Fundamentals of Psychrometry (part III)

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   Evaporation is the process of expelling moisture from materials into the air.  Moisture converts from a liquid state on a material's surface into a gaseous state as water vapor.  Air movers enhance the evaporation process by introducing airflow at the surface level of materials.  The airflow causes moisture to pass from surfaces into the air as vapor.  Airflow then replaces this highly saturated air with the dryer air produced by dehumidifiers removing water vapor from air.  Increases in temperature also increase evaporation, since warmer air can hold more moisture.


   Dehumidification is the process of removing or extracting excess moisture from the air.  To maintain a balanced drying system, the amount of moisture being removed from the air through dehumidification must equal or exceed the amount being added to the air through evaporation.  Stated another way, the amount of moisture being evaporated into the air should not exceed the amount of moisture being dehumidified from the air.


  High velocity airflow increases the rate of evaporation.  The air immediately adjacent to a wet surface becomes saturated as moisture evaporates from the surface.  High velocity airflow removes this cooler, moist air, replacing it with warmer, drier air.  Evaporation then occurs much more rapidly as moisture migrates from wet surfaces to dry air.  As moisture evaporates, the surface becomes cooler.


   Condensation is the process in which water vapor converts into the denser form of liquid.  The process occurs when warm, humid air comes into contact with cooler surfaces.  As the air reduces to its dew point temperature, water vapor condenses from the air and is deposited as moisture droplets on surfaces.  During the drying process, the temperature inside the structure should not be lowered to the dew point, allowing moisture to condense on window frames, glass, doors and other surfaces. 

Primary and Secondary Damage

   Primary damages in a water loss situation result when materials come into direct contact with water.  Examples of primary water damages to materials are:

* delamination of carpet backings
* swelling and buckling of hardwood floors
* rusting of metal surfaces
* weakening of adhesives and glues

   Secondary damages in a water loss result when materials are exposed indirectly to water.  When moisture evaporates into the air more quickly than moisture is being dehumidified from the air, secondary damages can result as higher humidity slows the rate of drying.  Examples of secondary water damages to materials are:

* absorption of water that has migrated from other areas of the structure
* absorption of water from the air as a result of high humidity
* absorption of water by hygroscopic materials causing swelling
* mold and mildew growth

Comments (1)

Edward & Celia Maddox
The Celtic Connection Realty - Queen Creek, AZ

Thanks for posting. We learn a lot from Active Rain Blogs. Best Regards

Feb 24, 2010 12:40 AM