OMM 1130-2 49
• A normal stage down occurs, and only one compressor on the circuit is running
• Unit mode = Ice AND the ice set point is reached
Pump Down 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 pump down pressure, then stop compressor
• If evaporator pressure does not reach pump down pressure within two minutes, stop compressor and
record a Failed Pumpdown event in the Event Log.
Rapid Circuit Shutdown
A situation may arise that requires a circuit to shut down immediately, without doing a pumpdown. This
rapid shutdown will be triggered by any of the following:
Unit State = Off
Circuit Alarm
Low ambient start attempt failed
All compressors, hot gas, and liquid line outputs should be turned off immediately for a rapid shutdown.
Liquid Line Solenoid
The liquid line output shall be on any time a compressor on the circuit is running and the circuit is not
performing a pump down. This output should be off at all other times.
Hot Gas Bypass Solenoid
This output shall be on when one compressor on the circuit is running and the circuit is not performing a
pump down. The output should be off at all other times including the delay time described below.
The hot gas bypass valve opening will be delayed for Hot Gas Bypass Time set point seconds (the default
is 30 seconds) when the first compressor starts on each circuit.
EXV Control
The EXV control logic is active regardless of the expansion valve type setting. While a circuit is in the
run state, the EXV controls suction superheat. The superheat target is 8
o
F. PID logic will be used to
control the superheat to the target value.
Any t
ime the circuit is not in the run state, the EXV position should be 0.
EXV Control Range
The table below shows the EXV range based on the number of compressors running and number of
compressors on unit.
Number of
Compressors
Valve
Position
Compressors Running
1 2 3
4
EXV Min 8% 8% -
EXV Max 60% 100% -
6
EXV Min 8% 8% 8%
EXV Max 35% 45% 65%
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