Applications Guide Tracer Graphical Programming - Trane

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Writing the discharge air control program
Controlling the cooling valve
Take a closer look at the sequence of operation with regard to the cooling
valve.
Modulate the cooling valve to maintain the discharge air temperature at
the discharge air cooling setpoint when all ofthefollowingaretrue:
• The supply fan is on.
• The air handler is in cooling mode.
• The economizer function is not enabled.
Or when all ofthefollowingaretrue:
• The supply fan is on.
• The air handler is in cooling mode.
• The economizer function is enabled.
• The outdoor air damper is open at least 90%.
Close the cooling valve when any of the following is true:
• The air handler is in heating mode.
• The supply fan is off.
• The discharge air temperature sensor is failed.
It may help to think of control of the cooling valve in two parts. First, programthedecisiontooperatethecoolingvalveasshowninFigure115.
• Use a combination of logic blocks to determine whether the cooling
valve operates.
• Apply the output of the economizer decision here.
• Use a Deadband block and the position of the outdoor air damper.
Figure 115: Operate the cooling valve?
The output of this decision feeds into the actual control of the cooling
valve using a PID loop (Figure 116 on page 124).
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