When free cooling available and the operating space temperature is greater than the operating cooling setpoint, the dampers modulate to control the upper SAT setpoint +/-0.5°F.
If the economizer position remains at 100% for 5 consecutive minutes, then the dampers modulate to control to the lower SAT setpoint +/-0.5°F.
If the economizer position remains at 100% for 5 consecutive minutes and the SAT is greater than the lower SAT setpoint +5°F, the first compressor output energizes. If the economizer position remains at 100% for another 5 minutes and the SAT is greater than the lower SAT setpoint +5°F, the second compressor output energizes. This process repeats every 5 minutes until all the compressors energize.
If the economizer position drops below 100% and does not reach minimum position then returns to 100%, remains at 100% for 5 minutes, and the SAT is greater than the lower SAT setpoint +5°F, the next available compressor energizes. If the economizer position remains at 100%, the process repeats every 5 minutes until all the compressors energize.
Any time the economizer remains at the minimum position for 5 consecutive minutes or the SAT is lower than the lower SAT setpoint -5°F, the last energized compressor turns off. If it remains at the minimum position or the SAT remains lower than the lower SAT setpoint -5°F, the compressors de-energize every 5 minutes until all the compressors are off.
If all compressor outputs de-engerize, the economizer modulates to control to the upper SAT setpoint +/-0.5°F.
When the cooling demand ends the compressors de-energize immediately and the dampers return to the operating minimum position.