Honeywell AUTOMATIC CONTROL Engineering Manual page 331

LOW LOAD CONDENSER WATER TEMPERATURE CONTROL:
TOWER FAN STOPS, VARIABLE SPEED CONTROL IS DISABLED, AND ON-OFF FAN
CONTROL IS ENABLED IF LOAD DROPS AFTER FAN DROPS TO
ON-OFF CONTROL: FAN RESTARTS IF TEMPERATURE RISES TO SETPOINT PLUS
DEGREES. FAN STOPS IF TEMPERATURE DROPS TO SETPOINT, BUT RUNS A MINIMUM OF
MINUTES. DURING ON PERIODS OF ON-OFF CONTROL, THE FAN RUNS AT
7
MINIMUM SPEED.
VARIABLE SPEED CONTROL RECOVERY: IF, AFTER CYCLING FAN ON FOR 3 MINUTES, THE
TEMPERATURE RISES
THE FAN RESUMES VARIABLE SPEED CONTROL.
VALVE CONTROL: VALVE MODULATES WATER TO SUMP AS NECESSARY TO MAINTAIN
VALVE CONTROL SETPOINT
FREEZING CONTROL: IF OA IS BELOW FREEZING, VALVE SHALL SWITCH TO TWO
POSITION CONTROL. VALVE SHALL OPEN TO SUMP AT VALVE CONTROL SETPOINT,
AND SHALL REVERT BACK TO TOWER WHEN TEMPERATURE RISES TO SETPOINT
PLUS DIFFERENTIAL. DIFFERENTIAL SHALL VARY FROM
AS OA VARIES FROM
Features
1. Precise PI condenser water control above minimum load.
2. User friendly monitoring and adjustment.
3. Optimized condenser water temperature.
4. Freeze prevention cycle.
5. Fan motor light-load overheating protection.
Conditions For Successful Operation
1. Control network, software, and programming to advise
tower controller of chiller demands.
2. Interlock and control wiring coordinated with the chiller
manufacturer.
3. Good OA RH sensor and WB calculation.
4. Proper project-specific setpoint and parameter settings.
5. If tower must operate with OA temperature below
freezing, the diverting valve must be controlled two
position, during freezing conditions.
Specification
Cooling tower controls shall be enabled anytime the
condenser water pump runs. The cooling tower fan speed shall
be modulated under EPID control down to a minimum 25%
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
DEGREES ABOVE THE "RESTART" TEMPERATURE
2
DEGREES.
69
TO DEGREES.
32 0
Fig. 30. Cooling Tower Load Control Dynamic Display.
25
TO
5
speed to maintain a condenser water temperature setpoint equal
to the OA WB temperature plus 7F, or a value equal to the
minimum temperature acceptable to the chiller plus 2F,
whichever is higher, but not to exceed 85F. If the tower capacity
is still excessive at minimum speed, control shall revert to an
on-off algorithm, cycling the fan off at the minimum speed at a
setpoint equal to the above setpoint, and back on at the minimum
speed at a temperature equal to the off temperature plus 6F,
with on cycles of no less than 7 minutes duration. If, after cycling
the fan on for 3 minutes, the temperature rises above the "back
on" setpoint by 2F, the EPID function shall be reinstated with a
start value of 25% and a ramp duration of 90 seconds.
The tower diverting valve shall modulate to divert tower water
to the sump if required to prevent the condenser water
temperature from dropping below the minimum temperature
acceptable to the chiller. When the OA temperature drops below
freezing, the valve shall switch to two-position control, wherein
the valve shall open to the sump when the water temperature
drops to the setpoint and close when the temperature rises a
differential which shall vary from 5 to 12F degrees as the OA
temperature varies from 32 to 0F.
321 ENGINEERING MANUAL OF AUTOMATIC CONTROL
SPEED.
6
DEGREES
12
M15047