McQuay AWS, Operating Manual

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KOMAC00607-09EN 33
Capacity will not be increased above 25% until discharge superheat has been at least 22
degrees F for a time of at least 30 seconds.
Manual capacity control
The capacity of the compressor can be controlled manually. Manual capacity control is
enabled via a set point with choices of auto or manual. Another set point allows setting
the compressor capacity from 25% to 100%.
The compressor capacity is controlled to the manual capacity set point. Changes will be
made at a rate equal to the maximum rate that allows stable circuit operation.
Capacity control reverts back to automatic control if either:
•
the circuit shuts down for any reason
•
capacity control has been set to manual for four hours
Slide Control Solenoids
The required capacity is achieved by controlling one modulating slide and one non-
modulating slide. The modulating slide can control 10% to 50% of the total compressor
capacity, infinitely variable. The non-modulating slide can control either 0% or 50% of
the total compressor capacity.
Either the load or the unload solenoid for the non-modulating slide is on any time the
compressor is running. For compressor capacity from 10% up to 50%, the non-
modulating slide unload solenoid is on to keep that slide in the unloaded position. For
capacity from 60% to 100%, the non-modulating slide load solenoid is on to keep that
slide in the loaded position.
The modulating slide is moved by pulsing of the load and unload solenoids to achieve
the required capacity.
An additional solenoid is controlled to assist in moving the modulating slide in certain
conditions. This solenoid is activated when the pressure ratio (condenser pressure
divided by evaporator pressure) is less than or equal to 1.2 for at least
5 seconds. It is deactivated when pressure ratio is more than 1.2.
Capacity Overrides – Limits of Operation
The following conditions override automatic capacity control when the chiller is in
COOL mode. These overrides keep the circuit from entering a condition in which it is
not designed to run.
Low Evaporator Pressure
If the Low Evaporator Pressure Hold event is triggered, the compressor will not be
allowed to increase in capacity.
If the Low Evaporator Pressure Unload event is triggered, the compressor will begin
reducing capacity.
The compressor will not be allowed to increase in capacity until the Low Evaporator
Pressure Hold event has cleared.
See the Circuit Events section for details on triggering, reset, and unloading action.
High Condenser Pressure
If the High Condenser Pressure Hold event is triggered, the compressor will not be
allowed to increase capacity.
If the High Condenser Pressure Unload event is triggered, the compressor will begin
reducing capacity.
The compressor will not be allowed to increase in capacity until the High Condenser
Pressure Hold event has cleared.
See the Circuit Events section for details on triggering, reset, and unloading action.