AEC ACOA Series Air-Cooled Central Chillers - Manual (Page 26)

4 Sequence of Operation
4-1 General Information
The operating thermostat (opstat) labeled 4TAS monitors return chilled water/air temperature
and activates various steps of capacity control to maintain controlled leaving chilled water
temperature or return air temperature.
Each model has similar control logic. Refer to your wiring diagram located on the inside control
panel door for your control layout for more information.
4-2 Circuit Operation
Typical 2 compressor control sequence of operation 4 step capacity control, continuous
pumpdown. Refer to the typical wiring diagrams.
1. Unit on Standby
• Main field disconnect switch closed
• 115 VAC control power on
• S1 control switch on
• 4TAS-1 through 4TAS-4 opstat contacts open or customer control contacts open
• 2TAS freezestat closed
• FLS flow switch closed
• CWP chiller pump auxiliary contacts closed
• Water flow rates nominal
• Compressor control switches closed
• All safety switches closed
• Pumpdown during standby
Any one of the compressors may come on for a short time and pump down if any low side
pressure buildup occurs during the standby period. The
1TR cycle timer locks out for five (5)
minutes after a pumpdown before the compressor can restart. Pumpdowns can be expected
immediately following a normal
ON cycle as residual liquid in the low side vaporizes, causing
the 4PS pumpdown switch to close momentarily.
ACOA Series Air-Cooled Central Chillers
Page 25