McQuay AGS 206A Operation Manual Download

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AGSB-5

Stopping

Staging down, at least two compressors running:

For staging off compressors, the LWT must be less than the active LWT setpoint minus the
Stage Delta-T. Based on default settings, the active setpoint would be 44

F, and stage delta

of 2 degrees F. So the LWT must be less than 42

F to stage off a compressor. This is a

sufficient condition to trigger a stage down.

For 3 compressor units, the Stage Down Delay Timer must time out before the second lag
compressor will stage down, unless LWT minus Stage Down Delta-T is exceeded, or the
LWT rises above the setpoint.

Staging down, one compressor running:

With one compressor left running, the stage off requires that the LWT be less than the active
LWT setpoint minus the Stop Delta-T. With default settings, the active setpoint is 44

F, and

the Stop Delta is 3 degrees F, so the LWT must be less than 41

F to stage off the last

compressor. This is a sufficient condition to trigger a stage down.

History Storage

The number of starts and total compressor run hours is maintained in non-volatile memory
and can be viewed on the Unit Controller or corresponding Circuit Controller.

Compressor Capacity Control

Compressor capacity is determined by calculating a slide position target. Adjustment to the
slide target for normal running conditions occurs every 10 seconds. For loading, a
maximum change of 1% is allowed, and for unloading, a maximum change of 2% is
allowed. During alarm conditions, the slide target may be reduced to satisfy alarm limits.
The change to the target is calculated as follows.

Capacity Overrides – Limits of Operation

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.  As previously noted, any compressor that is running with capacity
limits because of these conditions will be considered to be at full load in the compressor
staging logic.  An important point to realize is that a particular chiller’s components are
designed for a specific range of capacity and chilled water flow. Varied conditions such as
high water temperature or low condenser pressure can cause higher refrigerant flow than the
chiller is designed to handle, therefore the chiller control may limit unit operation to
maintain system integrity at the highest compressor load possible.

Low Evaporator Pressure

If the compressor is running and the evaporator pressure drops below the Low Evaporator
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 will be active until the evaporator pressure reaches the
hold setpoint plus 2-psi.

If the compressor is running above minimum load capacity and the evaporator pressure
drops below the Low Evaporator Pressure-Unload setpoint, 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. The slide target will then be limited to the current value until the evaporator
pressure rises to the unload setpoint plus 2-psi. If the pressure drops to the Unload Setpoint,
the EXV will switch to pressure control.