# Thread: CFM's per duct size

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CFM's per duct size....Am I correct in saying a 10" round will deliver 400 cfm's
Then 40 cfm's per inch???
8" = 320

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Originally posted by whtmntmgmt
CFM's per duct size....Am I correct in saying a 10" round will deliver 400 cfm's
Then 40 cfm's per inch???
8" = 320
whtmntmgmt, here are the resultants of your stated conditions:

400 cfm in 10" duct yields .08in. SP/100 ft. equivalent length at a velocity of 725 feet per minute.

320 cfm in 8" duct yields .19in. SP/100 ft. equivalent length at a velocity of 925 feet per minute.

good residential design usually uses velocities of 700 to 900 fpm for supply and 600 to 700 fpm for return.

air distribution design takes into account the volume of air to be moved, the desired velocity of the air, and the static pressures developed at the chosen conditions of volume and velocity. without working with all paramaters involved, volume (cfm), speed (velocity), and static pressures (resistance), poor designed system is the final outcome.

hope this helps in your understanding

[Edited by re2ell on 05-22-2006 at 01:49 PM]

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re2ell,is basicly correct in what he stated,but for that to be the actual "job conditions" is not very likely.Note ,he states at 100 ft. equivalent length,most duct system will be a lot more then that,so more resistance and you'll move much less air.If less then 100 ft., then you'll move more air.

You need Manual D ,from wwww.acca.org and many hours of study or better yet a class to learn to use it .

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These figures are round metal duct. Flex has a higher friction loss. Correct?

5. Originally posted by whtmntmgmt
CFM's per duct size....Am I correct in saying a 10" round will deliver 400 cfm's
Then 40 cfm's per inch???
8" = 320
The 8" will only deliver 260 cfm's at the same static pressure.
You cant go by just the diameter of the pipe, a 10" will carry 4 times as much as a 5".
You need the sq. in. size, radius squared times Pi.
For 10" thats 5 x 5 x 3.14 = 78.5 square inches
For 8" thats 4 x 4 x 3.14 = 50.24 square inches
400 cfm / 78.5 sq. in. = 5.09 cfm per sq. in.
5.09 x 50.24 = 255.7 cfm for the 8" pipe.

Like Dash said there's much more involved in figuring ductwork.

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If you really want to become somewhat of an expert with this airflow and duct topic then go to http://www.lamabooks.com and purchase the following two books;

Airflow In Ducts
Math For Technicians

These two easy to read, short and to the point books are highly illustrated and not at all difficult to understand.

The math book applies all the math to HVAC problems and solutions including duct sizing examples. Everything is well organized and each chapter builds nicely on the previous chapters.

7. This is Turtle.

On a short run a 10" flex duct will carry about 400 cfm.

These are guesstimates to close to what they are at short runs.

4" Duct 40 cfm
5" duct 60 cfm
6" duct 100 cfm
7" duct 150 cfm
8" duct 200 cfm
9" duct 300 cfm
10" duct 400 cfm
12" duct 600 cfm
14" duct 900 cfm
16" duct 1,400 cfm
18" duct 1,800 cfm
20" duct 2,300 cfm.

TURTLE

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