IOMM AGZB1 AGZ 010B through 034B 47
In the event of a tie on number of starts, the lower numbered compressor will start first. In
the event of a tie on run-hours, the lower numbered compressor will shut off first. Run
hours are compared in terms of tens of hours.
Next On = 1 if compressor 1 starts <= compressor 2 starts or compressor 2 not available,
and compressor 1 available
Next On = 2 if compressor 1 starts > compressor 2 starts or compressor 1 not available,
and compressor 2 available
Next Off = 1 if compressor 1 run hours > compressor 2 run hours
Next Off = 2 if compressor 1 run hours <= compressor 2 run hours
Compressor State
A compressor will start when all of the following conditions exist:
• The compressor is “next on”
• Stage Up Now is set
• The compressor is available to start
A compressor will stop when any of the following conditions exist:
• Unit state = Off
• Low Ambient start attempt failed
• Stage Down Now is set, both compressors are running, and the compressor is “next off”
• Pumpdown is complete
Normal Shutdown
If a condition arises that requires the unit to shut down, but it is not an emergency situation,
then a pumpdown will be performed. A normal shutdown will be initiated when any of the
following occur:
• Unit State = Pumpdown
• Circuit Switch = Off
• Low Ambient Lockout
• A normal stage down occurs, and only one compressor is running
• Unit mode = Ice AND the ice setpoint is reached
Pumpdown Procedure
• If both compressors are running, shut off the appropriate compressor based on
sequencing logic
• With one compressor left running, turn off hot gas output and liquid line output
• Keep running until evaporator pressure reaches the pumpdown pressure, then stop
compressor
• If evaporator pressure does not reach pumpdown pressure within two minutes, stop
compressor and log pumpdown failure alarm
Rapid Shutdown
A situation may arise that requires the unit to shut down immediately, without doing a
pumpdown. This rapid shutdown will be triggered by any of the following:
• Unit State = Off
• Stop Alarm
All compressor and liquid line outputs are turned off immediately for a rapid shutdown.
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