Return Duct Sizing Tips

[CuriouslyGeorge]

Are there general guidelines for what pressure drop is acceptable in a return duct (for commercial use) and isn't there also a dB related to this?

Bonus: Has anyone found a link to an online ductulator?

Thanks,
Curiously George

 

[lzh007]

For commercial and residential system, speed and noise is more seneitive than pressure drop if the return duct is not very long.

 

[GBAILEY]

Try these tools from McQuay:

http://www.mcquay.com/eprise/main/mcquaycom/designsolutions/designtools/MQC_designtools

I try and size return ductwork at .05/100ft and 500-700 fpm at the air device inlet as a general rule. Your milage may vary, of course

 

[62hog]

In my part of the world, the accepted PD for return duct is .08"/100'. Just for kicks, the accepted PD for supply is .1"/100'

 

[lilliput1]

I second 62hog.

Get your Carrier rep to give you a duct calculator.

 

[CountOlaf]

I third 62hog. Those pd's are my usual starting points, then I realize that duct is going to be way too big, and I start increasing allowable pd and adding motor hp to compensate!

Anyway, if you want the "real" Ductulator (registered trademark of Trane), you should get your Trane rep to give you one.

 

[CuriouslyGeorge]

CountOlaf, you said "I third 62hog. Those pd's are my usual starting points, then I realize that duct is going to be way too big, and I start increasing allowable pd and adding motor hp to compensate!"

...Doesn't that lead into noise control issues?
CuriouslyGeorge

 

[cme]

low pressure .10"/100' supply, return at .08"/100'

residential type units ..... .08 supply .06 return

 

[walkes]

George,

The noise issue you may be concerned with would more likely be attributed to velocity and the duct construction. If you have round/spiral duct that is fairly stiff you shouldn't have much of a problem.

If you are concerned about air noise within the duct you can acoustically line the duct(or install silencers). If you are concerned with radiated noise you could use external insulation and/or ensure appropriate stiffness to avoid the "oil canning" affect.

 

[CountOlaf]

George,

I haven't really experienced noise issues by increasing the allowable pressure drop. There are other "rules of thumb" regarding duct velocity limits. For instance, for office buildings, the recommended duct velocity is 2200 fpm for main rectangular ducts, 2400 fpm for round (with maximums of 2500 and 3200 respectively) and 1600 fpm rectangular branch ducts, 2000 round branch ducts (with maximums of 2000 and 2500 respectivley), also for office buildings.

According to my Ductulator..."these velocities generally produce sound levels within the occupies spaces that are considered acceptably quiet...the maximum velocities should be observed when the duct size must be reduced to pass through a space restricted area."

If you "play with" the Ductulator, you'll see that if you select ducts based on these velocity limits, you will get 2-3 times as much airflow through a given duct as you would selecting at the starting friction point (e.g., 0.1/100' for supply air). You will also of course quadrouple (or some other amount) your pressure drop. The point is, you have a pressure drop starting point and the "end" point should be based on velocity limitations. Then of course you should make sure your motor hp/esp doesn't get astronomically large.

It's analagous to pipe size selection. You should select sizes around 4ft/100' head loss for optimum economic design (hp vs noise), which will result in modest velocities (4 fps or so); but you can go up to 8 fps for 8" and smaller piping and 9 or 10 fps for 10" and larger piping.