Application of excel in psychrometric analysis

Abstract

In undergraduate courses in applied thermodynamics or heating, ventilation and air conditioning, mechanical engineering students are introduced to psychrometric principles. Thermal analysis of systems involving dry air and water vapor mixture requires a good understanding of psychrometric concepts and definitions of such parameters as humidity ratio, relative humidity, dew point temperature, and mixture enthalpy. Formulas defining the psychrometric parameters are usually employed in conjunction with property values from the steam tables and ideal gas tables (or equations) in design and analysis of air conditioning systems, cooling towers, and other processes involving the control of water vapor content in the air. Alternatively, psychrometric charts are employed to reduce the time and ease the effort necessary for such analysis. However, human error in reading values off the charts reduces the accuracy of the analysis. In addition, solving open-ended problems usually involves many steps which may require repeated use of psychrometric charts at each step, making the solution process tedious and time consuming. We have found that Microsoft Excel is a useful tool for teaching students the fundamental psychrometric concepts and its application in solution of problems requiring repeated evaluation of psychrometric parameters or recurring use of psychrometric charts. To reinforce students' understanding of the fundamental concepts we have designed a series of exercises requiring students to use simple equations available for ideal gases in Excel spreadsheets to evaluate such psychrometric parameters as relative humidity, humidity ratio, dew point temperature, and the enthalpy of air-water vapor mixtures. Goal Seek, or Solver functions of Excel are introduced to aid the solution process for problems requiring iterative processes. This paper provides several examples to demonstrate the effectiveness of Excel in teaching and learning the fundamentals of psychrometric principles and its application in solution of problems requiring the recurring evaluation of psychrometric parameters. © American Society for Engineering Education, 2010.