OM AGSB-5
17
High Condenser Pressure
If the compressor is running and the condenser pressure rises above the High Lift Pressure
Hold setpoint, the compressor will not be allowed to increase capacity. The slide position
target will be limited to a maximum value equal to the target at the time the hold condition
was triggered. This limit shall be 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 Condenser Pressure Unload setpoint, 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 slide target will then be limited to the current value until the condenser pressure drops
to 10 psi below the unload setpoint.
High Leaving Water Temperature
If the LWT is above 50
°
F, then the Max Slide is limited to 80% to avoid overloading.
Low Discharge Superheat
If the compressor is running, and the discharge superheat is less than 22
o
F, the compressor
will not be allowed to increase capacity. This limit will be active until the superheat is more
than 22
o
F.
If the compressor is running above minimum load capacity, and the discharge superheat is
less than 20
o
F, then the compressor will begin reducing capacity. The slide target will be
adjusted down 2% every 5 seconds, as long as the superheat remains below 20
o
F.
Maximum LWT Pulldown Rate
The maximum rate at which the leaving water temperature can drop is limited by the
Maximum Rate setpoint. 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, the slide adjustment will be made equal to the slope unload
factor.
Unit Capacity Overrides
Unit capacity limits can be used to limit total unit capacity in COOL and COOL w/
GLYCOL modes only. Multiple limits may be active at any time, and the lowest limit is
always used in the compressor capacity control.
These limits represent a limit on the unit capacity as a whole. Therefore, an estimate of the
current unit capacity is needed. Any circuit that is off is considered to be running at 0% of
its capacity. A running circuit is assumed to be running at a minimum of 20% capacity, and
the assumed capacity will very linearly from 20% to 100% as the slide position varies from
0% to 100%. The unit capacity is calculated using the following formula:
Unit Capacity = (Cir1 Ca Cir2 Ca Cir3 Capacity) / Number of Circuits
The estimated unit capacity and the active capacity limit are sent to all circuits for use in
compressor capacity control.
The active capacity limit values can be viewed at View Unit Status (2) and consist of the
following:
Soft Load
Soft Loading is a configurable function used to ramp up the unit capacity over a given time.
The set points that control this function are:
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