Thermodynamics

Chapter 14GAS–VAPOR MIXTURES AND AIR-CONDITIONINGAt temperatures below the critical temperature, the gasphase of a substance is frequently referred to as avapor. The term vapor implies a gaseous state that isclose to the saturation region of the substance, raising thepossibility of condensation during a process.In Chap. 13, we discussed mixtures of gases that are usuallyabove their critical temperatures. Therefore, we were notconcerned about any of the gases condensing during aprocess. Not having to deal with two phases greatly simplifiedthe analysis. When we are dealing with a gas–vapor mixture,however, the vapor may condense out of the mixture during aprocess, forming a two-phase mixture. This may complicatethe analysis considerably. Therefore, a gas–vapor mixtureneeds to be treated differently from an ordinary gas mixture.Several gas–vapor mixtures are encountered in engineering.In this chapter, we consider the air–water-vapor mixture,which is the most commonly encountered gas–vapor mixturein practice. We also discuss air-conditioning, which is the primaryapplication area of air–water-vapor mixtures.ObjectivesThe objectives of Chapter 14 are to:• Differentiate between dry air and atmospheric air.• Define and calculate the specific and relative humidity ofatmospheric air.• Calculate the dew-point temperature of atmospheric air.• Relate the adiabatic saturation temperature and wet-bulbtemperatures of atmospheric air.• Use the psychrometric chart as a tool to determine theproperties of atmospheric air.• Apply the principles of the conservation of mass and energyto various air-conditioning processes.| 717