58 WGS 130A to 190A OM WGS-5
Internal Capacity Overrides
The following conditions override the automatic slide control when the chiller is in COOL
mode or ICE mode. These overrides keep the circuit from entering a condition in which it is
not designed to run. Any compressor running with capacity limits because of these
conditions is considered to be at full load in the compressor staging logic.
Low Evaporator Pressure
If the evaporator pressure drops below the Low Evaporator Pressure Hold setpoint while the
compressor is running, the Low Evaporator Pressure Inhibit (Hold) event is triggered.
When triggered, the compressor will not be allowed to increase in capacity.
If the evaporator pressure drops below the Low Evaporator Pressure Unload setpoint while
the compressor is running, the Low Evaporator Pressure Unload event is triggered. When
triggered, the compressor will begin reducing capacity. The maximum allowed slide target
will be adjusted down 5% every 5 seconds until the evaporator pressure rises above the Low
Evaporator Pressure-Unload setpoint.
These events are logged to an event log when they occur. Both remain active until the
evaporator pressure rises above the hold setpoint plus 2 psi, or the circuit goes to a state
other than Run.
High Lift Pressure
If the compressor is running and the condenser pressure rises above the High Lift Pressure
Hold setpoint, the High Lift Pressure Inhibit event is triggered. When this happens, the
compressor will not be allowed to increase capacity. This event is active until the condenser
pressure drops 10 psi below the hold setpoint.
If the compressor is running above minimum load capacity and the condenser pressure rises
above the High Lift Pressure Unload setpoint, the High Lift Pressure Unload event is
triggered. When this happens, the compressor will begin reducing capacity. The maximum
allowed slide target will be adjusted down 5% every 5 seconds until the condenser pressure
drops below the High Condenser Pressure Unload setpoint. The compressor will not be
allowed to increase in capacity until the condenser pressure drops to 10 psi below the
unload setpoint.
Maximum LWT Pulldown Rate
The maximum rate at which the leaving water temperature can drop is limited by the
Maximum Rate setpoint, only when the LWT is less than 60
o
F and the unit mode is Cool. A
slope unload factor is used to reduce the slide target if the pulldown rate exceeds the
Maximum Rate setpoint.
Slope Unload Factor: Maximum Rate + LWT slope
If the pulldown rate is too fast, then the slide adjustment made will be equal to the slope
unload factor.
High Water Temperature Capacity Limit
If the evaporator LWT exceeds 55F, compressor slide position will be limited to a
maximum of 75%. Compressors shall unload to 75% or less if running at greater than 75%
slide position when the LWT exceeds the limit. This feature is to keep the circuit running
within the capacity of the condenser coil.
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