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  1. #1
    Join Date
    Oct 2008
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    Trying to calculate latent load from pool evaporation. help?

    I've got a therapy pool at 15x12ft. =180sqft
    They want to maintain the pool at 90F and the air temp at 85.F.

    I'm trying to determine the latent load my unit will see just from the pool evaporation portion of the space load.

    I think its Delta grains x4.5 x CFM = lb/hr then I need to convert that to BTU/hr.

    I think I'm missing how to get the grains and how to get to BTUs.
    I've done this before but its been a while.

    Any help appreciated.

  2. #2
    Join Date
    Jun 2003
    Madison, WI/Cape Coral, FL
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    Here is a worksheet that helps figure the moisture load from a pool at the different conditions you find with applications.
    This save a lot of time to dicuss the possibilities.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  3. #3
    Join Date
    Oct 2008
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    Thread Starter
    In case anyone stumbles across this thread looking for the same thing, here is what I found.

    The calculation is Latent load (kbtu/hr)= 0.1 * surface area (sqft) * differential vapor pressure * Activity factor.

    0.1 is a constant.
    Differential pressure comes from Steam tables of saturated air. In my case I have 90 degree water and 85 degree air. The tables show that 85 degree air at 50% RH has a vapor pressure of about 0.68.
    The vapor pressure of 90 degree air at saturation is about 1.42
    1.42-0.72=0.7 in this case.
    I realize I used the 90degree saturated air vapor pressure. This is because right at the surface of the water, the particules of water are trying to go into the atmosphere and the pressure of the air is resisting that. Nonetheless, those air particles right at the surface are fully saturated. Thats why differential pressure is key here.

    Activity factor is a value based on the type of water body it is. A therapy pool or hot tub has an activity factor of 1.0.
    A swimming pool has an activity factor of 0.5
    A wave pool has a 1.5 factor.
    This pool is 15ftx12ft

    In my case this resulted in the following 0.1*.7*180*1= 12.6 MBH =12,600 btus of latent heat.

    I also think that when a latent btu is created, a sensible btu is absorbed so I think that the overall btu load remains constant but I wasn't sure how to factor that in. I left my space load as was and just added another 13,000 btus of latent for the system to have to handle.

  4. #4
    Join Date
    Jun 2004
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    Last edited by jpsmith1cm; 07-13-2013 at 07:13 PM.

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