Questions about flexible duct

[wyblueheart]

Hi, everyone,
I am an entry-level engineer. I have never doing any projects before. So I have a few problems.
1) After I calculate the loads, load of some room is small, only 4 or 5 inch flexible duct are enough. So can I use a 4 or 5 inch flexible ducts, then connect the duct to a 24*24 diffuser( diameter maybe 6 inch) ????
2) Most runouts diameters are 7 or 8 inches, so can I connect 7 inch flexible ducts to 8 inch diameter diffuser(24*24) through a transition? This is a similar question as question 1.
I really need your advice. I am waiting online.
Thanks a lot.

[Edited by wyblueheart on 06-10-2005 at 08:54 AM]

[ralphtheplumber]

You're not going to find too many diffusers with necks smaller than 6". I'd just use that as a minimum size.

If you're specifying vav boxes, the smallest size is 6". Order a 4" or 5" box, and you'll get a 6" with a reducer on the inlet. Why bother?

Something else.. if you're coming up with tiny loads in some spaces, you may want to consider just transferring some air through there to keep the temperature reasonable. A toilet room with a 75 cfm exhaust fan doesn't really need a supply grille. Nobody cares if their 3'x6' storage room is conditioned to 72º, even if it's on an outside wall.

[jemawalton]

Is odd number size duct even available? Any job that we design/engineer we only use even numers for round duct sizes. If it requires a 7" duct, we use 8" etc.

[wyblueheart]

For office room 10*12 without exterior wall, only 80CFM is needed according to my calculation.

[sean88]

Here's a little chart we used in our engineering firm

5" 0 - 100 CFM
6" 101 - 150 CFM
8" 151 - 300 CFM
10" 301 - 400 CFM
12" 401 - 600 CFM
14" 601 - 800 CFM
16" 801 - 1200 CFM
18" 1201 - 1600 CFM

We limit our flex duct runs to 10 FT and they are supported every 3 FT. Hard elbows must be used when turning down into a diffuser.

Check out any diffuser catalog and you'll see only even numbered sizes are available. Limit the velocity to below 500 FPM on the discharge of the diffuser. You'll be fine.

[wyblueheart]

Thanks a lot.

Yes, I see that only even number diameter diffusers are available. How about flexible ducts? Still need to be even number?

I am using Elite Ductsize software to calculate, and the result shows 4,5,6,7,8,9 inches diameter ducts.

[Edited by wyblueheart on 06-10-2005 at 02:46 PM]

[bigtime]

It is best to stick with 6,8,10 etc. size flex duct. There is negligible cost difference to use the smaller odd sized duct and it makes it harder on the installing contractor. The air flows sean88 uses seem a little high to me, especially on the 6,8,and 10 inch ducts. We use a 6 for airflows up to 100cfm, 8 up to 225, and 10 up to 325. The higher airflows will create more noise. A hard 90 at the diffuser is not necessary in my opinion.

[john dalton]

Afternoon Gentlemen,

I have no idea where you are all from, but here in Los Angeles, CA the flexible ducting starts at 4 inch round and continues in 1 inch increments to 10 inch round and then continues in 2 inch increments to 24 inches.

We use the size of ducting that is required and regularly use odd sizes if there’s a call for it. Lastly, the capacity of a given size flexible duct is a function of two factors, the length of the duct and the static pressure of the air inside the duct.

Respectfully Submitted,
John J. Dalton

[hvacbear]

Originally posted by wyblueheart
Thanks a lot.

Yes, I see that only even number diameter diffusers are available. How about flexible ducts? Still need to be even number?

I am using Elite Ductsize software to calculate, and the result shows 4,5,6,7,8,9 inches diameter ducts.

[Edited by wyblueheart on 06-10-2005 at 02:46 PM]

Most likely the software is set for metal round ductwork. There are diffrent friction losses for metal vs flex so you will need to account for that as well.