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Thread: concentric diffusers

  1. #1
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    concentric diffusers

    Is there a good way to air balance or get the total CFMs of a RTU with concentric duct and diffusers?

  2. #2
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    use manufacturer cfm charts and your TESP readings to find total CFM'S,
    I'm not young enough, to know everything...

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  4. #3
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    Also need the fan curve charts

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  6. #4
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    Traverse the drops, or a velocity grid the grills. Need MFG data to know true area of the openings.


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  8. #5
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    [QUOTE=heatingman;26114457]Traverse the drops, or a velocity grid the grills. Need MFG data to know true area of the openings.

    Well thats what Im asking, how do you use a hood or traverse the duct when its a concentric duct and diffuser.

  9. #6
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    Quote Originally Posted by heatingman View Post
    Traverse the drops, or a velocity grid the grills. Need MFG data to know true area of the openings.

    Well thats what Im asking, how do you use a hood or traverse the duct when its a concentric duct and diffuser.
    Drill a hole/few holes in the duct before the diffuser. Usually on the main drop /plenum so you get the whole unit at once. To get each diffuser, you'd need a diffuser hood that's larger than them and hopefully a ceiling above to seal against. (open concept ductwork is going to make it difficult.)

  10. #7
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    Quote Originally Posted by supers_5 View Post
    Drill a hole/few holes in the duct before the diffuser. Usually on the main drop /plenum so you get the whole unit at once. To get each diffuser, you'd need a diffuser hood that's larger than them and hopefully a ceiling above to seal against. (open concept ductwork is going to make it difficult.)
    Again, its a concentric duct, which means the return is in the center and the supply is on all four sides. These unit are in a factory, so they just drop through the roof. The curb could probably be drilled and traversed, but Im not sure if that would work. I wouldnt be doing it regardless, it would be one of our Balancers. But their manager said there is no way to do it.

  11. #8
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    Can you get to where the supply and return duct is connected to the register?

    Usually, they come down quite a bit from the unit, then into the concentric fitting. The ducts are separate coming from the unit, before they go into the concentric.

    If not, you need the fan curve chart, check static, and rpm. That will give you cfm.

    There's no way to balance each register off the four sides. But there is a way to measure the unit total CFM. Well shouldn't say that, you could make an adaptor for a flow hood, out of cardboard I guess....I've done that before for odd size registers.

    What exactly are your trying to accomplish?

    Balancing the air out of each supply of the concentric, OR simply checking unit CFM?

  12. #9
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    Quote Originally Posted by hands View Post
    Again, its a concentric duct, which means the return is in the center and the supply is on all four sides. These unit are in a factory, so they just drop through the roof. The curb could probably be drilled and traversed, but Im not sure if that would work. I wouldnt be doing it regardless, it would be one of our Balancers. But their manager said there is no way to do it.
    Oh you mean the whole duct, not just the diffuser. My bad.

    You still need a hood, but round and only fitting the center of the register. That or you drill through both and still do the transverse. To get area face area of the outer duct, you find the area of the larger duct and subtract the inner duct.

    TESP of the unit still applies.

  13. #10
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    Do a traverse on the supply grille, using a vel-grid. Same for return, then do some math to find OA percent. Confirm with temperature. OA% = MA-RA / OA - RA

    For the temp method to work most accurately, need a reasonable difference between RA and OA.

    http://www.shortridge.com/velgrid.html

    You need to know the true area of the grill

    IE is a 48 x 48, but only 75 percent is free area, so you need to correct the area for that.

    The MFG of the grill can tell you what it is.

    Find another balancer.

    These get done all the time.

    Not ideal, but possible.

    They will also need to double check the results against static pressure of the unit and RPM of the blower against MFG charts.

    Also, they could do a pressure drop across the evaporator coil pending layout of the unit, which the MFG of the unit can provide.

    Balance companies know all of this.

    As installed there is nothing to balance.


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