Variable Speed Cooling Tower Fan Control; Single Cooling Tower Variable Speed Fan Control - Honeywell AUTOMATIC CONTROL Engineering Manual

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
Specification:
Condenser water temperature setpoint shall be equal to the OA
WB plus 7F, or the minimum temperature acceptable to the
chiller, whichever is higher. Fan shall stop when condenser
water temperature drops to setpoint. Fan shall start at low speed
when the temperature rises to the setpoint plus 6F, and shall
not stop for a minimum of 9 minutes. Fan shall transition from
low to high speed anytime it has run at least 3 minutes at low
speed and the temperature is above the low-speed start point
plus 2F or 85F, whichever is lower. Fan shall run a minimum of
9 minutes on high speed, and shall transition from high speed
to low speed if the temperature drops to a value equal to the
switch-to-high-speed value minus 6F.

SINGLE COOLING TOWER VARIABLE SPEED FAN CONTROL

Functional Description
1
7
100
PERCENT
OPEN TO
TOWER
Item No. Function
1 Indicates percent maximum tower fan speed.
2 Sets tower minimum speed.
3 Current OA conditions.
4 Sets minimum and maximum condenser water
temperature setpoints and WB approach.
5 Icon to select tower low load control dynamic
sequence display (Fig. 30).
ENGINEERING MANUAL OF AUTOMATIC CONTROL
TOWER FAN CONDENSER WATER TEMPERATURE
SETPOINT EQUALS OA WET BULB TEMPERATURE PLUS
4
71
MINIMUM
6
12
8
69
10
Fig. 29. Single Cooling Tower Variable Speed Fan Control Graphic
If required, use a valve control method similar to the one
used for single speed fan control.

Variable Speed Cooling Tower Fan Control

For variable speed cooling tower fans, an EPID control
function with WB optimized setpoint or fixed setpoint works
well until the fan drops to a minimum safe speed, below which
results in motor overheating. If the tower capacity is still
excessive at minimum fan speed, the fan control mode reverts
to on-off. During this mode, disable the PI function or when
the fan is restarted it will start at a speed greater than the
minimum, although the minimum speed was too great. This
produces an undesirable short on time. The following is a
possible application and specification:
7.0
DEGREES
78
85
CURRENT MAXIMUM
LOW LOAD/
FREEZING
CONTROL
5
CHILLER
86
6 Condenser water control sensor.
7 Displays valve position.
8,9 Dynamic pump symbols denote pump
operation.
10,11 Operator information.
12 Sets valve control setpoint.
320
OUTSIDE AIR
TEMPERATURE
87 71
DRY
WET
BULB
BULB
3
TOWER FAN MINIMUM
SPEED EQUALS
2
25
PERCENT
11
45
9
M15046