Trane CVGF500 Installation, Operation & Maintenance Manual page 106

Machine Protection and Adaptive Control
SoftLoading
Softloading stabilizes the startup control during the initial chiller pulldown. Soft load-
ing is used to bring the building loop temperature from its start value to the Chilled
Water or Hot Water Setpoint in a controlled manner. Without soft loading, the chiller
controls will load the chiller rapidly and use the full chiller capacity to bring the loop
temperature to setpoint. Although the start temperature of loop may have been high,
the actual system load may be low. Thus, when the setpoint is met the chiller must
unload quickly to the system load value. If it is not able to unload quickly enough, the
supply water temperature will drop below setpoint and may even cause the chiller to
cycle off. Soft loading prevents the chiller from going to full capacity during the pull-
down period. After the compressor has been started, the starting point of the filtered
setpoint is initialized to the value of the Evaporator Leaving Water temperature and the
percent RLA.
There are three independent Softload setpoints:
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•
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Note: UT ™ provides
access to these three setpoints.
Minimum and Maximum Capacity Limit
A Minimum Capacity can be set to limit the unloading ability of the compressor forc-
ing differential stop to be reached cycling the chillers. Minimum capacity limit will
be displayed when in this limit mode. This indicates when the chiller is running fully
unloaded. Similarly a maximum capacity can be set to limit normal chilled water tem-
perature control. The maximum capacity relay is energized which is a signal used by
generic BAS systems to start another chiller.
The minimum (default at 0 percent) and maximum (default at 100 percent) capacity
are adjustable using UT ™.
Evaporator Limit
Evaporator refrigerant temperature is continuously monitored to provide a limit func-
tion that prevents low refrigerant temperature trips. This allows the chiller to continue
to run at a reduced load instead of tripping off at the Low Refrigerant Temperature Cut-
out Setpoint (LRTC).
Evaporator limit could occur with an initial pulldown of a loop temperature where the
condenser is colder than the evaporator (inverted start), and the evaporator refriger-
ant temperature may drop below the LRTC. This limit prevents the unit from shutting
down on a diagnostic during this type of pulldown. Another example is a chiller that
is low on refrigerant charge. It will run with low evaporator refrigerant temperatures.
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Capacity Control Softload Time default is to 10 minutes and settable from 0-120
minutes. This setting controls the time constant of the Filtered Chilled Water Set-
point.
Current Limit Control Softload Time default is 10 minutes and settable from 0-120
minutes. This setting controls the time constant of the Filtered Current Limit Set-
point.
Current Limit Softload Starting Percent default is 40 percent RLA and settable from
20-100 percent. This setting controls the Starting point of the Filtered Current Limit
Setpoint.
CVGF-SVX03B-EN