You have to memorize the value of cpa which is 1. 006 kJ/kgC since this is constant. T must be given in the problem, or it can simply be measured with a thermometer or other temperature measuring devices.
Get ha. Use the equation given above and the given T in the problem. Get hg. Use the given T and your steam table. Get h. Use your calculated values of ha and hg, and the given value of H. If your answer is 76. 09 kJ/kg or something very close to that, you got the right answer!
You must memorize the value 2501 kJ/kg, which is the specific enthalpy of water vapor at 0°C. Therefore, to get the specific enthalpy at any temperature, you must add the term cpw*T; where cpw = 1. 84 kJ/kg°C, which is the specific heat of water vapor at constant pressure; and T is the temperature of air.
