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Staging cooling-tower fans
CNT-APG002-EN 37
®
Staging cooling-tower fans
Staging applications organize individual pieces of equipment into a group
to accomplish a single task. For example, several fans might be used to
maintain the supply water temperature in a cooling tower. Staging appli-
cations control a series of binary outputs on and off at specific times based
on an analog value. This value can be generated by a linear equation, a
PID calculation, a reset block, and so on.
The advantage of using PID control for staging applications is that you
can use the error deadband to optimize the system so that stages cycle
less often (see “Adjusting error deadband for staged outputs” on page 20).
Another advantage is that PID control is built into Trane controllers,
making settings easy to enter and adjust.
This section describes how to use a proportional-only PID loop to control
supply water temperature in a cooling tower with several fans instead of
a variable-frequency drive.
Setting up the PID loop
Proportional-only control works well in staging applications because the
output is linear and predictable, and therefore easy to manage. Integral
control can also be used but is much more complex to set up and tune.
To use proportional-only control in process control language (PCL), set
the integral and derivative gains to zero. In Tracer graphical program-
ming (TGP) editor, select Proportional Only in the PID Properties dialog
box (see “Programming in TGP” on page 26).