June 20, 2010
June 20, 2010
June 23, 2010
15.183.1 - 15.183.20
Application of Excel in Psychrometric Analysis
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.
All mechanical engineering degree programs require either a single course consisting of 3-4 semester credit hours (SCH) or a two-course sequence in thermodynamics, each consisting of 3 SCH. The fundamental concepts, including the evaluation of properties using tables or formulas are covered in the early stages of a single required course or in the first course for those programs requiring a two-semester course sequence. At the latter stages of the course coverage, students are introduced to thermodynamics cycles, including the air-standard power and refrigeration cycles. The coverage also includes an introduction to chemical reactions, mixtures of ideal gases, including psychrometric analysis of moist air. The moist air properties are usually obtained from using steam tables in combination with the properties of air.
Many modern thermodynamic textbooks provide a software packages to aid students with property evaluation1-3. Some of these software packages such as Interactive Thermodynamics (IT) or Engineering Equation Solver (EES) have programming capabilities4, 5. These programs are general purpose, non-linear equation solvers with built-in property functions. They are
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