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Thread: Moisture content of cool air in room diffuser

  1. #1
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    Moisture content of cool air in room diffuser

    Is it true that the moisture content of the air when it enters the conditioned room via supply air diffuser would be around 90%. I have designed the room at 50% RH but am bit confused after one of my college insiste that when cool air leaves the cooling coil of the AHU, it has 100% moisture content and by the time it reaches the room after travelling through the length of the duct, moisture content would remain around 90%. Something hard for me to agree. Please share your experience. Thanks

  2. #2
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    Do you question the change from 100% to 90% or are you questioning how the room can be at 50% RH?
    The 100% to 90% is simply the fact thaat the air heats up a little in traveling through the duct and at higher temperatures the air can hold more moisture before it saturates.

  3. #3
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    @zeke,
    I am just enquiring whether in practical does it happen that air at diffuser is at 90% RH. I personally think that it is much lower than what my colleague is insisting at. I have my room designed at 50% RH. The air coming from AHU at 100% RH is supposed to lose a lot of moisture content on the way while travelling the whole length of the duct unti
    L it reaches the diffuser.hadbit been 90% RH at diffuser, most of the diffuser would be sweating with moist in the room.

  4. #4
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    That's not the way it works.
    The 50% humid air goes over the cooling coils where the moisture condenses on the coils and drops to the intake of the condensate pump
    where it is pumped out. The cold air leaving the AHU is almost saturated ( RH=100%) passes through the duct with SAME amount of moisture
    which doesn't condense out since the air heats up a little and warmer air holds more moisture. So, if your colleague says that it dould be 90% RH, I would agree, depending on the slight heatup in its course through the duct.
    Now the cold air out of the diffuser mixes with the room air + the infiltration air plus internal cooling load to sustain your 50% Rh .design requirement.

  5. #5
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    Thanks a lot zeke, much clear now.....have a little extension to the same subject.....I have also been designing stream humidifiers / humidistats in the discharge side of the supply air ducts of AHU which served in an area with very low humidity levels. And it was done to ensure that the cool air reaches the room at the desired 50% RH, not that a higher humidity level was required. Don't you think based upon your explanation, I could have achieved the desired humidity levels inside the room even without the use of humidifiers.

  6. #6
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    Warmer air holds more moisture ??? Is it exactly so, my understanding is that when cooler air mixes with room air which is at relatively higher temperature, RH comes down. Correct me if I go wrong.

  7. #7
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    "


    Thanks a lot zeke, much clear now.....have a little extension to the same subject.....I have also been designing stream humidifiers / humidistats in the discharge side of the supply air ducts of AHU which served in an area with very low humidity levels. And it was done to ensure that the cool air reaches the room at the desired 50% RH, not that a higher humidity level was required. Don't you think based upon your explanation, I could have achieved the desired humidity levels inside the room even without the use of humidifiers. "




    No, unless your cooling load has a moisture component.You see, if the Humidity is low, you need more moisture to get to 50%. The AHU can only remove moisture .

    If you need 50% RH in a room and have to cool the room, then for low humidity climate , you must add moisture to the cooled air ( if it is not saturated)or add moisture to the room directly .
    Last edited by zeke; 12-03-2012 at 10:54 AM.

  8. #8
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    Quote Originally Posted by Hasny View Post
    Warmer air holds more moisture ??? Is it exactly so, my understanding is that when cooler air mixes with room air which is at relatively higher temperature, RH comes down. Correct me if I go wrong.
    Usually true.

    Exception when the room air is at very low RH and is holding the same amount of moisture of the cool air coming out of the AHU.

    Look at it this way, for high humidity rooms, the air goes thru the AHU which cools it, condensing out moisture and and after mixing with room air drops room humidity.But as the room RH gets lower as in dry climates, the amount of moisture in the air could be less than the amount of moisture of the saturated cool air at which point you obviously get no reduction of RH.

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