Basic Operating Sequence; Fig 28 - Chiller Control (Cooling) Range - York YCAV0157 Installation Operation & Maintenance

OPERATION
Start Sequence and Loading
To initiate the start sequence of the chiller, the following
conditions must be satisfied before the precharge of the
DC bus will take place:
· UNIT SWITCH must be ON.
· At least one System Switch is ON
· Run permissive inputs (Remote Cycling Contacts)
must be closed.
· No unit faults exist.
· No unit start inhibits exist.
· At least one system not faulted or inhibited.
· The Daily Schedule is calling for the chiller to
run.
· The Flow Switch is closed.
· Leaving Chilled Liquid Setpoint is above the
Setpoint + CR (Setpoint High Limit).
48˚F
46˚F
44˚F
42˚F ----------
40˚F
38˚F
FIG. 28 - CHILLER CONTROL (COOLING) RANGE
172

BASIC OPERATING SEQUENCE

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---------
Setpoint + CR ( Setpoint High Limit)
---------- ---------------------------------
Setpoint
Setpoint – CR (Setpoint Low Limit)
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Once the precharge takes place, if the anti-recycle timer
is timed out the chiller control system on the Chiller
Control Board will select the number of compressors
to start and begin operation of the compressors. The
compressor(s) speed will be ramped to the minimum
start frequency and increase speed as needed in an effort
to regulate the leaving chilled liquid temperature to meet
the desired Setpoint.
When a compressor starts, the Feed and Drain Valves on
the system will immediately begin to control superheat
and the liquid level in the Flash Tank and the Chiller
Control Board micro will begin to regulate the speed
on the VSD to bring the chilled liquid temperature to
within the Control Range (CR). The micro will regu-
late the speed of the compressor(s) primarily based on
temperature offset as the loading timer permits.
The Setpoint is the Leaving Chilled Liquid Temperature
midpoint of the Control (Cooling) Range. The Setpoint
High Limit is the Setpoint plus the Control Range. The
Setpoint Low Limit is the Setpoint minus the Control
Range. The chiller will attempt to control within the
temperature range programmed by the Setpoint +/-
CR.
Starting and stopping of compressors will be handled by
the Standard or High IPLV Capacity Control Routine.
Loading and unloading will be controlled by temperature
offset and rate by the Fuzzy Logic Control Routine.
A graphical representation of the Setpoint and high and
low limit (+/- CR) are shown below in FIG. 28:
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Programmed
Control (Cooling) Range
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FORM 201.21-NM1 (1223)
LD10625
JOHNSON CONTROLS