Thermodynamics

elow 50°C. Therefore, the enthalpy of water vapor in air can be taken to beequal to the enthalpy of saturated vapor at the same temperature. That is,(14–3)The enthalpy of water vapor at 0°C is 2500.9 kJ/kg. The average c p value ofwater vapor in the temperature range 10 to 50°C can be taken to be 1.82kJ/kg · °C. Then the enthalpy of water vapor can be determined approximatelyfromor(14–4)(14–5)in the temperature range 10 to 50°C (or 15 to 120°F), with negligibleerror, as shown in Fig. 14–3.14–2 SPECIFIC AND RELATIVE HUMIDITY OF AIRThe amount of water vapor in the air can be specified in various ways.Probably the most logical way is to specify directly the mass of water vaporpresent in a unit mass of dry air. This is called absolute or specific humidity(also called humidity ratio) and is denoted by v:The specific humidity can also be expressed asorh v 1T, low P2 h g 1T2h g 1T2 2500.9 1.82T1kJ>kg2T in °Ch g 1T2 1060.9 0.435T1Btu>lbm2T in °Fv m vm a1kg water vapor>kg dry air2v m vm a P vV>R v TP a V>R a T P v >R vP a >R a 0.622 P vP av 0.622P vP P v1kg water vapor>kg dry air2(14–6)(14–7)(14–8)where P is the total pressure.Consider 1 kg of dry air. By definition, dry air contains no water vapor,and thus its specific humidity is zero. Now let us add some water vapor tothis dry air. The specific humidity will increase. As more vapor or moistureis added, the specific humidity will keep increasing until the air can hold nomore moisture. At this point, the air is said to be saturated with moisture,and it is called saturated air. Any moisture introduced into saturated airwill condense. The amount of water vapor in saturated air at a specifiedtemperature and pressure can be determined from Eq. 14–8 by replacing P vby P g , the saturation pressure of water at that temperature (Fig. 14–4).The amount of moisture in the air has a definite effect on how comfortablewe feel in an environment. However, the comfort level depends moreon the amount of moisture the air holds (m v ) relative to the maximumamount of moisture the air can hold at the same temperature (m g ). The ratioof these two quantities is called the relative humidity f (Fig. 14–5)f m vm g P vV>R v TP g V>R v T P vP g(14–9)–1001020304050Chapter 14 | 719WATER VAPOR2482.12500.92519.22537.42555.62573.52591.3h g ,kJ/kgT,°C Table A-4 Eq. 14-42482.72500.92519.12537.32555.52573.72591.9Difference,kJ/kg–0.60.00.10.10.1–0.2–0.6FIGURE 14–3In the temperature range 10 to 50°C,the h g of water can be determinedfrom Eq. 14–4 with negligible error.AIR25°C,100 kPa(P sat,H 2 O @ 25°C = 3.1698 kPa)P v = 0P v < 3.1698 kPaP v = 3.1698 kPadry airunsaturated airsaturated airFIGURE 14–4For saturated air, the vapor pressure isequal to the saturation pressure ofwater.