Thermodynamics

Chapter 14 | 721Analysis (a) The partial pressure of dry air can be determined from Eq. 14–2:whereThus,P a P P vP v fP g fP sat @ 25°C 10.752 13.1698 kPa2 2.38 kPaP a 1100 2.382 kPa 97.62 kPa(b) The specific humidity of air is determined from Eq. 14–8:v 0.622P vP P v(c) The enthalpy of air per unit mass of dry air is determined fromEq. 14–12: 63.8 kJ/kg dry air10.622212.38 kPa21100 2.382 kPa 0.0152 kg H 2O/kg dry airh h a vh v c p T vh g 11.005 kJ>kg # °C2125°C2 10.01522 12546.5 kJ>kg2The enthalpy of water vapor (2546.5 kJ/kg) could also be determined fromthe approximation given by Eq. 14–4:h g @ 25°C 2500.9 1.82 1252 2546.4 kJ>kgwhich is almost identical to the value obtained from Table A–4.(d ) Both the dry air and the water vapor fill the entire room completely.Therefore, the volume of each gas is equal to the volume of the room:V a V v V room 15 m2 15 m213 m2 75 m 3The masses of the dry air and the water vapor are determined from the idealgasrelation applied to each gas separately:m a P aV aR a T 197.62 kPa2 175 m 3 210.287 kPa # m 3 >kg # K21298 K2 85.61 kgm v P vV vR v T 12.38 kPa2175 m 3 210.4615 kPa # m 3 >kg # K21298 K2 1.30 kgThe mass of the water vapor in the air could also be determined fromEq. 14–6:m v vm a 10.01522185.61 kg2 1.30 kg14–3 DEW-POINT TEMPERATUREIf you live in a humid area, you are probably used to waking up most summermornings and finding the grass wet. You know it did not rain the night before.So what happened? Well, the excess moisture in the air simply condensed onthe cool surfaces, forming what we call dew. In summer, a considerableamount of water vaporizes during the day. As the temperature falls during the