VFD Controls

[Grendil]

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??

 

[willard3]

You can start here....read fans in parallel

http://www.term.ucl.ac.be/recherche/tunnels_ routiers/vt1997b.pdf

 

[simsd]

I can't get through to the link that willard3 put up, but these are my thoughts.

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.

 

[lilliput1]

Both fans must be controlled in unison and the pressure drop across each balanced. Both fan should run unless one is disabled or out of service. It will be difficult to start one with the other fan running because the lag fan may be spinning backwards and will overload once started. I recommend putting a heavy duty gravity operated backdraft damper at the discharge of each fan and an automatic damper at the inlet of each fan to minimize the backspinning. Even with the dampers I recommend operation of both fans to avoid stressing the dampers and causing early failure.
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.

 

[simsd]

Yes, lilliput1 makes some good points about back spinning. A rotating fan without the VFD running can cause the VFD to trip on fault. Everything you want to do is achievable, but at how much do you want to spend is the magic question.