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CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
than 7 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 1 kelvin, 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 2 kelvins, but to no higher than 29.5 C,
and shall run no less than 7 minutes. If either fan is on and the
water temperature drops to the valve control setpoint plus 0.5
kelvins, 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 + 0.5 kelvin.
Both fans Off greater than
2 minutes & water
temperature less than
Setpoint + 0.5 kelvin.
One set of valves open to
tower (Seq. 5) greater
than 2 minutes, & water
temperature less than
Setpoint.
Decreasing
Seq. 1: Modulates both fans in unison to control
at Setpoint + 1 kelvin.
Seq. 2: Fan running longest stops.
Remaining fan modulates to maintain
Setpoint + 1 kelvin.
Seq. 3: Fans alternately cycled:
At Setpoint + 2.5 kelvin first fan On at
minimum speed.
At Setpoint + 1 kelvin 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 + 0.5 kelvin to sump.
At Setpoint + 1.5 kelvin 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 + 1.5 kelvin 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 0.5 kelvins, 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 1.5 kelvins 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 1.5 kelvins differential. If, after one valve is open to the
tower for 2 minutes, and the water temperature rises above the
valve control setpoint plus 2 kelvins, 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 0.5 kelvins,
and opening to the tower after a 1.5 kelvins differential. If, after
both valves position open to the tower for 2 minutes, the
temperature rises to the valve control setpoint plus 2 kelvins,
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 1 kelvins, 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:
ENGINEERING MANUAL OF AUTOMATIC CONTROL
322
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 + 2 kelvin:
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 + 2 kelvin:
Revert to Seq. 3.
If one valve set positions open to the
tower greater than 2 minutes &
water temperature greater than
Setpoint + 2 kelvin:
Revert to Seq. 5.
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