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Thread: Check CFM

  1. #27
    Join Date
    Mar 2012
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    West Chester, PA
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    511
    You can use the heat rise formula CFM = btu out/delta t x 1.08 which is essentially the same as the electric heat conversion. The BTU out is most cases will be taken from the name plate. of course we have to acknowledge that we may not truly be getting that name plate rating. If the unit is natural gas and the manifold pressure is at the manufacturer's spec, then the reading should be close enough for most applications. If the unit is propane, then the method can still be used but may be offset by the fact that propane is unregulated and can vary in heat content between suppliers. Again, the reading will likely still get you into the ball park. Another important thing to remember is that the blower speed must be the same between the heat mode and cooling mode for this to work or at least for the numbers to apply to cooling airflow (which is usually our main concern). Running the furnace with cooling air flow can be tricky on some newer equipment. The upside is that most new equipment is variable speed and you shouldn't be needing to use heat rise to confirm airflow. Simply check your programmed CFM and take a static reading to ensure that you aren't grossly out of range and you should be getting that programmed airflow. variable speed/premium ECMs will usually deliver programmed airflow to about .7-.8" water column. Beyond that that, the delivered airflow may begin to drop off.

  2. #28
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    Mar 2012
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    West Chester, PA
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    As someone else mentioned, i think, be sure to take your supply temp reading out of direct line of sight of the heating elements or heat exchanger. usually in the supply about 10" from the plenum is where i take my reading.

  3. #29
    Join Date
    Aug 2001
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    Mechanicsville, Virginia
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    1,097
    Beenthere, you are correct the return air in my example should be 612.59 CFM. I will make the correction ASAP.

    "If perfection is your goal, you may end up with good enough, what might you end up with when good enough is your goal?"
    Wayne Pendergast, efficientcomfort.net

  4. #30
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    Using Carrier pyschrometric software. Converting mass flow(2683 lbs/hour) to CFM, I come up with 620 return, and 678 supply.

    I did that on a scratch pad(punched it in a spread sheet quickly) so could be off a percent or 2.
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  5. #31
    Join Date
    May 2013
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    22
    Awsone thanks for the help

  6. #32
    Join Date
    Aug 2001
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    Mechanicsville, Virginia
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    With trepidation that I have hijacked this thread, I proceed...

    Quote Originally Posted by beenthere View Post
    Using Carrier pyschrometric software. Converting mass flow(2683 lbs/hour) to CFM, I come up with 620 return, and 678 supply.

    I did that on a scratch pad(punched it in a spread sheet quickly) so could be off a percent or 2.
    Beenthere, Our results are very close and many would not find the discrepancy significant. It looks like the main difference in our calculations stems from the density equation. In my equation, I factor the humidity ratio.

    CFM = ṁ * 60 * ρ

    ρ = 1/v * (1+W)

    where:
    ṁ = mass air flow
    ρ = Density
    v = Specific Volume
    W = Humidity Ratio

    From what I understand the term "Specific" in specific volume is a refers to "Dry Air Specific" and factoring the humidity ratio into the common density equation (ρ=1/v) accounts for humidity.

    Equation (6) in this link http://www.engineeringtoolbox.com/de...air-d_680.html is a variation of the equation I use. It is also found in this document (bottom left page 2) http://www.tcf.com/docs/fan-engineer...?Status=Master

    From what I have found, using the dry air specific equations is more common, simpler and doesn't make a great enough difference to worry about. In some cases it may even add a "fudge factor" to the design process.

    For my calculators, which are geared toward measuring actual equipment performance rather than calculating for equipment design, I have looked for and implemented the most precise (even though more complicated) equations and methods that I can understand. The psychrometric calculations used in my HVAC Performance Calc are from a Psychrometric and Coil performance calculator I also developed. It shows more detailed results. I'd be pleased to know if you approve.

    "If perfection is your goal, you may end up with good enough, what might you end up with when good enough is your goal?"
    Wayne Pendergast, efficientcomfort.net

  7. #33
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    Jan 2004
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    Lancaster PA
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    I've used those before. I usually come up slightly different when I do them with a spread sheet(I don't adjust density for altitude since most of the time I'm under 750 foot). Your calcs forms are sooo much easier to use then the way I do it. I always use mass air flow. So in heating its real easy as the mass never changes. In cooling of course its a bit more difficult since mass does change due to latent heat.



    PS: Carrier psych software does pretty much what your psych calc does. Just you can't change to exact altitude in it like you can in yours.
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  8. #34
    Join Date
    Jan 2007
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    Mn the state where absolutey nothing is allowed
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    density calcs

    is altitude a valid factor in density caculations?. seems to me many caculators use altitude as a direct correlation to atmospheric pressure = barameteric pressure.

    yet depending on current weather conditions, local barometric pressure here at home on a stormy day( 940 ft above sea level) can be lower than a beautifull sunny day in Denver Co.

    ive uses barameteric pressure divided by dry bulb absolute times 1.325

    (Bp/DBabs) x 1.325 = air density


    btw. im also not that worried over using correction factor other than 4.5 and 1.08 due to the low altitude around here unless the return air temps is quite high or low
    my boss thinks its possible to repeal the laws of physics

  9. #35
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    From sea level to 2500 foot it only makes a difference.
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