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  • Dehumidification Basics and Glossary of Terms


    Actual Vapor Pressure—the partial pressure exerted by the water vapor present in a parcel, measured in millibars. Water in a gaseous state (i.e. water vapor) exerts a pressure just like the atmospheric air.

    Dehumidification—the process of removing moisture from the air or other materials.

    Desiccant—a substance such as calcium oxide or silica gel that is used as a drying agent.

    Dew Point—the temperature air must be cooled to in order for saturation to occur, producing water in the form of dew or condensation.

    Dry Bulb Temperature—the actual air temp.

    Evaporation—the change of liquid water into water vapor. Moisture evaporates due to differential vapor pressure—the larger the vapor pressure differential, the faster the drying.

    Humidistat—an instrument that indicates or controls the relative humidity of the air.

    Hygrometer—an instrument that measures atmospheric humidity.

    Relative Humidity—the ratio of the amount of water vapor in the air at a specific temperature to the maximum capacity of the air at that temperature. Relative humidity is expressed as a percentage: divide the actual vapor pressure by the saturation vapor pressure and then convert to a percent.

    Saturation of Air—the condition under which the amount of water vapor in the air is the maximum possible at the existing temperature and pressure. Condensation or sublimation will begin if the temperature falls or water vapor is added to the air.

    Saturation Vapor Pressure—the maximum partial pressure that water vapor molecules would exert if the air were saturated with vapor at a given temperature. Saturation vapor pressure is directly proportional to the temperature.

    Wet Bulb Temperature—the lowest temperature that can be obtained by evaporating water into the air at constant pressure. Wet bulb temperatures can be used along with the dry bulb temperature to calculate dew point or relative humidity.

    Calculating Requirements


    Normally two to five air changes per hour (ACH) are recommended for water damage restoration applications.

    1. Calculate the volume of the structure in cubic feet (L x W x H).
    2. Calculate the SCFM for one air change (volume ÷ 60).
    3. Calculate the dehumidification capacity required (SCFM x ACH).

    Example: A multi-story building measuring 100'L x 50'W x 50'H has received water damage due to flooding. Provide dehumidification capacity for three air changes per hour (ACH).

    1. Volume = 100 x 50 x 50 = 250,000 cu. ft.
    2. SCFM = 250,000 ÷ 60 = 4,167 SCFM
    3. Dehumidification capacity = 4,167 SCFM x 3 ACH = 12,501 SCFM

    This capacity could be provided using three 5,000-CFM dehumidifiers or one 15,000-CFM dehumidifier.

    Industrial Condensation Prevention

    Use desiccant dehumidifiers in combination with air conditioning units. Size dehumidifier flow for approximately one-half the air conditioner flow. Use 400 SCFM per ton of A/C capacity. If large amounts of outside air (above 10% of total flow) are required, use an air conditioner to pre-cool the dehumidifier inlet.

    Example: Provide temporary dehumidification for a commercial building with a rooftop 20-ton air conditioner.

    Dehumidification capacity = 400 SCFM x 20 tons ÷ 2 = 4,000 SCFM

    Industrial Preparation and Surface Coatings

    Temporary dehumidification inside tanks being prepared for painting or industrial coating eliminates the potential for condensation.

    Procedure: Purge the tank with 100% dehumidified air. Return air is not used due to the heavy dirt loading, usually 2 to 4 ACH is sufficient.

    Example: Provide temporary dehumidification for a 100' diameter tank 30' high. The tank has (3) 3' diameter manholes and (8) 1' diameter vents.

    Sizing Method #1 (Volume)

    1. Calculate the volume of the tank in cu. ft. (3.14 x Radius2 x H).
    2. Calculate the SCFM for one air change (volume ÷ 60).
    3. Calculate the dehumidification capacity required (SCFM x ACH).

    Dehumidification capacity = 235,500 cu. ft. ÷ 60 = 3,925 SCFM @1 ACH

    This computes to 7,850 SCFM @ 2 ACH or 15,700 SCFM @ 4 ACH.

    Sizing Method #2: (Leakage)

    1. Calculate the leakage in square feet from manholes and vents.
    2. Calculate the dehumidification capacity required (Leakage x 250 fpm).

    Leakage = 21.13 sq. ft. (manholes) + 9.42 sq. ft. (Vents) = 30.60 sq. ft. total

    Dehumidification capacity= 30.60 sq. ft. x 250 fpm = 7,650 SCFM

    Note: The above calculations are for estimating purposes only. The actual dehumidification requirements for specific applications may vary.
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