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  1. #1
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    Exhaust hood testing

    Hey guys, I've been asked to verify airflow to a lab hood before they call for certification.

    I just need to check the air entering the front plexiglass panel.

    Would I just use a Hotwire anemometer and traverse the face, at the plane of the glass, and average the grid?

    It's too big for a flow hood.

  2. #2
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    For what they want you to do testing the face velocity and converting to CFM is ok. Hotwire and electronic anemometers are both allowed. If allowed to drill the duct for a pitot traverse that is the best way to do the job. Inspect the duct carefully especially in an old system.

  3. #3
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    Thread Starter
    This hood has a downflow fan also. Right now the velocity and CFM entering the front is all they want to check.

    Is there a standard for testing? I.e. Placement in the free area, and number of test points per sq.ft.?

  4. #4
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    There is more than one standard but all end up the same place. The EPA has acceptable methods including instruments used as does NEBB, AABC and others. You can access the EPA requirements on line. The easiest acceptable method is the velocity grid. You take enough readings to cover the entire opening even if you overlap some of the readings. You need to know the position of the sash for the necessary average velocity. It doesn't sound like they are giving you enough info. Traverse of the duct will tell you if you have the minimum exhaust required and that will tell the owner if the hood can be set up to meet design.

  5. #5
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    You might want to check with the manufacturer. They may have a specific way to test it.

  6. #6
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    Thread Starter
    They've been somewhat helpful. Although they have conflicting data regarding the hood. The hood manual states one design CFM while another design data sheet states less. When I questioned which to follow, the answer was that the higher end "should be fine".

  7. #7
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    Could you get a tachometer on the wheel or blower pulley with all the panels on or just pull the cover off the drive train,,, then check tesp and use the blower performance curve

  8. #8
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    If they want to use the hood better call in the company that is going to test it. Why expose yourself to possible liability when they are going to be required to fully test the hood before using it? There is no method acceptable for determining airflow other than measuring it. If the airflow is wrong the testing agency will correct it as part of the testing.

  9. Likes heatingman liked this post
  10. #9
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    Quote Originally Posted by coolerinfrederick View Post
    Could you get a tachometer on the wheel or blower pulley with all the panels on or just pull the cover off the drive train,,, then check tesp and use the blower performance curve
    Most lab hoods are connected to plume fans I believe their called. They are the type the heavily dilutes the hood air with outside air at the fan.

    Sort of like a venturi effect.

    As an example, the exhaust fan may be moving 10000 cfm, but may only pull 1000 from the lab hood.


    Sent from my iPhone using Tapatalk

  11. #10
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    heatingman,
    It depends on what the hood is used for. Some chemicals require both dilution and dispersal. For dispersal the fan discharges large quantities of air and sends the mixed air plume high enough in the air to enhance safety. It is not something a novice should screw with.
    If something goes wrong what happens if a legal eagle looks at this site?

  12. #11
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    Quote Originally Posted by WAYNE3298 View Post
    heatingman,
    It depends on what the hood is used for. Some chemicals require both dilution and dispersal. For dispersal the fan discharges large quantities of air and sends the mixed air plume high enough in the air to enhance safety. It is not something a novice should screw with.
    If something goes wrong what happens if a legal eagle looks at this site?
    Your absolutely right. I was just explaining why rpm at the fan probably would not work to find lab hood air flow.

    I personally wouldn't touch that stuff with a ten foot pole, as far as qualifying whether the hood is working or not.

    As you know, even within the TAB world, lab hood measurement and certifying is a specialist market.

    Sent from my XT1254 using Tapatalk

  13. #12
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    heatingman,
    I understood where you were going and was re-enforcing what you said with some additional information. I tested fume hoods until the certification program was initiated. I never liked doing it because the manufacturers will not commit to anything when it comes to performance. The traditional method of measuring face velocity is with the velocity grid (vel-grid). The problem with the vel-grid is that to be reliable a correction multiplier needs to be determined. The correction multiplier is established by comparing the measured velocity to the velocity measured by pitot traverse. You probably see where that is going. Hood certification testing does not include certifying whether the hood passed or failed. I couldn't accept that concept and never tested another hood. I have tried to get a couple of hood users that trust my work to pressure the manufacturer to provide some assurance the hood was set up properly and was functioning per design when tested. No luck because nobody wants to create waves.

  14. #13
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    Thanks heatingman and wayne,, always learning

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