cen84959_ch14 - cen84959_ch14.qxd 6:05 AM Page 737 Chapter...

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Chapter 14 GAS–VAPOR MIXTURES AND AIR-CONDITIONING | 737 A t temperatures below the critical temperature, the gas phase of a substance is frequently referred to as a vapor. The term vapor implies a gaseous state that is close to the saturation region of the substance, raising the possibility of condensation during a process. In Chap. 13, we discussed mixtures of gases that are usu- ally above their critical temperatures. Therefore, we were not concerned about any of the gases condensing during a process. Not having to deal with two phases greatly simplified the analysis. When we are dealing with a gas–vapor mixture, however, the vapor may condense out of the mixture during a process, forming a two-phase mixture. This may complicate the analysis considerably. Therefore, a gas–vapor mixture needs to be treated differently from an ordinary gas mixture. Several gas–vapor mixtures are encountered in engineer- ing. In this chapter, we consider the air–water-vapor mixture, which is the most commonly encountered gas–vapor mixture in practice. We also discuss air-conditioning, which is the pri- mary application area of air–water-vapor mixtures. Objectives The objectives of Chapter 14 are to: Differentiate between dry air and atmospheric air . Define and calculate the specific and relative humidity of atmospheric air. Calculate the dew-point temperature of atmospheric air. Relate the adiabatic saturation temperature and wet-bulb temperatures of atmospheric air. Use the psychrometric chart as a tool to determine the properties of atmospheric air. Apply the principles of the conservation of mass and energy to various air-conditioning processes.
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14–1 DRY AND ATMOSPHERIC AIR Air is a mixture of nitrogen, oxygen, and small amounts of some other gases. Air in the atmosphere normally contains some water vapor (or mois- ture ) and is referred to as atmospheric air . By contrast, air that contains no water vapor is called dry air . It is often convenient to treat air as a mixture of water vapor and dry air since the composition of dry air remains rela- tively constant, but the amount of water vapor changes as a result of con- densation and evaporation from oceans, lakes, rivers, showers, and even the human body. Although the amount of water vapor in the air is small, it plays a major role in human comfort. Therefore, it is an important consideration in air-conditioning applications. The temperature of air in air-conditioning applications ranges from about 10 to about 50°C. In this range, dry air can be treated as an ideal gas with a constant c p value of 1.005 kJ/kg · K [0.240 Btu/lbm · R] with negligible error (under 0.2 percent), as illustrated in Fig. 14–1. Taking 0°C as the ref- erence temperature, the enthalpy and enthalpy change of dry air can be determined from (14–1 a ) and (14–1 b ) where T is the air temperature in °C and T is the change in temperature. In air-conditioning processes we are concerned with the changes in enthalpy h , which is independent of the reference point selected.
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  • Spring '09
  • Y
  • Atmospheric thermodynamics, Water vapor

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