When commissioning multi-stage HVAC equipment, remember to test the staging all the way up and all the way back down. A recurring issue has come up on a couple of our jobs recently and has also been noticed during past commissioning projects. Compressor staging logic is commonly set-up without all set-point overshoot possibilities in mind.
A recent test called for the supply air temperature set-point of an AHU to be lowered in order to witness the compressors stage on one by one. Then the supply air temperature set-point would be increased to witness the compressors stage off one by one.
On AHU-1, after the SAT set-point was lowered from 55F down to 50F, the compressors staged on, with their appropriate minimum time-delay between staging, and a SAT of 50F was achieved and maintained. The SAT was then set back to 55F. The compressors immediately cycled to all off as the logic had only considered “minimum run times” of 5 minutes and not staging. All compressors had been on for 5 minutes and therefore the run time “if statement” was satisfied. The SAT slowly moves up past 55F, past 60F, and past 75F as the minimum compressor off time has to be satisfied before the unit can cycle back on. By this time the SAT was so high that 5 minutes after C1 stages on, C2 is called on because the SAT was only able to make it down to 68F in the window given for C1 to try and make set-point (though the SAT was still falling and might have continued down to set-point if given enough time). Now two compressors were running and working the SAT down toward 55F even faster, but not fast enough as C3 comes on in another 5 minutes. Now, with all compressors running the SAT shoots past 55F and down to 51.9F when the logic says shut down all of the compressors again, and the cycle repeats itself. We move to AHU-2 and the compressor death cycle is already in progress, compressors are cycling on to try and lower the current SA temp of 67F to the set point that was recently over shot.
Compressors do not last long if this type of operation goes unnoticed or if this system is not commissioned. This particular case had missing logic and probably run-times that were a little short. Another project, with a similar issue had the proper staging logic in place, but the run-time condition before staging the next compressor was set at only 3 minutes. Here the overshooting was still present but not as drastic since at least there was some form of staging. Either way, compressor life suffers and space humidity levels may be high even if a single compressor is being forced to remain on with hot gas bypass or variable capacity.
Compressor staging logic should cover all operating conditions to ensure that compressors don’t overshoot their set-points and cycle repeatedly. The run-times should be set long enough to avoid overshoot and to avoid short cycling during “tweener times”, (where capacity stages do not match current load conditions), but short enough to not affect occupant comfort. And all of this logic should be field verified to work for each size/capacity of equipment at the project.
-Joe Fleming, PE, LEED AP BD+C, BEMP