Applications Guide Tracer Graphical Programming - Trane

®
Chapter 5 Cooling tower with variable-speed fan example
To calculate the approach temperature:
1. Place a Subtract block in the design space and connect it so that the
Wet-Bulb Temp is subtracted from the Supply Temp.
2. Place a Variable block in the design space and assign the analog variable,
Approach Temp, to it.
3. Connect the Subtract block to the Approach Temp variable block so
that the calculated value is written to the variable (Figure 89).
Figure 89: Calculations module complete
Writing the cooling tower fan module
The new sequence of operation calls for modulation of the fan speed to
maintain the condenser water supply temperature.
Upon successful confirmation of condenser water flow, and when the
cooling tower water supply temperature exceeds the setpoint by 2.5°F,
turn on the fan. Modulate the fan to maintain the condenser water
supply temperature according to setpoint. When the cooling tower
supply water temperature is 2.5°F below the setpoint, turn off the
fan. Note that the setpoint is adjustable from the operator display
and is limited to a minimum of 65°F and a maximum of 95°F.
Starting and stopping the fan
As in Chapter 4, a deadband is required to start and stop the fan. Use a
centered deadband to start the fan when the water temperature is 2.5°F
above the setpoint and to stop the fan when the water temperature is
2.5°F below setpoint. Remember that flow must be confirmed in order for
the fan to start.
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