Abstract
Traditional approaches to the thermodynamic analysis of processes involving air-water vapor mixtures are often limited to a psychrometric analysis and a first-law analysis. The substances involved are moist air (a mixture of dry air and moisture), and water (either as vapor, or as a liquid). A complete thermodynamic analysis must, however, include a second-law analysis.
This article presents a complete outline for the thermodynamic analysis and evaluation of processes involving air-water vapor mixtures. With regards to exergy analysis, in particular, it is noted that the published and widely accepted equation for the exergy of moist air is indeterminate when the humidity ratio of the ambient air is zero. This raises questions about the appropriate reference environment for water or water vapor when the atmospheric air does not contain moisture. The alternative of an expanded reference environment, which includes a large pool of water co-existing with ambient air is proposed in this article. The resulting equations for the exergy of moist air are determinate regardless of the humidity ratio of the atmospheric air in the reference environment.
Composite desiccant material “CaCl2/Vermiculite/Saw wood”: a new material for fresh water production from atmospheric air