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  1. #1
    Join Date
    Jul 2014
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    Confused Humidity Control in Tropics

    Got new assignment working in the tropics. The world where the design
    condition (98%) is 88F DB & 80F WB (70.5% rH, 77.2F Dew Point).

    The AHU Multizone gear is old that has uses tried & true Honeywell Traneline
    standalone controls (simple but dumb). The owners wants to upgrade
    to modern AHUs with VFDs and VAVs at each zone. And they want the best
    energy saving controls. I tell em' that humidity is critical and pressure control.

    Looking in the control books I see humidity control broken down to two methods:
    1) Indirect RH control - using dewpoint temp sensor by the coil. Old school way?
    2) Direct RH control RH detector located in space zone.

    Then there's the Face & Bypass damper control system. But isn't that from the old
    CV days?

    Someone mentioned to build the new AHUs with Heat Pipe technology to cut the OA humidity
    and save energy. The extra fan energy is offset by being allow to raise the room setpoint
    (due to lower humidity).

    Anyone got any ideas or experience on dealing with humidity design & controls in high
    humid locations. What about using heat pipes?

    Also, don't want to use reheat (energy waster)


  2. #2
    Join Date
    Jun 2003
    Madison, WI/Cape Coral, FL
    Post Likes
    The most basic step often missed is setting up the a/c to provide the desire %RH/dew point in the conditioned space. Ideal 75^F, 50%RH, 55^F dew need a 45^F coil to reduce the cold supply dew point to <50^F. During evenings, rainy days, and low/no cooling loads but high outdoor dew point, supplemental dehumidifier reduces indoor humidity while supplying free reheat. This eliminates overcooling for humidity control which reduces condensation problems on ducts and in cool walls/insulation.
    The next big energy saver is to not cool space when unoccupied for +4 hours, but dehumidify the space with high efficiency dehumidifiers to maintain minimal humidity levels. Raising indoor temps reduces the amount of dehumidification needed to maintain 50%RH in the space by factors of X4. The high efficiency dehumidifiers remove 2-3X more moisture per kwh verses using the a/c to remove moisture when unoccupied.
    A/c of style is not able to maintain low %RH during low/no cooling loads and high outdoor dew points. This includes heat pipe a/cs. Heat pipes do increase the latent cooling providing there a plus moderate cooling load. Check out the new generation high efficiency dehumidifiers from Ultra-Aire/Quest/High E-Dry/Santa Fe
    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"

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