Using grossly oversized ductwork for much less airflow by capping air outlets with orifice openings

[BronYrAur]

I have a situation where a training facility that used to have large equipment running is now only being used for training without the equipment running. The facility used to have 50,000 CFM of ventilation air being supplied and exhausted. I would like to reduce that number to 2,500 CFM, but I would like to re-use the existing ductwork.

All of the air outlets are currently just open round ducts of various sizes (12", 16", 20", etc.). My thought is to cap all of the openings and drill holes to allow some air to escape. This will allow for some pressure to build up in the ductwork, which will drive the air to the remote outlets. I found an online calculator for airflow through an orifice, and it was coming up with approx 230 CFM through a 4" orifice. This was with 80 deg F air, at 1"WC pressure, discharging to atmospheric pressure.

Does this sound feasible? Since my ductwork is so over sized, I don't anticipate any significant friction loss in the duct. I'm thinking that a makeup air unit with 1.5"WC external static should be more than enough to give me 1" WC pressure at the orifice openings.

I would appreciate any comments on this. Thank you.

 

[EnergyProfessional]

it is feasible, but you should properly balance (measure the flow and adjust the dampers) and possible have balancing dampers. Where you balance you may need smaller duct since such large dampers likely leak much.

You easily come into the situation that the "first "spaces get all the air, and the "last" ones don't' get any. Depending on your exhaust system and if is one open space, this might not matter much.

 

[DRWeig]

I have run into this once before. It should be OK. My old mentor told me to envision a fully inflated balloon with many tiny holes in it. If all the holes are identical, equal air will come out of each even if the flow in the big balloon part is extremely slow. If you stop up the big holes effectively, you'll do fine. You might need to tweak as HerrKaLeun said.

Best to you,

Goober Dave

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