On-Off Cooling Tower Fan Control; Two-Speed Cooling Tower Fan Control - Honeywell AUTOMATIC CONTROL Engineering Manual

FANS ON IN SEQUENCE
ON TEMPERATURE RISE
COOLING
TOWER
CONDENSER
WATER
TEMPERATURE
CONTROL CWS
Fig. 27. Cooling Tower Control.
For minimum chiller energy usage, the condenser water
temperature setpoint should be as low as can be safely used by
the refrigeration system or as low as can be provided by outdoor
air conditions. The approach value of the evaporative cooling
tower indicates how close the cooled water temperature can
come to outdoor air wet-bulb temperature at design conditions.
When outdoor air is at a lower wet-bulb temperature than design,
the tower can cool water to a lower temperature, but never to
the wet-bulb temperature. Therefore, the controller setpoint
should be at the lowest temperature attainable by the cooling
tower to save chiller energy and not waste fan energy trying to
reach an unobtainable value. Figure 28 is a reset schedule of
condenser water setpoint as a function of outdoor air wet-bulb
temperature with the minimum safe temperature indicated.
CONDENSER
WATER
TEMPERATURE
SETPOINT
MINIMUM SAFE
TEMPERATURE
OA WET BULB TEMPERATURE
CONDENDER WATER SETPOINT = OA WET BULB + MINIMUM DIFFERENCE
Fig. 28. Condenser Water Temperature
Reset from Outdoor Wet-Bulb.
When wet-bulb reset is not used, a condenser water
temperature setpoint around midpoint between the design
(usually 85F) and the minimum value acceptable to the chiller
is satisfactory.
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
BYPASS
VALVE
CWR
C2695
MINIMUM TEMPERATURE
DIFFERENCE THAT CAN BE
ATTAINED (APPROACH)
OUTDOOR AIR
WET-BULB
TEMPERATURE
C1480

On-Off Cooling Tower Fan Control

On-off fan control is satisfactory where the load is always
high and where several towers are banked together for
multistage control.
On-off cooling tower control with a single setpoint for a PI
control function produces questionable performance. A
preferred solution starts the fan on a rise in condenser water
temperature to 80F, stops the fan on a drop to 73F, and limits
on periods to a minimum of 5 minutes. The 5 minute minimum
on time can still produce water too cool for the chiller during
light loads and low OA WB temperatures. The use of a
modulating valve can prevent condenser water temperature from
dropping below the lowest that the chiller can tolerate (valve
control would be PI with a single setpoint). During below
freezing OA conditions throttling condenser water into the tower
can cause icing, cycling the valve open and closed can reduce
freezing. The specification following provides warmer ice-
melting water during colder weather.
Specification:
On a rise in condenser water temperature to 80F the cooling
tower fan shall start and on a drop in condenser water
temperature to 73F the cooling tower fan shall stop. When
started, the cooling tower fan shall run a minimum of five
minutes. When the OA temperature is above freezing, the bypass
valve shall modulate if required to prevent the condenser water
from falling below 71F. When the OA temperature drops below
32F, the bypass valve shall open to the bypass on a drop in
condenser water temperature to a minimum acceptable
temperature and open to the tower upon a rise in temperature.
The temperature rise shall vary between 5 and 12F as the OA
temperature varies from 32F to 0F.
NOTE: The variable temperature rise allows warmer water to
enter the tower during extreme weather to encourage
melting and reduce icing.

Two-Speed Cooling Tower Fan Control

For two-speed cooling tower fans, a single setpoint for a PI
control function also produces unpredictable performance. A
better solution would be one of absolute setpoints similar to
that noted for on-off fans. If wet-bulb reset is used, the control
objective is to turn the fan off at the optimum setpoint but at no
less than the minimum temperature acceptable to the chiller
plus 1 degree, and cycle to and from low and high speeds above
this value with minimum on times for each speed and with no
more than 7 speed changes per hour for fans under 10 feet in
diameter or as required by the tower manufacturer. The
specification following is typical.
319 ENGINEERING MANUAL OF AUTOMATIC CONTROL