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TRG-TRC016-EN 85
period four
Chiller-Plant Control
notes
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.
load indicators
Capacity
flow
flow
met er
met er
chiller
chiller
-
-
plant
plant
controller
controller
supply
supply
-
-
water
water
temperature
temperature
return
return
-
-
water
water
temperature
temperature
Figure 93