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  1. #1
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    Oct 2016
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    Most accurate cfm

    I’ve run into this before...

    Let’s say you have a fan/duct exhaust system with ten grilles. Your fan is centrifugal and installed correctly. Nice straight duct into and out of fan. Let’s just assume the duct system is completely sealed and has no leaks.

    You read the grilles with a calibrated hood. Take a traverse in a nice long straight section. Read the inlet/outlet pressure on the fan, and plot flow on a factory curve.

    The three values are different, which do you believe?

    Variable...what if the grilles are 50 cfm 6x6s and you have a 2’x2’ hood? Does this make the hood readings less reliable?

  2. #2
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    https://hvac-talk.com/vbb/showthread...he-books/page2
    Post 18 starts talking about the same issues. I’m not a tab guy but from what I’ve learned traverse is the most accurate in almost any situation
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  3. #3
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    Duct traverse is what you would go by. Thats the accepted standard.


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  4. #4
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    Thread Starter
    Thanks. I don’t understand why the fan curve is not more correct, as long as the fan is installed with straight duct as shown in iom. Have never trusted this value, as you guys state also. But wonder why.

  5. #5
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    Feb 2016
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    The main reason plotting on the fan curve is less accurate than a pitot traverse is the unreliability of measured static pressures. A flow hood reading 50 CFM is nowhere close to their range of peak of accuracy. Flow hoods are also susceptible to side loading of the airflow if the distance between the duct and the grille is short enough to force most of the air out one side of the grille. The hood can't compensate for side loading of the grid but will readout flow based on it's ability to average the velocity pressure and for side loading that isn't good. The pitot traverse is the base line of accuracy because it averages the velocity profile. Readings are taken across the duct and if plotted you can get a good idea of the expected error of the total airflow.
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  6. #6
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    Factory curves are interesting because they don't always represent real world situations. They are made under lab conditions with no return restrictions (ducts) You can't duplicate these conditions in the field.
    T&B reports include them because it's expected. With a hood sometimes something has to be used to cause the hood to measure all the air. Like cardboard covering all but where the grill is. Probably not very accurate because of the hood not a good fit. Maybe a lo flow hood would fit better. Most times I ignore the fan curve and put trust in the traverse in a duct run like that. There are other ways to measure the grills also.
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  7. #7
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    The 2x2 hood might not be accurate on air flows that small. Might check the manual. Maybe you could cross check the results with an anemometer or velometer.
    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

  8. #8
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    Thread Starter
    Thanks for all the responses.

  9. #9
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    Aug 2019
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    good information

  10. #10
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    Aug 2019
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    Agreed , the pitot tube duct traverse is usually the most accurate . However , you have to be real careful when looking at your individual velocity readings as they should be within about 10% of each other . When your traverse includes zeros or other very low readings it is not as accurate and you have to make a call . Your hood readings , traverse and fan curves are almost like a 3 -headed monster . Sometimes you have to pick the 2 that are close to each other . If it was an exhaust fan I would include looking at the running amps vs the full load amps . If it was a dx air handler I would look at my temperature split on the coil and throw that into the mix . Sometimes you have to guess , in that case I would say something like ……" Total airflow was determined by ____ and ____ ." Then if you are challenged you can back up your numbers .

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