Applications Guide Tracer Graphical Programming - Trane

®
Chapter 4 Cooling tower with two-speed fan example
4. Place a Variable block in the design space and assign the analog variable,
Supply Temp Setpoint (sourced from the operator display/service
tool), to it (Figure 77).
Figure 77: Input blocks for the cooling tower fan module
Adding the output blocks
The cooling tower fan module requires two outputs.
To add the output blocks:
◆
Place two Output (Hardware) blocks in the design space and assign
the following binary outputs to them (Figure 78):
• Fan Start/Stop/Low
• Fan High
Figure 78: Output blocks for the cooling tower fan module
Starting the fan at low speed
Dissecting the sequence of operation into ever smaller sections, you find
that starting the fan at low speed depends on two criteria.
• Condenser water flow is established.
• The condenser supply temperature is 2.5°F above the setpoint.
Use a Deadband block to start and stop the fan. Apply a centered deadband
with a deadband value of 5.0°F. According to the sequence, the
upper limit, or setpoint, for the Deadband block is equal to the condenser
water setpoint plus 2.5°F.
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