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CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
Features
1. Production of chilled water via condenser water and a
HX in lieu of chiller operation.
2. User friendly monitoring and adjustment.
3. Automatic selection of free cooling mode.
4. Maintains full flow and minimum temperature condenser
water during free cooling pull-down period.
Conditions For Successful Operation
1. Control network, software, and programming to advise
chilled water plant of AHU chilled water demands.
2. Interlock and control wiring coordinated with the chiller
manufacturer.
3. Appropriate cooling tower and chiller control.
4. Accurate calculation of OA WB from OA DB and OA RH.
5. Pumps selected for HX pressure drops and chiller/
condenser water flow balancing.
6. Flow switches and starter auxiliary contacts provided to
prove flow in lieu of current sensing relays for the chiller
and condenser pumps.
Specification
NOTE: This control scheme assumes that the chilled water
plant must continue to operate while the condenser
water temperature is dropped from the minimum
temperature acceptable to the chiller to the temperature
required to produce chilled water. It is also assumed
that full condenser water flow is required at all times
(otherwise, chiller pressure could be maintained by
modulating the tower-side changeover valves to reduce
condenser flow).
Chilled water pump:
Anytime at least one chilled water pump is operating and the
OA WB drops below the free cooling setpoint for greater than ten
minutes, the condenser water temperature setpoint shall be lowered
to a value equal to the chilled water setpoint minus 1.5K, but no
lower than 5.5 C. The OA WB free cooling changeover setpoint
shall be the chilled water temperature setpoint minus 5 kelvins.
Condenser water:
During the condenser water temperature pull-down period,
the condenser entering water temperature shall be prevented
from dropping below the minimum acceptable value by
modulating open a valve between the condenser leaving water
and the pump inlet.
When the condenser water temperature reaches the free
cooling setpoint, the changeover valves for the chiller that has
operated longest shall position open to the HX and the chiller
shall stop. If the free cooling condenser water temperature
setpoint is not reached within eight minutes, the condenser water
setpoint shall be returned to the chiller cooling value and a
descriptive system alarm shall be issued.
Five minutes after the first chiller changes over to free cooling,
the second chiller shall changeover as noted for the first chiller.
In the free cooling mode, the tower controls shall operate to
maintain the condenser water temperature setpoint, which shall
be reset from 5.5 C to 13.5 C by the chilled water temperature
setpoint demand for cooling.
Anytime the system is running in the free cooling mode and
the chilled water temperature is greater than 1 degree above
setpoint for greater than 5 minutes, the changeover valves for the
chiller that has been off longest shall position open to the chiller,
the condenser water temperature setpoint shall be raised to the
value specified for chiller operation, and the chiller controls shall
be enabled. If both chilled water pumps are operating, the
remaining chiller shall changeover and start 2 minutes later. As
the chillers start, their respective low-limit condenser water
temperature valves shall be placed under EPID control with a
start value of 75% open and a ramp duration of 60 seconds.
After reverting back to the chiller cooling mode, during the
condenser water warm-up period the cooling tower fans shall
remain on at no less than minimum speed, to prevent short
cycling, and the condensers entering water temperature shall
be controlled by the low-limit condenser water temperature
pump inlet valves.
A software CHILLER-AUTO-TOWER selection function
shall be provided to lock the system in either mode if desired.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
326
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