VFD Controls
VFD Controls
(OP)
I have 2 AHU's common Duct.With 2 Split Condensing Units.
60 Ton Each DX. pnumatically controlled fan inlet dampers
25 HP Supply and 10HP Return fans. Looking to install VFD drives on both fans. Concerned with CFM at different % cooling and fans working at different speeds. They are older units no $$ to change out. ANy ideas??
60 Ton Each DX. pnumatically controlled fan inlet dampers
25 HP Supply and 10HP Return fans. Looking to install VFD drives on both fans. Concerned with CFM at different % cooling and fans working at different speeds. They are older units no $$ to change out. ANy ideas??





RE: VFD Controls
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RE: VFD Controls
I'm assuming these are two fans and coils because of size restrictions vs. CFM requirements. Given this, I would treat the unit as one AHU in terms of how I think of it.
So option 1:
Install one VFD, large enough to accommodate both fans (save some $$$). Use one input signal from the controller into the VFD in order to modulate both fans in sequence with each other.
So option 2:
Install two VFD's. Use one input signal from the controller into the VFD in order to modulate both fans in sequence with each other.
On the cooling end, I would assume the same and use a common signal to trigger both condensing units as required.
One of the issues, is making sure that the VFD's don't throttle down too much so as to reduce the flow across the coils to a critical point. You may need to se what this minimum is.
RE: VFD Controls
As simsd said you have to limit the minimum VFD speed to not have less than the minimum recommended airflow across the DX coil to not have icing problem at the coil. This would be around 250 FPM face velocity,. Check with the manufacturer. Either set the minimu turndown volume of supply air terminal boxes with reheat or add recirculoation duct around the fans with automatic damper to open to keep airflow through coils above minimum while fan speed is modulated to maintain minimum static presure at remote duct run.
RE: VFD Controls