Application of exergy analysis to various psychrometric processes

Abstract

The relation between work and changes in entropy generation arises from the simultaneous treatment of the first and second laws referred to as exergy (or available energy) analysis. In this paper, we discuss thermodynamic analysis of various psychrometric processes using the concept of exergy. A parametric study of each of the processes is carried out to determine the variation of second-law efficiency as a function of mass flow rate, relative humidity and temperature. Other trends such as variation of temperature with relative humidity are also shown where applicable. Irreversible losses are calculated by applying an exergy balance on each system. In this regard, an engineering equation solver (EES) programme is used, which is unique because it has built-in functions for most thermodynamic and transport properties; removing the need for approximate equations. The concept of total exergy as the sum of thermomechanical and chemical parts is employed in calculating the flow exergies for air and water vapor mixtures. It is shown for some processes investigated that an increase in the relative humidity of the incoming air stream increases second-law efficiency. We notice that a decrease in mass flow rate of fresh air (second incoming stream) in the case of adiabatic mixing decreases the second-law efficiency of the process. Also, it is shown that the mass flow rate (of both water and steam) has almost a linear relationship with relative humidity in the range investigated. Copyright © 2003 John Wiley & Sons, Ltd.