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Thread: T&B on a loop.

  1. #1
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    T&B on a loop.

    Another thread reminded me of an interesting problem/solution. It was a small AH 20000 cfm.
    The installed duct wouldn't deliver on the end terminals.
    The engineers asked to do several things but eventually told the tinner to create a loop. Branch the supply with a pair of pants and connect the two together at the end of the runs.
    Pressurize the duct in two directions.

    I can remember homes that had a perimeter loop. I think the practice went away because of the extra ducts required.
    This was the first time I had seen it used but it did solve the air problem. I'm not sure how to figure things like friction and predict cfm.
    Anyone ever run into this design today?
    Give me a relay with big enough contacts, and I'll run the world!

    You can be anything you want......As long as you don't suck at it.

    If a person wants to create a machine that will be more likely to fail...Make it complicated.

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  2. #2
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    Ive never worked on one, but seen the design at a chipotle. I think mainly it was for looks in that case, as the duct was exposed.

    But near the ends of the runs, the only measurements that would be accurate would be from the grills.

    Maybe 75% way down the run, a traverse may prove dubious.

    Curious what Wayne thinks.




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  3. #3
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    most the trane intellipak vav systems I saw were ducted like this
    Keep it simple to keep it cool!

  4. #4
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    The duct loop theoretically cuts the velocity in half and thus the friction losses. What actually happens depends on the load distribution and the load proximity to the origin of the air supply to the load. The main place I have seen this design is in gymnasiums. They work close to the design theory because the relative location of the diffusers and airflows are all the same. I haven't seen this design implemented on a VAV system. If this has to be used on a VAV system the diversity would probably be 25 to 30% or maybe more. What makes undersized ducts a problem on VAV'S is the air may be available to the VAV but at too low a pressure for the VAV to deliver airflow close to design. VAV'S and FPB'S should have at least 1/2 inch of static at the box inlet.
    No man can be both ignorant and free.
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  5. #5
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    Thread Starter
    I might think this system could save metal because the initial run would be smaller but more fo it. Smaller ducts overall are cheaper in metal and labor. The design in the post was to solve a problem but I also know it's a valid design.

    Off the plenum the runs would be smaller but could be staged like an equal friction or how it might be an advantage for static regain. I'm not familiar with the velocity reduction method. I also wonder if /how it might affect dynamic losses.
    I also wonder if this idea might be used for other poor designs that failed to deliver on end runs. If running another small duct to the end runs rather than demoing or modifying the whole duct system. Just pondering.
    Give me a relay with big enough contacts, and I'll run the world!

    You can be anything you want......As long as you don't suck at it.

    If a person wants to create a machine that will be more likely to fail...Make it complicated.

    USAF 98 Bomb Wing 1960-66 SMW Lu49

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