Air intake duct design

[sab1]

I work at a hospital and we're having an air intake duct of 24"x16" with an approx. length of 125' and 3 elbows which will need to handle 48,500 CFM ( 3 different air handlers) and I think it's undersized. Any help?

Thanks

 

[mechanicaldude]

A 24"x16" duct can handle between 2200 - 3600 cfm depending on desired velocities, but the velocity on the intake duct shouldn't exceed about 1000 feet per minute. So at 1000 feet per minute the most a 24 x 16 duct can handle is 2,400 cfm. So to answer your question, the duct that you described is way too undersized to handle 48,500 cfm.

 

[BronYrAur]

Are you sure that the CFM you posted is for the outside air, or is that the total CFMs of the units? Either way, your required quantity of OA is no doubt much higher than 2,400CFM. Watch your louver sizing by the way. I would not go higher than 500FPM at the louver to avoid pulling in snow, rain, etc.

 

[ChrisConley]

Just a quick look and some of those numbers have to off. 48,500 cfm should be travelling through a duct that is ~62" in diameter... unless you are using a high velocity system. Even then the friction loss for 125' with 3 elbows would be on the order of 15" w.c.

Just a comment on BronYrAur's post, 500 fpm is a good rule of thumb for louvre sizing, but I generally recommend that you review the louvre manufacturer's sizing charts. They will list a point of 'rain penetration' on their charts which can vary significantly depending on louvre design.

 

[Golestan]

Sab1,
It may be that your total AHU has the 48500 cfm, the OA flow at about 10 -15% of that amount (old rules of thumb) would make the OA cfm at about 4000-6000 cfm. Even with this senario, the 24X16" is too small. I believe 1000 fpm is max velocity I would bring air in. This will be about 2400 cfm outside air.

Going back to your question: yes the duct seems to be bery undersized no matter which way we look at it.