ENGINEERING MANUAL OF AUTOMATIC CONTROL
PNEUMATIC CONTROL FUNDAMENTALS
94
DISCHARGE AIR TEMPERATURE
CONTROL SEQUENCE
Any time the AHU (Fig. 79) operates in the non-warm-up
mode, Switching Relay SR-4 operates to allow the normal
Discharge Air Temperature Controller TC-3 to modulate the
hot water valve closed (through Switching Relay SR-1, Fig. 77)
and the chilled water valve open in sequence, on a rising cooling
load, to maintain the Temperature Controller TC-3 setpoint.
Controller TC-3 is a PI (proportional plus integral) controller.
NOTE: In this constant 1.9 m
3
/s outside air system, if the return
air is 22°C and the outside air is -25°C, the mixed air
temperature will drop below 13°C if the AHU airflow
drops below 52 percent of the design airflow.
OFF/FAILURE MODE CONTROL
SEQUENCE
If compressed air fails, both control valves open, the outside
air damper closes, and the return air damper opens.
When the fan is off, Switching Relay SR-3 (Fig. 78) positions
to close the hot water valve, Switching Relay SR-5 (Fig. 79)
positions to close the chilled water valve, the outside air damper
closes, and the return air damper opens.
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