for example, cold outdoor air with given temperature and relative humidity is heated and humidified with steam. whether to take dead state conditions for both fluids separately?
You need to take the dead state for each pure substance separately, and then calculate the exergy of the mixture in the way outlined for example by Cengel 'Thermodynamics' 7e, example 13-4. When ideal components at the same temperature and pressure are mixed, the temperature and pressure of the mixture is the same as the starting values, but the partial pressures of each component has reduced. This leads to a value of destroyed exergy.
Here is a publication that I found that contains psychrometric charts showing exergy, which might give some more pointers:
M. Salazar-Pereyra, M. Toledo-Velázquez, G. Eslava, R. Lugo-Leyte and C. Rosas, "Energy and Exergy Analysis of Moist Air for Application in Power Plants," Energy and Power Engineering, Vol. 3 No. 3, 2011, pp. 376-381. doi: 10.4236/epe.2011.33048.
I am agree with Dr.Ravindran
If the substance exist in atmosphere 1 bar and 25 centigrade are the dead state. For other substances a chemical reaction to substances that form components of air has to be considered in order to calculate exergy value of a flow or substance.
If the thermal system directly interacts with atmospheric environment, in an exergy-based analysis, you can suppose the atmospheric condition is the dead condition. If the system, however, dose not directly interact with atmospheric environment, an exergy-based analysis is not applicable anymore because environment-related dead states are required. It's unreasonable that the thermodynamic performance of a thermal system that does not interact with the atmospheric environment depends on the atmospheric environment. Entropy-based analysis should be more reasonable, as illustrated by Alefeld.
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