Honeywell AUTOMATIC CONTROL Engineering Manual page 333

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. 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
57 or 58F), 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 7F, but to no lower than the chiller can tolerate and no higher
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
than 85F. During tower free cooling mode, the 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 2F. 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:
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 6F, but to no higher than 85F 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 2F, 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:
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 6 to 12F as the OA temperature
varies from 32F to 0F. 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:
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 4F, but to no higher than 85F, and shall run no less than 7
323
For both towers; valves shall position open to the tower and
This mode of fan operation shall be the same as for above
For both towers; valves shall position closed to the sump and
ENGINEERING MANUAL OF AUTOMATIC CONTROL