User's Manual

Staging cooling-tower fans
CNT-APG002-EN 41
®
For staging applications, the result of the PID calculation controls binary
outputs rather than an analog output. For this kind of staging applica-
tion, it is typical to use the deadband to make sure that the binary output
state is maintained for some specific range. Figure 30 illustrates the stag-
ing points for two cooling-tower fans. The three lines indicate (from bot-
tom to top): the number of fans versus the control value, fan 1 on and off
points, and fan 2 on and off points. Fan 1 is turned on at 63% and off at
10%. Fan 2 is turned on at 90% and off at 36%.
Figure 30: Cooling tower fan on and off points
Control value (%)
Fan 2
Fan 1
No. of
fans
2
1
0
0 10 20 30 40 50 60 70 80 90 100