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  1. #1

    Question Relative Humidity - ASHRAE v ACCA

    I'm adding the ASHRAE Outdoor Design Conditions as a lookup table to my ACCA MJE8 spreadsheet and I notice that ASHRAE don't represent RH in the same way as ACCA.

    The ACCA manual gives you the RH figures for 55%, 50% and 45% for any given city. However, the ASHRAE table presents the Humidity Ratio "humidity ratio, grains of moisture per lb of dry air" (which I understand is the same thing as RH), as 0.4%, 1% and 2%. E.G.

    ACCA
    Birmingham. AL
    55% 34

    50% 41

    45% 47

    ASHRAE
    Birmingham. AL
    0.4% 135

    1% 131

    2% 127

    Now both seem to describe RH in the same way, i.e. a percentage of total saturated air (grains) at a given temperature. But they represent the 3 comfort levels on a different scale. How can I convert the ASHRAE RH data into the same scale as the ACCA MJE8?

  2. #2
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    ACCA Manual J = grains difference between outdoor grains at design dry and wet bulb conditions for given geographic region and indoor grains at 75*F dry bulb/50% or 55% relative humidity.

    Example: Dallas, Texas summer design dry bulb = 100*F dry bulb/75*F coincident wet bulb, which equates to ~ 91 grains per pound of dry air moisture content. Indoor design conditions to be met are 75*F dry bulb @ 50% relative humidity, which equates to ~ 64 grains per pound of dry air moisture content. Difference between indoor grains and outdoor grains is 27 grains.

    Not sure about ASHRAE's approach. Will need to study it to offer definitive response. Others may chime in with correct info.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  3. #3
    Thanks Shophound, impressive knowledge of the subject.
    The ASHRAE figures are still a mystery to me tho...

  4. #4
    Here's more info on the ASHRAE data for Birmingham, AL if that helps?

    Dehumidification DP/MDB and HR
    .4% | 1% | 2%
    DP HR MDB DP HR MDB DP HR MDB
    75 135 83 | 74 131 82 | 73 127 81

    DP = dew-point temperature, F
    HR = humidity ratio, grains of moisture per lb of dry air
    MDB = mean coincident dry-bulb temp., F

    The ACCA RH figures for Birmingham, AL look like this:
    Summer
    Design Grains 55%RH|Design Grains 50%RH|Design Grains 45%RH
    34 41 47

    Now does anyone know how to convert the ASHREA data into the same range as the ACCA tables?

  5. #5
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    Quote Originally Posted by unclebingo View Post
    Here's more info on the ASHRAE data for Birmingham, AL if that helps?

    Dehumidification DP/MDB and HR
    .4% | 1% | 2%
    DP HR MDB DP HR MDB DP HR MDB
    75 135 83 | 74 131 82 | 73 127 81

    DP = dew-point temperature, F
    HR = humidity ratio, grains of moisture per lb of dry air
    MDB = mean coincident dry-bulb temp., F

    The ACCA RH figures for Birmingham, AL look like this:
    Summer
    Design Grains 55%RH|Design Grains 50%RH|Design Grains 45%RH
    34 41 47

    Now does anyone know how to convert the ASHREA data into the same range as the ACCA tables?
    Off the cuff, it strikes me what you wish to do is an apples/orange deal. The ASHRAE data is weather data, the ACCA data is geared specifically to the ACCA Manual J or N method of heat gain/loss calculations for structures using their heat transfer multipliers and assumptions. The design dry bulb for M J/N is sensible heat gain (for summer comfort cooling), and the grains difference figures into the infiltration rate the structure might experience under design conditions in the summer (only reason to be concerned with what the moisture levels outside are is how much of a load it imposes on the structure's interior on a design day in reference to the interior target conditions).

    To quote a section of ASHRAE's "Humidity Control Design Guide" :

    .4% Humidty Ratio -

    These data represent the humidity ratio that is higher than all but .4% of the observations during the years of the period of record. It is the same as the peak dew point values contained in the Handbook - Fundamentals... When a year is typical of the long term record, the humidity is not likely to be higher than this value, except for about 35 hours out of the year's total of 8760 hours.
    Translation: ~35 hours out of a year, the dew point in Birmingham, Alabama, might exceed 75*F (equal to 135 grains of moisture per pound of dry air), meaning the relative humidity can range from 100% at 75 degrees dry bulb to 53% at 94 degrees dry bulb, which is the ACCA design dry bulb temperature for Birmingham. This differs from the ACCA data that has a coincident wet bulb temperature of 75, which renders 104 grains per pound of dry air, 67 degrees dew point, and 42% relative humidity.

    The ACCA method would state that if Birmingham's summer design is 94 db/75 wb, rendering gr/lb at 104, the grains difference when indoor target conditions are 75 db @ 50% RH is 37 (104 - 65). You can use a psychrometric chart or calculator to find the grains delta for indoor humidity targets at 45% or 55% (75 dry bulb constant for all three ranges) easily.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  6. #6
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    The ASHRAE data is most likely dehumidification data

    back around 1993 or so they realized that the most humid time of year typically does not occur when you are at a "design dry bulb and mean coincidental wet bulb" condition
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  7. #7
    Thanks again Shophound.. checking into Psychrometric calcs now.

