IOM 1206-1
41
Unit Controller Operation
T3 – Run to Pumpdown
Any of the following are required:
•
Last compressor on circuit is commanded to stop
•
Unit State = Pumpdown
•
Circuit switch is open
•
Circuit mode is disable
•
Circuit Pumpdown alarm is active
T4 – Pumpdown to Off
Any of the following are required:
•
Evaporator Pressure < Pumpdown Pressure Value
•
Unit State = Off
•
Circuit Rapid Stop alarm is active
T5 – Run to Off
Any of the following are required:
•
Unit State = Off
•
Circuit Rapid Stop alarm is active
•
A low ambient start attempt failed
T6 – Preopen to Off
Any of the following are required:
•
Unit State = Off
•
Unit State = Pumpdown
•
Circuit switch is open
•
Circuit mode is disable
•
Circuit Rapid Stop alarm is active
•
Circuit Pumpdown alarm is active
Pumpdown Procedure
Pumpdown is performed as follows:
•
If multiple compressors are running, shut off the appropriate
compressors based on sequencing logic and leave only one
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
Low Ambient Starts
A low OAT start is initiated if the condenser refrigerant
saturated temperature is less than 29.5°C (85.1° F) when the
first compressor starts. Once the compressor starts the circuit
is in a low OAT start state for a time equal to the Low OAT
Start Time set point. During Low OAT Starts, the freezestat
logic for the low evaporator pressure alarm as well as the low
evaporator pressure hold and unload alarms are disabled. The
absolute limit for low evaporator pressure is enforced and the
low evaporator pressure alarm should trigger if the evaporator
pressure drops below that limit.
When the Low OAT Start Timer has expired, if the evaporator
pressure is greater than or equal to the Low Evaporator
Pressure Unload set point, the start is considered successful
and normal alarm and event logic is reinstated. If the
evaporator pressure is less than the Low Evaporator Pressure
Unload set point when the Low OAT Start Timer expires, the
start is unsuccessful and the compressor will shutdown.
Multiple Low Ambient Start attempts are allowed. On the third
failed Low Ambient Start attempt, the Restart Alarm is
triggered and the circuit will not attempt to restart until the
Restart alarm has been cleared.
The restart counter should be reset when either a startup is
successful, the Low OAT Restart alarm is triggered, or the unit
time clock shows that a new day has started.
