Honeywell AUTOMATIC CONTROL Engineering Manual page 334

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
minutes. The operating tower valves shall modulate to maintain
the above noted valve control setpoint at anytime regardless of
fan operation. If, after a fan is 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 the single fan operates to a speed exceeding
60%, the "off" fan shall start at an initial speed of 25% and shall
ramp into unison control over a 240 second duration (during which
time the operating fan will slow down as the starting fan slowly
assumes part of the load).
Dual pump operation with below freezing OA temperature:
For both towers; valves shall position closed to the sump and
the tower fans shall modulate in unison, to maintain common
condenser water temperature, under EPID control down to a
minimum fan 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 two tower fans operating at 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
minutes. If either fan is on and the water temperature drops to the
valve control setpoint plus 1 degree, the valves of the tower in
which the fan is on shall position open to the sump until the fan
stops, at that time the valves shall position back to the tower.
Table 1. Temperature Control Sequence for Dual Pumps Running in Below Freezing OA.
Load:
Between both fans running at
minimum speed and full load.
Less than both fans at
minimum speed, greater than
one fan at minimum speed.
Less than one fan at minimum
speed, greater than both fans
Off.
One fan running at minimum
speed greater than 2 minutes
and water temperature less
than Setpoint + 1 degree.
Both fans Off greater than 2
minutes & water temperature
less than Setpoint + 1 degree.
One set of valves open to
tower (Seq. 5) greater than 2
minutes, & water temperature
less than Setpoint.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
Decreasing
Seq. 1: Modulates both fans in unison to
control at Setpoint + 2F.
Seq. 2: Fan running longest stops.
Remaining fan modulates to maintain
Setpoint +2F.
Seq. 3: Fans alternately cycled:
At Setpoint + 5F first fan On at
minimum speed.
At Setpoint + 2F first fan Off.
Fans run 6 minutes minimum.
Repeat cycle for second fan.
Seq. 4: Positions valve set of the operating
tower to sump until fan stops.
Repositions valve set back to tower.
Seq. 5: Alternately positions valve sets:
At Setpoint + 1 degree to sump.
At Setpoint +3F back to tower (one
set of valves always open to tower).
Fans locked Off.
Seq. 6: At setpoint positions valve sets of
both towers to sump.
At Setpoint + 3F alternately positions
valve sets to tower (one set of
valves always closed to sump).
If both fans are off and the water temperature drops to the valve
control setpoint plus 1 degree F, the valves of Tower 1 shall position
open to the sump, and the Tower 2 valves shall alternately cycle
open to the sump with a 3 degree temperature differential. If,
while one valve is open to the sump, the water temperature drops
to the valve control setpoint, the other valve shall position open
to the sump, and the valves shall alternate open to the tower with
a 3 degree differential. If, after one valve is open to the tower for
2 minutes, and the water temperature rises above the valve control
setpoint plus 4F, the remaining valve shall position open to the
tower and the valves shall revert to alternately opening to the
sump at the valve control setpoint plus 1 degree, and opening to
the tower after a 3 degree differential. If, after both valves position
open to the tower for 2 minutes, the temperature rises to the valve
control setpoint plus 4F, control shall revert back to alternately
cycling the fans on at minimum speed as before. If, after a fan is
running greater than 2 minutes at minimum speed 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 the single fan operates to a speed exceeding 60% for
greater than 3 minutes, the "off" fan shall start at an initial speed
of 25% and shall ramp into unison control over a 120 second.
NOTE: This "Dual pump below freezing OA temperature"
sequence gets quite complex as the condenser water load
increases and decreases. Table 1 may help understand
the shifting sequence.
Load Sequence
Seq. 10: If one fan runs greater than 60%
Seq. 9:
Seq. 8:
Seq. 7:
324
Increasing
—
speed greater than 2 minutes:
Remaining fan ramps on in unison
from 25% speed over 240 seconds.
Revert to Seq. 1.
If either fan is on at minimum speed
greater than 2 minutes & water
temperature is greater than
Setpoint + 4F:
Revert to Seq. 2 via EPID with start
value of 25% and ramp duration of
120 seconds.
—
If both valve sets position open to the
tower greater than 2 minutes &
water temperature is greater than
Setpoint + 4F:
Revert to Seq. 3.
If one valve set positions open to the
tower greater than 2 minutes &
water temperature greater than
Setpoint + 4F:
Revert to Seq. 5.