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  1. #1
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    Free air calculation.


    Hi all, I'm in the process of servicing an AHU system as part of a building refurb that our company is doing. Does anyone know roughly the free air area of a 600x600 4 way supply diffuser? You know the sort, aluminium painted white, that sits in t-bar ceiling grid.
    I need this to apply to my anemometer m/s readings to give client the volumes.
    I know they will vary by manufacturer but a ball park figure will help a lot.
    Thanks



  2. #2
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    Re: Free air calculation.

    Best/easiest way is to make up a cardboard air collection hood, calibrate it and then take all of your readings with the hood.

    edit: Just checked with Lindab catalogue, they don't give free areas (damn). Looks like you'll have to measure the slots yourself.
    Last edited by Brian_UK; 05-02-2010 at 08:08 PM.
    Brian - Torquay, Devon, UK
    I have to stop saying "how stupid can you be?" to my co-workers.
    They're starting to take it as a challenge...

    BASIC MAINTENANCE. If it doesn't move and it should then use WD40. If it moves but it shouldn't then use Duct Tape.

  3. #3
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    Re: Free air calculation.

    Hello

    If the air distribution system incorporates a moderate number of air terminals, I would recommend hiring a propriety 'flow measuring hood' for the job. These instruments employ balometer technology to measure the volume flow rate and normally include digital displays with high resolution. It is important to examine the calibration certificate before you leave the store to verify the instruments accuracy. It can be very difficult to attain accurate and consistant readings without the appropriate equipment.
    Depending on the quantity of terminals to be measured, alot of time can be saved if you have an assistant to identify the terminal and record the respective flow rates while comparing against design figures.

    Hope this helps.

    Solas

  4. #4
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    Re: Free air calculation.

    46% according to the downloads here

    ooooooPs, I've always used 60% in my calcs , my systems always work though.

    Eggs

  5. #5
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    Re: Free air calculation.

    0.11m2
    Milenko

  6. #6
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    Re: Free air calculation.

    use an anomometer and a tape measure.
    Lineal metres flow x metres squared is total flow.
    Lots of lineal flow readings and average. Not rocket science stuff.

    magoo

  7. #7
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    Re: Free air calculation.

    Quote Originally Posted by Magoo View Post
    Not rocket science stuff.
    magoo
    Neither is reading the question.

    Quote Originally Posted by coolhandlew View Post
    Does anyone know roughly the free air area of a 600x600 4 way supply diffuser?
    Eggs

  8. #8
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    Re: Free air calculation.

    Quote Originally Posted by Magoo View Post
    use an anomometer and a tape measure.
    Lineal metres flow x metres squared is total flow.
    Lots of lineal flow readings and average. Not rocket science stuff.

    magoo
    I'm well aware of that, but there has to be a correction factor for restrictions ie. louvres etc

  9. #9
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    Re: Free air calculation.

    For "eggs "and everyone else there is a standard for a difuser hood testing forget about louvers .., what ever comes out is actual, my initial comment stands.
    For commissioning a/c diffusers a square test hood that reduces from square to round with a fixed discharge diameter, add anemometer will give the actual flow rates, the angle from square to round to meet standard is 11 degrees. Then use calcultor and flow rates. Like I said earlier not rocket science. The more cross reference readings the better, increases the accurracy.
    Last edited by Magoo; 15-02-2010 at 02:53 AM.

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