We have 3 AAON pre-manufactured packaged central plants piped to a common header. Each machine is roughly 150 tons with variable secondary CW flow/CV primary flow with 8 - 25ton compressors (copeland scrolls) and variable condenser fan speed control.
The sequence from day 1 has called for all three machines to run simultaneously.
When we are in low load conditions (180gpm demand @ 3-5deg detla T) all three machines lockout on barrel freeze protection. When we disable 1 of the machines the problem persists and the other 2 lockout on freeze protect.
When we get into these low load conditions (with just 1 compressor running) the condenser fans start to short cycle (ramp to 100% and back to 0% within 1 minute).
1. I believe that the condenser fan VFD PID loop is not tuned properly but AAON says it is supposed to work that way. Why?
2. The whole point of a primary secondary system is to enable the machine to run from 100% load to 0% load without freezing out. Why can't these machines handle it?