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Thread: Finding new cfm from old cfm and pressure drop across a filter

  1. #1
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    Finding new cfm from old cfm and pressure drop across a filter

    I can use a fan law calculation for this correct?

    The set up, I have pressure drop curves from an air filter manufacturer.

    At 1200 cfm the pressure drop is rated at .12

    If I wanted to know what the cfm was from a .5 pressure drop I could use this formula correct?

    (Cfm new/ cfm old) squared = Static new/static old


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  2. #2
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    It depends on why the pressure drop increased. If the pressure drop increased due to plugging of the filter the airflow will drop a lot (588 CFM) but if the increase in drop was due to increased airflow the airflow will increase a lot (2499 CFM). Your formula is good for increased drop due to added flow.

    In my opinion it is easier to use CFM_New=CFM_Old Times SQ_Root of (New_Static divided by Old_Static) (for drop due to added flow)

    For added drop due to dirty filter just swap places with the statics.

    These calcs are based on a flow of 1200 CFM. For an accurate answer you need actual measured CFM.
    No man can be both ignorant and free.
    Thomas Jefferson

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  4. #3
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    Thread Starter

    Finding new cfm from old cfm and pressure drop across a filter

    Thanks Wayne!

    Basically my duct system at the new place is not ideal for doing a traverse. And the registers are the baseboard type which are not ideal for use of a flow hood (not that I have one anyway)

    Precise accuracy is not required more close enough sort of measurements for relative change.

    The pressure changes are based on blower speed changes. The numbers I provided where more just examples, but I do recall that in the highest speed the pressure drop across the filter was higher then the chart shows.

    I plan to buy a new filter and use that for reference.

    I want to know approximately how much flow each speed of the blower is producing.


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  5. #4
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    If you can take reliable wet bulb temperatures before and after the coil we can use the compressor capacity chart and work backwards to get the airflow.

    If you can't get a reliable wet bulb temperature then you can take dry bulb temps and measure condensate output and temperature and we can do the same thing.

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  6. #5
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    Thread Starter
    Quote Originally Posted by shellkamp View Post
    If you can take reliable wet bulb temperatures before and after the coil we can use the compressor capacity chart and work backwards to get the airflow.

    If you can't get a reliable wet bulb temperature then you can take dry bulb temps and measure condensate output and temperature and we can do the same thing.

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    Yes. I have the means to do that. Would be a good means to confirm the pressure drop flow calcs.


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  7. #6
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    The most frustrating thing I ran into when I got into TAB was measuring static pressures. They are simple to measure but in residential systems are rarely accurate. The problem goes away if you can traverse the duct but as you said most residential systems don't have room for enough undisturbed flow for a traverse. I measured statics with instruments that varied in cost from $50, $90 and $4,000 with disappointing results.
    I admire your approach to this heatingman and am convinced you will end up close enough.
    No man can be both ignorant and free.
    Thomas Jefferson

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