How to Calculate the Enthalpy of Moist Air

Step-by-Step Guide

1. 1

   Step 1: Study the equation for calculating the enthalpy.

   Specific enthalpy (enthalpy per kg of dry air) of moist air is the sum of the specific enthalpy of dry air and the specific enthalpy of  the water vapor in the air.
   
   This is given by the equation: h = ha + H*hg; where ha is the specific enthalpy of dry air, H is the humidity ratio, and hg is the specific enthalpy of water vapor. , This is given by the equation cpa*T; where cpa is the specific heat of air, and T is the temperature of air.
   
   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. , In layman's terms, it gives the wetness of air.
   
   Along with the temperature of air, this must be given in the problem., Using a steam table, this can easily be found.
   
   Once you have the temperature, bring out your saturated steam table and look for the value of hg.
   
   This value has the units kJ/kg., Find the specific enthalpy of moist air at 25°C with
   0.02 kg/kg moisture.
   
   These are the T and H, respectively.
   
   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!
2. 2

   Step 2: Calculate ha.

3. 3

   Step 3: Examine H. Humidity ratio is the ratio of the mass of water vapor in the moist air and the mass of dry air.

4. 4

   Step 4: Get hg.

5. 5

   Step 5: Practice.

Detailed Guide

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