ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
308
Fig. 15. Dynamic Chiller Control Display.
3. Appropriate cooling tower and control is required. See
COOLING TOWER AND CONDENSER WATER
CONTROL.
4. Two-way AHU control valves. This allows good single-
chiller, single pump operation.
5. Appropriate chilled water flow and differential pressure
controlled bypass valve to keep the minimum required
flow through the chillers (varies with chiller
manufacturers).
6. The unoccupied period 80 percent load limiting
parameters are based on the assumption that VAV AHUs
are operating under a reduced maximum airflow setpoint
during all unoccupied cooling modes (see the Air
Handling System Control Applications section) to save
fan energy while simultaneously placing the chiller
operation in its maximum efficiency range.
Features
1. Automatic start-stop, staging, and setpoint optimization
of chillers.
2. User friendly monitoring and adjustment.
3. Optimized unoccupied operation.
4. Chiller that has run longest since last start is first to stop.
5. Chillers cannot start late (shortly before the unoccupied
period begins).
Conditions For Successful Operation
1. Control network, software, and programming advises
chiller controller of AHU chilled water demands.
2. Interlock and control wiring coordinated with the chiller
manufacturer.
ANYTIME ANY AHU VALVE IS OPEN GREATER THAN % FOR MORE THAN
MINUTES AND THE TIME IS BEFORE , THE LEAD CHILLED WATER PUMP STARTS.
ANYTIME THE LEAD CHILLER HAS RUN LONGER THAN MINUTES, THE CHILLED
WATER TEMPERATURE IS GREATER THAN DEGREES ABOVE SETPOINT FOR MORE
THAN MINUTES, THE BYPASS VALVE IS CLOSED, AND THE TIME IS BEFORE ,
THE OFF CHILLED WATER PUMP STARTS.
ANYTIME BOTH CHILLERS ARE RUNNING AND THE WATER DIFFERENTIAL TEMPERATURE IS
LESS THAN KELVINS FOR MORE THAN MINUTES, THE CHILLER WHICH HAS
RUN LONGEST STOPS AND REMAINS OFF AT LEAST MINUTES.
ANYTIME ALL AHU CHILLED WATER VALVES ARE LESS THAN % OPEN
THE CHW TEMP. SETPOINT INCREMENTS UP AT THE RATE OF KELVINS EVERY
MINUTES TO A MAXIMUM OF DEGREES.
ANYTIME ANY AHU CHILLED WATER VALVE IS FULL OPEN, THE CHW TEMPERATURE SETPOINT
DECREMENTS DOWN AT THE SAME RATE TO A MINIMUM OF DEGREES.
ANYTIME ANY CHILLER IS RUNNING AND ALL AHU VALVES ARE OPEN LESS THAN %
FOR MORE THAN MINUTES, AND THE CHILLER HAS RUN GREATER THAN
MINUTES, THE CHILLED WATER PUMP STOPS.
320
1545
90
4
1545
2.4
4.0
30
80
0.15
5
11
7
20
3
30
8
105.5
MINIMUM CURRENT MAXIMUM
CHILLED WATER SETPOINT
0.5
CHILLED WATER SYSTEM
M15256
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