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  1. #1
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    ACCA MANUAL D Psychometric airflow calculation question

    Hello Engineers, physicists, and HVACKERS. My question is this. In the ACCA manual D Psychometric Airflow Calculation equation:

    CFM=Load/1.1X(Altitude Correction Factor)X(Temp difference in supply air and room air). I dont get the supply air temp part. How is this number generated? Is it an assumed number, or a number that comes out in a manual J worksheet, or a rule of thumb, or does the equipment have to be existing? Is it one of those rules of thumb like 400 CFM per ton for cooling, and we assume a 55 degree supply temp? I know the equipment having to be installed already doesnt make any sense. That would be closing the barn door when the horse is already out. Can someone with a good understanding of this clarify? Im aware this method is rarely used, Im just trying to expand my understanding. Thanks in advance

  2. #2
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    It is whatever you are using as your design supply temperature. If you have a constant 55 degree supply for design just plug it in and go. If your design has a supply temperature reset then it will be a variable dependent on your reset.
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  3. #3
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    Quote Originally Posted by TheComfortGuru View Post
    Hello Engineers, physicists, and HVACKERS. My question is this. In the ACCA manual D Psychometric Airflow Calculation equation:

    CFM=Load/1.1X(Altitude Correction Factor)X(Temp difference in supply air and room air). I dont get the supply air temp part. How is this number generated? Is it an assumed number, or a number that comes out in a manual J worksheet, or a rule of thumb, or does the equipment have to be existing? Is it one of those rules of thumb like 400 CFM per ton for cooling, and we assume a 55 degree supply temp? I know the equipment having to be installed already doesnt make any sense. That would be closing the barn door when the horse is already out. Can someone with a good understanding of this clarify? Im aware this method is rarely used, Im just trying to expand my understanding. Thanks in advance
    That’s called the basic “sensible heat” formula.
    You calculated the load for the room in manual J. Then get your expected temperature difference from the equipment you chose. Then size the duct for the air volume required to meet the load.
    Last edited by mgenius33; 03-14-2018 at 11:34 PM.
    "The only true wisdom is in knowing you know nothing" Socrates

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  5. #4
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    It's supposed to be the Delta T between supply air and return air not necessarily room air.
    The formula is most often used in it's other form to determine sensible heating/cooling.
    BTUH= 1.08 (Corr alt) * CFM * Delta T
    Temperature is very linear.
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  6. #5
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    Supply air temperature is a function of the calculated temperature rise needed to provide the desired conditioned space dry bulb temperature and the desired corresponding relative humidity.
    In most commercial applications the design supply air temperature is 55 F. The design temperature rise is 17 to 20 F which gives a relative humidity of 50 to 55 percent.
    In most cases the heat added to increase the temperature is sensible.

  7. #6
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    Supply at the coil is often lower than the outlet - business opportunity there (insulation). Same is true on the return (insulation and tape/mastic). Grewat troubleshooting technique. Bring drywall saw.

  8. #7
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    Quote Originally Posted by TheComfortGuru View Post
    CFM=Load/1.1X(Altitude Correction Factor)X(Temp difference in supply ar and room air)
    The formula is supposed to start:

    BTUH=....

    Not CFM=.

    I can see why having a delta temp on a CFM calculation seems confusing. But it is really a BTU calculation. And as mentioned, it is for heating. Cooling uses enthalpy to catch the latent heat extraction.
    Hmmmm....smells like numbatwo to me.

  9. #8
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    Quote Originally Posted by numbawunfela View Post
    The formula is supposed to start:

    BTUH=....

    Not CFM=.

    I can see why having a delta temp on a CFM calculation seems confusing.
    Are you saying a derivation would be inaccurate?
    "The only true wisdom is in knowing you know nothing" Socrates

  10. #9
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    mgenius is correct. You can and often need to solve equations for the variable you want to quantify.
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  11. #10
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    Quote Originally Posted by hvacker View Post
    It's supposed to be the Delta T between supply air and return air not necessarily room air.
    The formula is most often used in it's other form to determine sensible heating/cooling.
    BTUH= 1.08 (Corr alt) * CFM * Delta T
    Temperature is very linear.
    For air delivery requirement to a room, it is based on desired room air conditions

    Example - sea level, so no elevation correction.

    I have a room design condition of 70 degrees 50% RH and a BTU heat loss of 1500 btu/hr

    How many CFM of air do I need to send to the room when I have a unit supplying 100 degrees?

    The calculation would be
    CFM= BTUH / 1.08 * (supply temp-room temp)
    CFM = 1500 /1.08 * 30

    result would be = 46.3 cfm

    Humidity is not taken into account, as the heating unit does not change the grains of humidity in the air.

  12. #11
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    Quote Originally Posted by mgenius33 View Post
    Are you saying a derivation would be inaccurate?
    I read this and was like WHA?

    But that happens when you Htalk when you first wake up on a weekend.

    My apologies, I took 5 seconds to read the post and not 10. I missed that the LOAD in the equation was the missing BTUH that I was looking for, and the whole thing was solved for the variable the OP was looking for, thus moving the pieces around.

    HVACKER and heatingman are using the formula I was thinking of.

    Good reminder to take the extra 5 seconds before posting.

    Hmmmm....smells like numbatwo to me.

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