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Thread: Duct Problems

  1. #14
    Join Date
    Jun 2010
    Location
    Philadelphia PA
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    2,190

    An interesting thought

    Quote Originally Posted by synergy View Post
    Sounds like you have no static pressure. Too many openings or too many air exits to allow for static pressure to build up and push the air out of the closest registors. Close down some of the dampers until you get static pressure in the trunk. I Should think you would need a 1/2 inch or so of pressure at 2/3 down the trunk run, on a unit that size.
    But don't you agree that close to the unit discharge we have all velocity pressure and won't have any static til the airstream slows down and we see the static regain we are looking for?
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  2. #15
    Join Date
    Jun 2010
    Location
    Philadelphia PA
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    2,190

    Hey Gibby, just remembered

    Quote Originally Posted by Gib's Son View Post
    I kinda missed the turbulence word first time around. That being said, I have seen it many times with high velocities that the air just rushes on by the first branch.
    A number of years ago we installed a make-up air unit for an industrial printing press. The mechanical insisted on a high velocity supply since the run was short and the we had plenty of fan on this extended plenum kind of design. well the air was moving around 1800 FPM and actually sucked air in on the first supply register. Just thought of that looking at your post. Funny huh?
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  3. #16
    Join Date
    Jul 2011
    Location
    Columbus, Ohio
    Posts
    136

    Static

    Static pressure should be at its higest at the fan exist and at its lowest at the end of the run, this is due to friction loss through the duct. The duct internal area and the number of exits need sized and balanced so as the air supplied to the duct is slightly higher than the air leaving the duct. This creates static pressure. Static pressure forces the air out of the registors. With out static pressure, the air will travel down the duct and the velocity pressure will force the air out at the end runs only. You need to have a pressurised duct to get even air flow to all supplies. How much static varies from type of system.

  4. #17
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
    Posts
    5,058
    Quote Originally Posted by genduct View Post
    But don't you agree that close to the unit discharge we have all velocity pressure and won't have any static til the airstream slows down and we see the static regain we are looking for?
    Velocity pressure only exists with static pressure.

    You can have static without velocity though.

  5. #18
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,190

    Hey Second

    Quote Originally Posted by second opinion View Post
    Velocity pressure only exists with static pressure.

    You can have static without velocity though.
    I agree with your quote but without motion what would be the point of a system that wasn't delivering anything?

    Me and Mr Bernoulli Think of TOTAL Pressure the same way you think of WATTS You know Volts time Amps. Key being the watts stay the same and as the velocity slows the static builds. Now that doesn't mean there is all velocity pressure at the discharge just mostly velocity pressure.

    That's my understanding of the Physics of the situation. If you see any flaws or have a simpler explanation, you know me, I am always open to being educated
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  6. #19
    Is this a new unit? Has the unit and duct been sized correctly? Has it been balanced?

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