R&AC Lecture 27 - 1 Lesson 27 Psychrometry Version...

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1 Lesson 27 Psychrometry
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2 The specific objectives of this lecture are to: 1. Define psychrometry and the composition of moist air ( Section 27.1 ) 2. Discuss the methods used for estimating properties of moist air ( Section 27.2 ) 3. Present perfect gas law model for moist air ( Section 27.2.1 ) 4. Define important psychrometric properties ( Section 27.2.2 ) 5. Present graphical representation of psychrometric properties on a psychrometric chart ( Section 27.2.3 ) 6. Discuss measurement of psychrometric properties ( Section 27.3 ) 7. Discuss straight-line law as applied to air-water mixtures ( Section 27.3.1 ) 8. Discuss the concept of adiabatic saturation and thermodynamic wet bulb temperature ( Section 27.3.2 ) 9. Describe a wet-bulb thermometer ( Section 27.3.3 ) 10. Discuss the procedure for calculating psychrometric properties from measured values of barometric pressure, dry bulb and wet bulb temperatures ( Section 27.4 ) 11. Describe a psychrometer and the precautions to be taken while using psychrometers ( Section 27.5 ) At the end of the lecture, the student should be able to: 1. Define psychrometry and atmospheric air 2. Use perfect gas law model and find the total pressure of air from partial pressures of dry air and water vapour 3. Define and estimate psychrometric properties 4. Draw the schematic of a psychrometric chart 5. Discuss the straight-line law and its usefulness in psychrometry 6. Explain the concepts of adiabatic saturation and thermodynamic wet bulb temperature 7. Differentiate between thermodynamic WBT and WBT as measured by a wet bulb thermometer 8. Estimate various psychrometric properties given any three independent properties 9. Describe a psychrometer 27.1. Introduction: Atmospheric air makes up the environment in almost every type of air conditioning system. Hence a thorough understanding of the properties of atmospheric air and the ability to analyze various processes involving air is fundamental to air conditioning design. Psychrometry is the study of the properties of mixtures of air and water vapour. Atmospheric air is a mixture of many gases plus water vapour and a number of pollutants (Fig.27.1). The amount of water vapour and pollutants vary from place to place. The concentration of water vapour and pollutants decrease with altitude, and above an altitude of about 10 km, atmospheric air consists of only dry air. The pollutants have to be filtered out before processing the air. Hence, what we process is essentially a mixture of various gases that constitute air and water vapour. This mixture is known as moist air .
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3 The moist air can be thought of as a mixture of dry air and moisture. For all practical purposes, the composition of dry air can be considered as constant. In 1949, a standard composition of dry air was fixed by the International Joint Committee on Psychrometric data. It is given in Table 27.1. Constituent
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This note was uploaded on 02/29/2012 for the course CHEMICAL 302 taught by Professor Nptel during the Spring '12 term at Birla Institute of Technology & Science.

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R&AC Lecture 27 - 1 Lesson 27 Psychrometry Version...

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