Thermodynamics CHANGE OF PHASEMoistureContentThe moisture content of a substance is the opposite of its quality. Moisture (M) is defined asthe ratio of the mass of the liquid to the total mass of both liquid and vapor. The moisture ofthe mixture in the previous paragraph would be 0.8 or 80%. The following equations show howto calculate the moisture of a mixture and the relationship between quality and moisture.(1-21)Mmliquid(mliquidmvapor)M = 1 - xSaturatedandSuperheatedVaporsIf a substance exists entirely as vapor at saturation temperature, it is called saturated vapor.Sometimes the term dry saturated vapor is used to emphasize that the quality is 100%. Whenthe vapor is at a temperature greater than the saturation temperature, it is said to exist assuperheated vapor. The pressure and temperature of superheated vapor are independentproperties, since the temperature may increase while the pressure remains constant. Actually, thesubstances we call gases are highly superheated vapors.ConstantPressureHeatAdditionConsider the plot on theFigure 7 T-V Diagramtemperature-volume diagram ofFigure 7, viewing theconstant-pressure line thatrepresents the states throughwhich the water of theprevious discussion passes as itis heated from the initial stateof 14.7 psia and 60°F. Letstate A represent the initialstate and state B represent thestart of the saturated liquid line(212°F). Therefore, line ABrepresents the process in whichthe liquid is heated from theinitial temperature to thesaturation temperature.Rev. 0 Page 35 HT-01
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