load has not been used as much as the simple and reliable methods discussed previously.
An alternate way to monitor chiller load is by measuring the current draw of the chiller motor. By itself, this does not provide an adequate control indicator, but when used in conjunction with other information, such as system supply-water temperature, it can be effective. System supply-water temperature is used to determine when to turn an additional chiller on, and operating chiller compressor-motor current draw is used to determine when a chiller can be turned off.
The most effective load indicator for any chilled-water system is dependent on the design of that system. Creative designers have used the control strategies as described here and in various combinations to effectively control a wide variety of chiller plants. It is highly recommended that one of the first tasks undertaken in the design process is to create a simplified flow diagram and a load model of the system that allows for the evaluation of various control strategies and sensor placements. This will help to ensure that effective chiller-plant control can be implemented.
