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Staging cooling-tower fans
CNT-APG002-EN 39
®
The TGP program follows this sequence of operation:
1. Chilled-water pump status is checked. If there is flow, the cooling
towers are allowed to operate.
2. Based on the error (the difference between the chilled-water setpoint
and the chilled-water temperature), the controller turns cooling-tower
fans on or off as needed to ensure efficient cooling tower operation.
3. If the chilled-water temperature sensor fails, all cooling-tower fans
are turned on.
Note that:
• A 2-input Or block (a TGP block) checks the status of the chilled-
water pumps. Both fan stages are turned off if neither chilled-water
pump is operating.
• A PID calculation generates an output based on the difference
between the chilled-water setpoint and the chilled-water tempera-
ture. If both pumps are off, the PID calculation is disabled and the
output set to the default of zero.
• If the chilled-water temperature sensor fails, the PID output defaults
to the fail-safe value of 100, which turns both fan stages on.
Specific settings are listed in Table 16.
The throttling range is fairly wide—from 10°F to 25°F (6°C to 14°C). The
large throttling range keeps control loose to prevent stages from cycling
too often. Assuming an output range from 0 to 100, the throttling range
translates to a proportional gain of 26 to 10 respectively (see “Calculating
the gains” on page 11). The proportional gain chosen for this application
is in the middle of that range at 17. The gain may need to be adjusted to
optimize the system.
The temperature of the water flowing through a cooling tower responds
fairly slowly, so the sampling frequency should be set to at least 1 minute.
The sampling frequency may need to be adjusted to a slower rate if the
temperature oscillates around the setpoint (see “Calculating the sampling
frequency” on page 14). The goal in this case is to effectively control the
water temperature while limiting equipment cycling.
Table 16: Settings for staging cooling-tower fans
PID setting Initial value
Proportional gain 17 (midrange between 10 and 26)
Integral gain 0
Derivative gain 0
Proportional bias 63% (set the same as the first stage enable value)
Error deadband 2.0°F (1.1°C)
Action Direct
Sampling frequency At least 1 minute