I was sent on a service call for "high discharge temp alarms" on a single story office building. I found a building with 4 roof mounted ahu's, each having a chilled water coil and high eff. "bag" air filters. each ahu had both supply and return fans controlled by vfd's, each ahu 20-30 tons ? They all are ducted into the same supply duct which makes a single loop around the building. They all pull return air from a plenum ceiling. The building operator had shut down 2 ahu's to save energy because the building was approx. 30% occupied. The 2 ahu's that were operating were running sf vfd at 100% and rf vfd at 80% moving approx. 14,000 cfm each.
chilled water valves were at 100%, but supply air temps were averaging 67deg. I started the other 2 ahu's and vfd's slowed to approx 28%, chilled water valves came under control as supply air temps dropped. All seems well. I've never seen this type of system before.
1) Do all 4 ahu's need to run even under low load?
2) should return fan vfd set point be a % of supply fan vfd? If so How much?
I would imagine all AHU have to run at the same time the supply duct would be oversized if you had shut some air handlers this would reduce your static and increase velocity across the coils. This increase in velocity would reduce heat transfer which is why you had the high supply air temp. You can verify this by taking an airflow reading in the supply duct downstream from where they tee together. Of course you could set the static pressure control for the VFD lower which would increase your coil performance but the down side is you will lose in duct performance (not enough static / CFM). As for the return fans at 80% most likely you are exhausting 20% of the air via exhaust fans so the the
(RETURN CFM= SUPPLY CFM-EXHAUST CFM)
If you want to shut down some of the AHUs, there would need to be some kind of damper setup to keep the running units from pushing air backwards through the stopped units. Similar to check valves on parallel pumps.
I have seen this type of system once. The supply pressure was used to stage on additional AHUs. As more zones open the duct pressure drops until it hits the low limit. Then an additional AHU starts. If all supply vfds drop to some low speed setting, one AHU would be stopped and its damper closed.