AEC ACOA Series Air-Cooled Central Chillers - Manual (Page 27)

2. Run Cycle Sequence (Increasing Load)
• Customer contacts close
Step 1
1. Opstat 4TAS-1 closes as return chilled water temperature starts to rise. Relay 8CR
bypasses pumpdown switch 4PS-1 to remove it from the control circuit.
Note: Go to Step 4 if Compressor 1 is off for more than five (5) minutes.
2. Cycle timer 1TR times out and closes (up to a five (5) minute delay).
3. Compressor 1 starts. Contactor auxiliary switch energizes liquid line solenoid 1SOL and
Circuit 1 refrigerant flow starts. Compressor 1 unload solenoid 3SOL energizes through
4TAS-NC2 and NC4 . Compressor 1 unloads one bank of cylinders.
4. Oil failure switch 3PS-1 starts to time open (120 sec. delay). Low pressure switch 9PS-1
starts to time open (120 to 240 sec. delay).
Note: If oil pressure or suction pressure is not established at a normal level within the above-
listed delay times, Circuit 1 shuts down.
5. Circuit 1 oil and suction pressure rise to normal levels, and 3PS-1 and 9PS-1 terminate
the time open sequence.
First stage capacity is now on line.
Step 2
6. Opstat 4TAS-2 closes as water temperature continues to rise. Compressor 1 unload
solenoid 1SOL de-energizes as 4TAS-NC2 opens. Compressor 1 loads up to 100%.
Second stage capacity is now on line.
Step 3
7. Opstat 4TAS-3 closes as water temperature continues to rise. Relay 9CR bypasses
pumpdown switch 4PS-2 to remove it from the control circuit. Staging timer 4TR-2
delays Compressor 2 start (10 to 30 seconds).
8. Staging timer 4TR-2 closes. If cycle timer 1TR-2 times out, Compressor 2 starts 100%
loaded. Contactor auxiliary switch energizes liquid line solenoid 2SOL and circuit 2
refrigerant flow starts. Oil Failure switch 3PS-2 starts to time open (120 sec. delay). Low
pressure switch 9PS-2 starts to time open (120 to 240 sec. delay).
Note: If oil pressure or suction pressure is not established at a normal level within previously-
listed delay times, Circuit 2 shuts down.
Page 26
ACOA Series Air-Cooled Central Chillers