glossary

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

Due point - the temperature air must be cooled to in order for saturation to occure, producing water in the form of dew or condensation. The dew point temperature assumes there is no change in the air pressure or moisture content of the air.

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.

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 expresed as a percentage and can be computed in a variety of ways. One way is to divide the actual vapor pressure by the saturation vapor pressure and then multiply by 100 to convert to a percent.

Satuation of air - the condition under which the amount of water vapor the air is the maximum possible at the existing temperature and pressure.

Condensation or sublimation wil begin if the temperature falls or water vapor is added to the air..

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

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: Prodivd temporay dehumidification for a commercial building with a rooftop 20 ton air conditioner

Dehumidification capacity = 400SCFM x 20 tons divided 2 = 4000 SCFM

Temporary dehumidification inside tanks being prepared for painting or industrial coating eliminates the potential for condensation. Two sizing methods are shown.

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 dehimidification for a 100' diameter tank 30' high. The tank has (3) 3' diameter manholes and (8) 1' diameter vents. Two sizing methods are given below to calculate the dehumidification capacity required;

Sizing Method # 1(Volume)

1, Calculate the volume of the tank in cu. ft.(3.14xRadius (2sq.) x H)

2. Calculate the SCFM for one air change(volume divided by 60)

3. Calculate the dehumidification capacity required (SCFMxACH)

Dehmidification capacity=235,500 cu.ft. divided by 60 = 3,925 SCFM @ 1 ACH This computes to 7,850 SCFM @ 2 ACH or15,7000 SCFM @ 4 ACH

Sizing method #2 (Leakage)

1. Calculate the leakage in sq.ft. from manholes and vents.

2. Calculate the dehumidification capacity required (Leakagex250FPM)

Leakagae=21.13 sq.ft. (manholes) +9,42sq.ft. (vents)=30.60 sq.ft.total Dehumidification capacity = 30.60 sq.ft.x250FPM = 7,650 SCFM

Saturation vapor pressure - the maximum partial pressure that water vapor molecules would exert if the air were saturated with vapor at a given tempeature. 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 con be used along with the dry blub temperature to calculate dew point or relative humidity.

Please Note: The above caluclations are for estimating urposes only. The actual dehumidification requirements for specific applications may vary.