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Slowing air velocity for coil install

Slowing air velocity for coil install

Slowing air velocity for coil install

(OP)
Hey guys I have 36" dia duct with a velocity of 1250 fpm and 9000 cfm flow. I want to slow the velocity down to 450 fpm by increasing the duct area. Can anyone provide an equasion on the length of duct need before the velocity will stabilize.

RE: Slowing air velocity for coil install

You need a 60" dia, duct whoch will give you a 0.003986"wc friction rate at 100 TEL to get 450 FPM

RE: Slowing air velocity for coil install

I have not seen a design guide with an equation of velocity profile as a function of angle a diverging flow. SMACNA does have loss a coefficient formula in A.21 - transition round to square but this will only give you the loss. SMACNA shows a max of 45 deg as a design standard. I look at the coil selection first, then make the duct work. Most coils are going to be rectangular. I think you would be in the range of a 60"w x 48" or 80"w x 36". Your inlet velocity is low for a 36". Use 1' to 2' straight duct in front of the coil to help equalize the velocity profile. Perforated panels upstream help but add fan hp. Make sure you have 3 to 10 duct dia. up stream of the transition.  

RE: Slowing air velocity for coil install

AMCA may have something, look under System Effects.

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