Vapor Pressure of Water and Humidity

Humidification involves the transfer of water from the liquid phase into a gaseous mixture of air and water vapor. Dehumidification involves the reverse transfer, whereby water vapor is transferred from the vapor state to the liquid state. Humidification and dehumidification can also refer to vapor mixtures of materials such as benzene, but most practical applications occur with water. To better understand humidity, it is first necessary to discuss the vapor pressure of water. 

Vapor pressure of water and physical states. Pure water can exist in three different physical states solid ice, liquid, and vapor. The physical state in which it exists depends on the pressure and temperature. 

Liquid and vapor coexist in equilibrium along the line AB, which is the vapor-pressure line of water. Boiling occurs when the vapor pressure of the water is equal to the total pressure above the water surface. For example, at 100°C (212°F) the vapor pressure of water is 101.3 kPa (1.0 atm), and hence it will boil at 1 atm pressure. At 65.6°C (150°F), from the steam tables in Appendix A.2, the vapor pressure of water is 25.7 kPa (3.72 psia). Hence, at 25.7 kPa and 65.6°C, water will boil. 

If a pan of water is held at 65.6°C in a room at 101.3 kPa abs pressure, the vapor pressure of water will again be 25.7 kPa. This illustrates an important property of the 

Dejinition of humidity. The humidity H of an air-water vapor mixture is defined as the kg of water vapor contained in 1 kg of dry air. The humidity so defined depends only on the partial pressure of water vapor in the air and on the total pressure P (assumed throughout this chapter to be 101.325 kPa, 1.0 atm abs, or 760 mm Hg). Using the molecular weight of water A) as 18.02 and of air as 28.97, the humdiity H in kg H20/kgdry air or in English units as IbHjO/lb dry air is as follows  

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VAPOR PRESSURE OF WATER AND HUMIDITY 93A Vapor Pressure of Water... 

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