PSYCHROMETRY

Psychrometrics. Psychrometrics is the study of the measurement of the moisture content of atmospheric air (moist air). Atmospheric air, or moist air is a mixture of many gases and pollutants plus water vapor. The water vapor (moisture) in atmospheric air exists in a superheated state at a very low pressure, usually less than 1 psia. One can also define atmospheric air as a mixture of dry air and water vapor (moisture). In 1949, a standard composition of dry air was defined by the International Joint Committee on Psychrometric Data as shown in Table 2-1.

In HVAC study, psychrometry is commonly taken to mean the study of atmospheric moisture and its effect on buildings and building systems.

Ideal Gas Approximation. Atmospheric air pressure of 14.7 psi obeys the ideal gas law with sufficient accuracy for most engineering applications. Errors in calculating the fundamental psychrometric parameters, such as enthalpy, specific volume, and humidity ratio of saturated air at 14.7 psi are less than 0.7% for a temperature range of 60 F to 120 F when ideal gas relationships are used. Accordingly, we will assume that atmospheric air behaves as ideal gases with constant specific heat. Table 2-2 gives the properties of some ideal gases.