Properties of Air and Other Gases
The thermodynamic and transport properties of gases and vapors are important in fan engineering. This chapter deals with the thermodynamic properties, especially pressure, temperature, humidity, density, and enthalpy Transport properties, such as viscosity, thermal conductivity, and diffusivity, are dealt with in subsequent chapters. The gaseous materials most frequently encountered in fan engineering are air and water vapor; accordingly, most of the data are for these substances. Some formulae have been written specifically for these materials, but most are generalized to accommodate any gas.
Atmospheric air is a mixture of dry air, water vapor, and impurities. Dry air is a mechanical mixture of gases, whose principal constituents are listed in Table 1.1. (The table values may be considered representative of the composition of normal outdoor air throughout the troposphere.) The amount of water vapor in atmospheric air will depend on weather conditions. The nature and amount of impurities in the atmosphere depend on the forces at work in producing and dispersing contaminants. Industrial, urban, rural, seaside, and other areas have characteristic atmospheres due to differences in impurities.
The reference for Table 1.1 lists neon, helium, krypton, hydrogen, xenon, ozone, and radon, totaling less than 0.0025 percent by volume, as the residual part of atmospheric air. also lists methane, nitrous oxide, sulfur dioxide, nitrogen dioxide, ammonia, carbon monoxide, and iodine, totaling 0.0003 percent by volume, as constituents of normal, clean, dry atmospheric air. ASHRAE considers all these gases in the calculation of the apparent molecular weight of clean, dry atmospheric air and obtains a value of 28.9645. Rounding off and lumping the residuals with the nitrogen, as has been done in Table 1.1, yields an apparent molecular weight of 28.964.