Question

write down the options of how to find the value of an enthalpy difference for air.Which one is most accurate??

Answer 1

If mass and temperature 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.

Calculate ha. 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.
Examine H. Humidity ratio is the ratio of the mass of water vapor in the moist air and the mass of dry air. In layman's terms, it gives the wetness of air. Along with the temperature of air, this must be given in the problem.

Get hg. 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.

Other method

Enthalpy= mcdT , m-mass , c-specific heat , dT - Temp difference.