Actual Vapor Pressure – the partial pressure
exerted by the water vapor present in a parcel,
measured in millibars. Water in a gaseous state
(ie 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

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

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 temperatures to calculate dew point or relative humidity
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 = 4,187 SCFM x 3 ACH = 12,501 SCFM
This capacity could be provided using three 5,000 CFM dehumidifiers or one
15,000 CFM dehumidifier.

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

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 aq. 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.




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