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does anyone know free air conversions for round ducting?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by TCaldwell, Jan 28, 2008.

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  1. TCaldwell

    TCaldwell Minister of Fire

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    I am trying to calculate with a 7'' round duct the air velocity is 2107 ft/min . what does that equate to for cfm, and a 6'' duct with a velocity of 2823 ft/min , I am told that you multiply the fpm x the free air number? to equal cfm, I hope somebody knows how to calculate free air for round ducts, thanks tom ps. these numbers are from the garn

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  2. heaterman

    heaterman Minister of Fire

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    Trying to determine CFM??

    Multiply FPM x PI x radius squared and divide by 12

    Take your 6" pipe......... radius is 3" x 3 x 3.14 = 28.26 sq inches in that 6" circle. The velocity is 2,823 FPM so 2,823 x 28.26 =79,777

    divide by 12 to get feet and you have 6,648 CFM

    This sounds REALLY high, are you sure about the velocity?


    Then again.................I shouldn't attempt any math at all this time of the day. :) Time for this kid to hang it up.
  3. vik455

    vik455 New Member

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    take the 79777and divide by 144. one square foot = 144 square inches. 79777/144=554 cfm
  4. eekster

    eekster New Member

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    I looked at my air duct calculator at work here and I think you be around that 550 cfm mark also. That"s alot of cfm"s through a 7" pipe.
  5. TCaldwell

    TCaldwell Minister of Fire

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    heaterman, vik and eekster are correct, duct dia squared x 3.14=square inches, square inches divided by 144= sqft take sqft x fpm=cfm. yes it sounds high but the burn rate is 425,000btu/hr. heaterman , in your testo garn results what differential pressures have you experienced , i am running 500cfm at 2 inches water column, 3/4 hp @ 3450 rpm thanks tom
  6. atlarge54

    atlarge54 New Member

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    Without looking it up I think it's the radius squared times 2pi for area, not diameter squared. I've been wrong before, so it wouldn't hurt to double check.
  7. TCaldwell

    TCaldwell Minister of Fire

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    you are correct i got lost, it is the radius squared not the diameter squared,who thought being a pyro would become so complicated!
  8. Jersey Bill

    Jersey Bill Member

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    2800 fpm seems awefull high for a chimney, if thats what it was.
    normally in ductwork design I keep the velocity below 1000 fpm,
    except for powered exhaust with entrained solids. in that case i try
    not to exceed 1500 fpm. At about 1000 fpm you can hear the air whistle as it
    goes through.

    Was the 2800 fpm a measured value.

    I have a ductulator which replaces those messy calculations. they don't use radius either.
  9. TCaldwell

    TCaldwell Minister of Fire

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    jersey bill this was a measured reading at the invert of the discharge of a 6'' flue stack, the air supply reading was 2107 fpm through a 7 inch duct . these were generated by a 3/4 hp draft inducee with a shrouded radial blade also static at 2 inch w/c this boiler fires at 425000btu/hr+ does that calculator know where i could find a blower for a 1 hp motor that would up the cfm to 650+/-?
  10. Jersey Bill

    Jersey Bill Member

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    OK, i get it. big stuff there.
    I ran thise numbers on the ductulator again and it came out to 590 cfm measured.
    No help on finding a bigger blower. You are right on with the horsepower though.

    If you reduce the resistance, like by increasing the duct size, the pressure drop would decrease,
    and the flow would increase, using the same fan. I dont know the big picture and what you are up against.
    You might be able to swap some 90's for some 45's, or change them to long sweep turns.
    good luck
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