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Thread: round vs square

  1. #1
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    round vs square

    i've heard conflicting statements about which is better round or square/rectangular ducts as far as air flow. also any comments of insulation - inside or outside???
    td55

  2. #2
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    I figure you are trying to get a specific answer as to which is better. But I think as long as you are getting the correct CFM it just doesn't matter if it is circular or rectangular.

  3. #3
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    Better to wrap(outside) sheet metal then to line it. Unless you also want sound attenuation.

    A 12" round and a 16x8 carry the same amount of air. its just the 12" round takes up 4" less space one way, and 4" more the other.
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  4. #4
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    Quote Originally Posted by beenthere View Post
    Better to wrap(outside) sheet metal then to line it. Unless you also want sound attenuation.

    A 12" round and a 16x8 carry the same amount of air. its just the 12" round takes up 4" less space one way, and 4" more the other.
    Also, the 12" round uses less metal then the 16X8.
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  5. #5
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    insulating inside requires higher sq" duct due to friction loss. depends on the location of the duct to determine if i insulate inside or out
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  6. #6
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    Round pipe uses less materials, is easier to install, easier to seal, easier to insulate, and is lower friction than rectangular duct.
    If more government is the answer, then it's a really stupid question.

  7. #7
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    Square duct looks better, and I'd like to see where it states that round has less friction

  8. #8
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    12" round pipe, 113.04 sq in, 700 CFM at a .1 FR.
    16x8=128 sq in, 700 CFM at a .1 FR.
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  9. #9
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    Quote Originally Posted by Catpower View Post
    Square duct looks better
    If you say so...
    Ducts are generally out of sight, but pretty much all of the best looking exposed duct systems I've seen were round spiral pipe.

    and I'd like to see where it states that round has less friction
    Just look at your ductulator. For the same air volume and velocity, rectangular duct has more crossectional area than round pipe. This is because there is more surface area inside the duct, therefore more friction created by the airflow over it.

    http://www.snipsmag.com/CDA/Archives...00f932a8c0____
    If more government is the answer, then it's a really stupid question.

  10. #10
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    Put your 30 inch round duct above a 24 inch suspended ceiling

  11. #11
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    Ah, leave it to an engineer, they'd expect 30" tall duct above a 12" ceiling lol
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  12. #12
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    Heat gain and loss and required metal gauge also argue in favor of round pipe. A 10" round has a circumference of about 31". Taking the rectangular duct to a very extreme condition, just for argument, a 1" x 80" would have the same area as the 10" round, but the circumference would be 162". This rectangular duct would have much more heat transfer. Since square or rectangular duct must be bigger in area than round for the same friction loss, and because of the flat sides, it will have to be a heavier gauge. Even duct with a 1 to 1 aspect ratio (ratio of height to width) will have a little more friction loss than round pipe of the same area. But I'm just a technician and this is really an engineering issue. Bottom line for me is there is not a huge difference in round pipe and square pipe, but extremely rectangular pipe requires careful engineering and I will take measurements if an otherwise ok system has low airflow. Pressure readings before and after the rectangular duct section can help the engineer specify corrective action. I have seen this low airflow problem where the duct had to transition around obstructions and the sheet metal guys made the rectangular transition with the same cross sectional area as the square duct when it should have been increased. If the sheet metal guys are going to take it on themselves to make changes in the design, they should first learn what ideal is from study and from determining equivalencies on the duct calculator.

  13. #13
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    Quote Originally Posted by Catpower View Post
    Put your 30 inch round duct above a 24 inch suspended ceiling
    Rectangular duct is good for limited space applications. If space isn't a concern, like an attic, round duct is a superior choice.
    Another consideration is that since round duct has less surface area, there is reduced opportunity for heat gain in an unconditioned space.
    If more government is the answer, then it's a really stupid question.

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