Above the surface of the earth is a layer of air called the atmosphere, or atmospheric air. The lower atmosphere, or homosphere, is composed of moist air, that is, a mixture of dry air and water vapor.

Psychrometrics is the science of studying the thermodynamic properties of moist air. It is widely used to illustrate and analyze the change in properties and the thermal characteristics of the air-conditioning process and cycles.

The composition of dry air varies slightly at different geographic locations and from time to time. The approximate composition of dry air by volume is nitrogen, 79.08%; oxygen, 20.95%; argon, 0.93%; carbon dioxide, 0.03%; other gases (e.g., neon, sulfur dioxide), 0.01%.

The amount of water vapor contained in the moist air within the temperature range 0 to 100°F changes from 0.05 to 3% by mass. The variation of water vapor has a critical influence on the characteristics of moist air.

The equation of state for an ideal gas that describes the relationship between its thermodynamic properties is given as:

pv = RTR
pV = mRTR

where p = pressure of the gas, psf (1 psf = 144 psi)
v = specific volume of the gas, ft3/lb
R = gas constant, ftlbf/lbm, °R
TR = absolute temperature of the gas, °R
V = volume of the gas, ft3
m = mass of the gas, lb

The most exact calculation of the thermodynamic properties of moist air is based on the formulations recommended by Hyland and Wexler (1983) of the U.S. National Bureau of Standards. The psychrometric charts and tables developed by ASHRAE are calculated and plotted from these formulations.

According to Nelson et al. (1986), at a temperature between 0 and 100 °F, enthalpy and specific volume calculations using ideal gas Equations show a maximum deviation of 0.5% from the results of Hyland and Wexler’s exact formulations.

Therefore, ideal gas equations are used in the development and calculation of psychrometric formulations. Although air contaminants may seriously affect human health, they have little effect on the thermodynamic properties of moist air. For thermal analysis, moist air may be treated as a binary mixture of dry air and water vapor.

Applying Dalton’s law to moist air:

pat=  pa + pw

Dalton’s law is summarized from the experimental results and is more accurate at low gas pressure. Dalton’s law can also be extended, as the Gibbs-Dalton law, to describe the relationship of internal energy, enthalpy, and entropy of the gaseous constituents in a mixture.

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