Thermodynamics

on this principle is called a sling psychrometer and is shown in Fig. 14–13.Usually a dry-bulb thermometer is also mounted on the frame of this deviceso that both the wet- and dry-bulb temperatures can be read simultaneously.Advances in electronics made it possible to measure humidity directly in afast and reliable way. It appears that sling psychrometers and wet-wicked thermometersare about to become things of the past. Today, hand-held electronichumidity measurement devices based on the capacitance change in a thin polymerfilm as it absorbs water vapor are capable of sensing and digitally displayingthe relative humidity within 1 percent accuracy in a matter of seconds.In general, the adiabatic saturation temperature and the wet-bulb temperatureare not the same. However, for air–water vapor mixtures at atmosphericpressure, the wet-bulb temperature happens to be approximately equal to theadiabatic saturation temperature. Therefore, the wet-bulb temperature T wbcan be used in Eq. 14–14 in place of T 2 to determine the specific humidityof air.Wet-bulbthermometerChapter 14 | 725Dry-bulbthermometerEXAMPLE 14–3The Specific and Relative Humidity of AirThe dry- and the wet-bulb temperatures of atmospheric air at 1 atm (101.325kPa) pressure are measured with a sling psychrometer and determined to be25 and 15°C, respectively. Determine (a) the specific humidity, (b) the relativehumidity, and (c) the enthalpy of the air.Solution Dry- and wet-bulb temperatures are given. The specific humidity,relative humidity, and enthalpy are to be determined.Properties The saturation pressure of water is 1.7057 kPa at 15°C, and3.1698 kPa at 25°C (Table A–4). The constant-pressure specific heat of airat room temperature is c p 1.005 kJ/kg · K (Table A–2a).Analysis (a) The specific humidity v 1 is determined from Eq. 14–14,where T 2 is the wet-bulb temperature and v 2 isThus,(b) The relative humidity f 1 is determined from Eq. 14–11a to bef 1 0.00653 kg H 2 O/kg dry airv 1 P 210.622 v 1 2P g1v 1 c p 1T 2 T 1 2 v 2 h fg2h g1 h f2v 2 0.622P g 2P 2 P g210.6222 11.7057 kPa21101.325 1.70572 kPa 0.01065 kg H 2 O>kg dry airv 1 11.005 kJ>kg # °C23115 252°C4 10.010652 12465.4 kJ>kg212546.5 62.9822 kJ>kg10.006532 1101.325 kPa2 0.332 or 33.2%10.622 0.006532 13.1698 kPa2FIGURE 14–13Sling psychrometer.Wet-bulbthermometerwick