Psychrometrics Thermodynamics of Moist Air Mixtures

Methods for calculating for nonreacting ideal gas mixtures, reacting ideal gas mixtures, liquids, and solids are based on determination of entropy and enthalpy changes in previous sections. Examples of the calculation are given in Section 3.6, where the Gibbs function is used to determine the expected open-circuit voltage (OCV) of a fuel cell. 

Psychrometrics is the study of nonreacting moist air mixtures and is critical to understand the water balance in low-temperature PEFCs. Nonreacting moist mixtures can be evaluated exactly like other nonieacting gas mixtures, but since engineering with moist air mixtures is so common, additional parameters and special charts have been developed to aid in calculation and analysis, hi most fuel cells, water is produced as a product and must be removed from the fuel cell as part of the effluent mixture. In low-temperature PEFCs, the water balance is critical to maintain proper electrolyte conductivity while avoiding electrode flooding. 

To begin, we will hrst assume a two-gas mixture of water vapor and another ideal gas mixture, air. In this case, the total atmospheric pressure is the sum of the air and vapor 

It is important to realize there can be many other moist gas-phase mixtures besides air. Consider a humidified anode fuel mixture of CO2, CO, H2, and H2O vapor. Then, 

The humidity ratio is defined as the mass of moisture per mass of dry mixture and can also be defined as the rate of moisture mass flow per dry mass flow  

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