Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual page 331

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 over-heating 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. Electrical heaters and insulation on tower piping where
water cannot drain when the tower is isolated or when the
pump stops, if required.
6. Towers operate satisfactorily down to 50% water flow.
7. The common tower water temperature sensor has a high
responsiveness and is in a well with heat conductive
compound to rapidly detect temperature variations.
8. Chiller or free cooling HX tolerates variations in condenser
water temperature when OA temperature is below freezing
and fans and valves are cycling under temperature
differential control and valves are not modulated because
of freezing danger.
9. Common piping from chillers to cooling towers.
Specification
The dual variable speed fan cooling tower control configuration
requires consideration of four control strategies:
– Single pump operation, above freezing OA.
– Single pump operation, below freezing OA.
– Dual pump operation, above freezing OA.
– Dual pump operation, below freezing OA.
NOTES:
1. If the towers are not required to operate when OA is
suitable for air-side economizer cooling (usually below
14 or 14.5 C), leave the fans running at minimum speed
(rather than switching to an on-off algorithm) and
modulate the sump valves when necessary to prevent
cooling to a temperature unacceptable to the chiller.
2. With both towers activated, the differentials used for
cycling fans and valves are half of the differentials used
when only one pump and tower are activated because
only half the water is affected by the change.
Tower valves:
Each tower supply piping shall be provided with two modulating
butterfly control valves; one normally open valve in the supply to
the tower header, and one normally closed valve in a line to the
tower sump. The valves shall operate inversely during temperature
control modes. When both pumps are off, both valves shall open
(to drain as much water as possible to tower sump), and both
shall close during respective periods of tower isolation.
Condenser water set point calculation:
During chiller operation, the common condenser water
temperature setpoint shall be reset to a value equal to the OA WB
plus 3.5 kelvins, but to no lower than the chiller can tolerate and
no higher than 29.5 C. During tower free cooling mode, the
ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
321
setpoint shall be determined by the tower free cooling control
module. The OA WB shall be calculated from OA RH (the RH
sensor shall be of no less than 2% accuracy) and OA DB. The
fans shall operate at the setpoint but no cooler than the minimum
temperature the chiller can tolerate plus 1 kelvin. The valves shall
operate to maintain a fixed minimum temperature setpoint of the
minimum value the chiller can tolerate or the tower free cooling
setpoint in the tower free cooling mode of operation.
Single pump operation with above freezing OA temperature:
For both towers; valves shall position open to the tower and
closed to the sump and the tower fans shall modulate in unison
under EPID control down to a minimum fan speed of 25% to
maintain common condenser water temperature. If the
condenser water load drops below the capacity of both fans
operating at minimum speed, the fan that has operated longest
shall stop and both its valves shall close, and the remaining
tower fan shall assume the full load. If the condenser water
load drops below the capacity of the remaining fan operating
at minimum speed, the fan shall stop and revert to an on-off
algorithm. In the on-off mode the fan shall stop at the fan control
setpoint noted, the tower fan that has been off the longest shall
restart after a rise in temperature of 3 kelvins, but to no higher
than 29.5 C and shall run no less than 7 minutes. The operating
tower valves shall modulate to maintain the common condenser
water setpoint at anytime regardless of fan operation. If, after
running greater than 4 minutes in the on-off mode of operation,
the water temperature rises to a value equal to the "fan on"
setpoint plus 1 kelvin, the fan shall revert back to EPID control
with a start value of 25% and a ramp duration of 120 seconds.
If only one fan is running in the EPID control mode and its
speed rises to 80 percent, the off tower valves shall position
open to the tower and the off tower fan shall start at 25 percent
speed and ramp into unison control over a 240 second duration.
Single pump operation with below freezing OA temperature:
This mode of fan operation shall be the same as for above
freezing, except that the valve control algorithm must be two
position (because modulating the tower water at below freezing
OA temperatures can cause ice formation). Thus the valves shall
position to full flow to the tower sump anytime the water
temperature drops to the valve control setpoint, and shall position
back to full flow to the tower after the water temperature has
risen a value that resets from 3 to 6 kelvins as the OA temperature
varies from 0 C to –18 C. The tower fan shall not be allowed to
start until the valves have been positioned open to the tower greater
than 90 seconds and the water temperature is rising.
Dual pump operation with above freezing OA temperature:
For both towers; valves shall position closed to the sump and
the tower fans shall modulate in unison, to maintain a common
condenser water temperature setpoint, under EPID control down
to a minimum speed of 25%. If the condenser water load drops
below the capacity of both fans operating at minimum speed, the
fan that has operated longest shall stop, and the remaining tower
fan shall assume the full load. If the condenser water load drops
below the capacity of the remaining fan operating at minimum
speed, the fan shall stop and revert to an on-off algorithm
alternating the fans operating at the minimum speed in the "on"
mode. In the on-off mode the fan shall stop at the fan control
setpoint noted above, and shall restart after a rise in temperature
of 2 kelvins, but to no higher than 29.5 C, and shall run no less