  8. #8
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    Quote Originally Posted by Carnak View Post
    The ASHRAE data is most likely dehumidification data

    back around 1993 or so they realized that the most humid time of year typically does not occur when you are at a "design dry bulb and mean coincidental wet bulb" condition
    The section I quoted from ASHRAE's "Humidity Control Design Guide" states that the data shown is from the Handbook of Fundamentals, and goes further to state the Handbook includes interpretation of the data pertaining to comfort cooling in addition to dehumidification. The Handbook is the one ASHRAE book I do not yet have, but if I wait patiently for June (and renew my ASHRAE membership) I expect the latest edition of the Handbook will be issued, since I understand it is refreshed every four years.

    Yeah, if I were designing a dry room, I think I'd be real interested in the weather bin data we've been discussing. For residential/light commercial comfort cooling/heating, Manual J or N should do just fine.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  9. #9
    ACCA Design Conditions Data

    ASHRAE Design Conditions Data



    I need to calculate the Design Grains 45%, 50%, 55% figures that appear in the ACCA table above by using the data contained in the ASHRAE tables. Is this possible?

  10. #10
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    Quote Originally Posted by shophound View Post
    The section I quoted from ASHRAE's "Humidity Control Design Guide" states that the data shown is from the Handbook of Fundamentals, and goes further to state the Handbook includes interpretation of the data pertaining to comfort cooling in addition to dehumidification. The Handbook is the one ASHRAE book I do not yet have, but if I wait patiently for June (and renew my ASHRAE membership) I expect the latest edition of the Handbook will be issued, since I understand it is refreshed every four years.

    Yeah, if I were designing a dry room, I think I'd be real interested in the weather bin data we've been discussing. For residential/light commercial comfort cooling/heating, Manual J or N should do just fine.
    You use the design dry bulb and coincidental wet bulb to design your temperature control system.

    Then you look at the dehumidification data, it gives a moisture content and a dry bulb temperature. You look and see if the cooling system you designed based on design dry bulbs and wet bulbs will get overwhelmed by humidity during times of peak humidity.

    Note that not only is the dry bulb temperature typically lower during times of peak humidity, that probably the sun is not shining either.This means there is less motivation for the thermostat to turn on the cooling.

    So if you live somewhere humid and think the ACCA numbers are fine for light commercial, then you may end up in here posting threads asking for help dealing with high humidity problems. You will get the standard responses 'like IAQ thermastats" , Inifinity Systems with thermidistats, a thermastor dehumidifer,upgrade to ECM single phase stuff, slow down the blower, auto fan, but the real problem is someone just assumed everything would be hunky dory using the ACCA data in the first place and never checked that they would end up with 70% RH.

    They started publishing the dehumidification data in 1993 for the very reasons I gave already.

    The data is not just for dry rooms or designing a museum archive, it is for making sure that everyday system can at least hold the RH under the bare minimum of 65% under the worst conditions
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  11. #11
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    Quote Originally Posted by Carnak View Post
    You use the design dry bulb and coincidental wet bulb to design your temperature control system.

    Then you look at the dehumidification data, it gives a moisture content and a dry bulb temperature. You look and see if the cooling system you designed based on design dry bulbs and wet bulbs will get overwhelmed by humidity during times of peak humidity.

    Note that not only is the dry bulb temperature typically lower during times of peak humidity, that probably the sun is not shining either.This means there is less motivation for the thermostat to turn on the cooling.

    So if you live somewhere humid and think the ACCA numbers are fine for light commercial, then you may end up in here posting threads asking for help dealing with high humidity problems. You will get the standard responses 'like IAQ thermastats" , Inifinity Systems with thermidistats, a thermastor dehumidifer,upgrade to ECM single phase stuff, slow down the blower, auto fan, but the real problem is someone just assumed everything would be hunky dory using the ACCA data in the first place and never checked that they would end up with 70% RH.

    They started publishing the dehumidification data in 1993 for the very reasons I gave already.

    The data is not just for dry rooms or designing a museum archive, it is for making sure that everyday system can at least hold the RH under the bare minimum of 65% under the worst conditions
    Thanks, Carnak, that's good info.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  12. #12
    Great description Carnak, and I can see your point.

    For the purposes of adding "rest of the world" design data to my ACCA table, is it possible to calculate the ACCA Design grains 45%, 50%, 55% from the data contained in the ASHRAE tables? As the ACCA spreadsheet requires that I enter a figure for design grains in order to perform a calc.

    By using ASHRAE data instead of ACCA data I would hope to arrive at a more accurate calc result, particularly regarding RH.

  13. #13
    Join Date
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    the accca data is most likely the difference between the grains that coincide between the indoor conditon of 75F & 50% RH and ther grains that coincide with the design dry bulb and wet bulb

    do this

    work out the grains if you maintain 45,50, & 55% RH inside


    work out the grains for the outdoor condition-- the dry bulb and wet bulb

    subtract the indoor grians at 45, 50, & 55% form the outdorr grains at the design dry bulb and wet bulb

    use those grains to design, the difference between indoor and outdoor if you are doing a manual j

    the 50% number should be the same as the ACCA number

    then if you want to make sure , re-run your load at the condition ASHRAE says for worst case humidity, it will be a cooler temperature and very humid

    if you know how to work out the SHR that your coil needs, if it is less than 0.7 under that worst case humidity condition , price a kenmore dehumidifier and you are gold
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

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