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Old 02-01-2010, 09:10 AM   #1
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Plenum Trunk Size


Could somene verify a trunk size question?

I need to re-work a trunk off the supply plenum. The new trunk will supply a 10" rigid branch pipe and 8" rigid branch pipe. The 10" branch needs to supply 360 cfm and the 8" branch 200 cfm.

The plan is to tap off the supply plenum with a 12" start collar then transition to a 12" x 12" x 8" Tee. The 8" on the Tee will go to the 8" branch. The other end of the 12" Tee will reduce to 10" to supply the 10" branch.

The information I have tells me that a 12" rigid pipe can supply both branches. 560 cfm with 12" rigid pipe @ approximately 715 fpm.

Will this work?

Thanks.

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Old 02-01-2010, 09:17 AM   #2
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Plenum Trunk Size


It would work better if you used a wye.

But that tee can work.

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Old 02-01-2010, 09:24 AM   #3
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Plenum Trunk Size


Thanks BT. I was planning on using an Air-Flow Tee.

http://www.standexadp.com/catproduct...lename=Airflow Tee

I'll look at the Wyes.

Also, do you see any issues with this take-off vs a round?

http://www.standexadp.com/catproduct...&stylename=Adh Takeoff - Square to Rnd

Last edited by Earnie; 02-01-2010 at 09:27 AM.
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Old 02-01-2010, 09:34 AM   #4
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Plenum Trunk Size


Quote:
Originally Posted by Earnie View Post
Thanks BT. I was planning on using an Air-Flow Tee.

http://www.standexadp.com/catproduct...lename=Airflow Tee

I'll look at the Wyes.

Also, do you see any issues with this take-off vs a round?

http://www.standexadp.com/catproduct...&stylename=Adh Takeoff - Square to Rnd
Those tee's aren't as good as a wye. but they are much better then just a bull tee.
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Old 02-01-2010, 09:38 AM   #5
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Plenum Trunk Size


Ok, I'll look at the Wye. Only reason I selected the Tee is that it transitioned at 90 degrees vs the 45 degree for the Wye.

Do you see any issues with the retangular start collar?
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Old 02-01-2010, 09:52 AM   #6
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Plenum Trunk Size


Its a bit more restrictive then a round start collar. But not generally a problem if you allowed for it in your pipe/trunk line sizing.
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Old 02-01-2010, 10:19 AM   #7
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Plenum Trunk Size


The supply plenum is the issue; it can only be 12" wide due to the width of the air handler's discharge opening.

Looks to be a problem installing a 12" round start collar in a 12" wide plenum box. The 12" retangular collar is 9" wide x 18" high so it will give me 3" for the flange.

Always something, isn't it.

UPDATE:

Did a bit of math on the issue.

12" dia = 113 sq. in.

9" X 18" = 162 sq. in.

9" x 13" = 117 sq. in.

Looks like the possible restriction is accounted for in the larger volume.

Last edited by Earnie; 02-01-2010 at 10:33 AM.
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Old 02-01-2010, 11:26 AM   #8
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Plenum Trunk Size


The area is increased. but, the turbulence sometimes is more then the area makes up for.
It takes air a little distance to stabilize at each direction change. And after each transition configuration change.

But. Sometimes you got to do what you got to do.

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