Thermodynamics

740 | ThermodynamicsThe enthalpy of water vapor in the air can be taken to be equalto the enthalpy of the saturated vapor at the same temperature:h v (T, low P) h g (T) 2500.9 1.82T (kJ/kg) T in °C 1060.9 0.435T (Btu/lbm) T in °Fin the temperature range 10 to 50°C (15 to 120°F).The mass of water vapor present per unit mass of dry air iscalled the specific or absolute humidity v,v m vm a 0.622P vP P v1kg H 2 O>kg dry air2where P is the total pressure of air and P v is the vapor pressure.There is a limit on the amount of vapor the air can holdat a given temperature. Air that is holding as much moistureas it can at a given temperature is called saturated air. Theratio of the amount of moisture air holds (m v ) to the maximumamount of moisture air can hold at the same temperature(m g ) is called the relative humidity f,where P g P sat @ T . The relative and specific humidities canalso be expressed asf f m vm g P vP gvP10.622 v2P gandv 0.622fP gP fP gRelative humidity ranges from 0 for dry air to 1 for saturatedair.The enthalpy of atmospheric air is expressed per unit massof dry air, instead of per unit mass of the air–water-vapormixture, ash h a vh g 1kJ>kg dry air2The ordinary temperature of atmospheric air is referred toas the dry-bulb temperature to differentiate it from other formsof temperatures. The temperature at which condensation beginsif the air is cooled at constant pressure is called the dew-pointtemperature T dp :T dp T sat @ PvRelative humidity and specific humidity of air can be determinedby measuring the adiabatic saturation temperature ofair, which is the temperature air attains after flowing overwater in a long adiabatic channel until it is saturated,v 1 c p 1T 2 T 1 2 v 2 h fg2h g1 h f2whereand T 2 is the adiabatic saturation temperature. A more practicalapproach in air-conditioning applications is to use a thermometerwhose bulb is covered with a cotton wick saturatedwith water and to blow air over the wick. The temperaturemeasured in this manner is called the wet-bulb temperatureT wb , and it is used in place of the adiabatic saturation temperature.The properties of atmospheric air at a specified totalpressure are presented in the form of easily readable charts,called psychrometric charts. The lines of constant enthalpyand the lines of constant wet-bulb temperature are verynearly parallel on these charts.The needs of the human body and the conditions of theenvironment are not quite compatible. Therefore, it oftenbecomes necessary to change the conditions of a living spaceto make it more comfortable. Maintaining a living space oran industrial facility at the desired temperature and humiditymay require simple heating (raising the temperature), simplecooling (lowering the temperature), humidifying (addingmoisture), or dehumidifying (removing moisture). Sometimestwo or more of these processes are needed to bring the air tothe desired temperature and humidity level.Most air-conditioning processes can be modeled as steadyflowprocesses, and therefore they can be analyzed by applyingthe steady-flow mass (for both dry air and water) andenergy balances,Dry air mass:Water mass:Energy:v 2 0.622P g 2P 2 P g2ainm # a aoutm # aainm # w aoutm # w or ainm # av aoutm # avQ # in W # in ainm # h Q # out W # out aoutm # hThe changes in kinetic and potential energies are assumed tobe negligible.During a simple heating or cooling process, the specifichumidity remains constant, but the temperature and the relativehumidity change. Sometimes air is humidified after it isheated, and some cooling processes include dehumidification.In dry climates, air can be cooled via evaporative cooling bypassing it through a section where it is sprayed with water. Inlocations with limited cooling water supply, large amounts ofwaste heat can be rejected to the atmosphere with minimumwater loss through the use of cooling towers.