R&AC Lecture 28 - Lesson 28 Psychrometric Processes...

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Unformatted text preview: Lesson 28 Psychrometric Processes The specific objectives of this lecture are to: 1. Introduction to psychrometric processes and their representation ( Section 28.1 ) 2. Important psychrometric processes namely, sensible cooling and heating, cooling and dehumidification, cooling and humidification, heating and humidification, chemical dehumidification and mixing of air streams ( Section 28.2 ) 3. Representation of the above processes on psychrometric chart and equations for heat and mass transfer rates ( Section 28.2 ) 4. Concept of Sensible Heat Factor, By-pass Factor and apparatus dew point temperature of cooling coils ( Section 28.2. ) 5. Principle of air washers and various psychrometric processes that can be performed using air washers ( Section 28.3 ) 6. Concept of enthalpy potential and its use ( Section 28.4 ) At the end of the lecture, the student should be able to: 1. Represent various psychrometric processes on psychrometric chart 2. Perform calculations for various psychrometric processes using the psychrometric charts and equations 3. Define sensible heat factor, by-pass factor, contact factor and apparatus dew point temperature 4. Describe the principle of an air washer and its practical use 5. Derive equation for total heat transfer rate in terms of enthalpy potential and explain the use of enthalpy potential 28.1. Introduction: In the design and analysis of air conditioning plants, the fundamental requirement is to identify the various processes being performed on air. Once identified, the processes can be analyzed by applying the laws of conservation of mass and energy. All these processes can be plotted easily on a psychrometric chart. This is very useful for quick visualization and also for identifying the changes taking place in important properties such as temperature, humidity ratio, enthalpy etc. The important processes that air undergoes in a typical air conditioning plant are discussed below. 28.2. Important psychrometric processes: a) Sensible cooling: During this process, the moisture content of air remains constant but its temperature decreases as it flows over a cooling coil. For moisture content to remain ) T T ( c m ) h h ( m Q A O pm a A O a c − = − = ( 2 8 . 1 ) DBT W O A Fig.28.1 : Sensible cooling process O-A on psychrometric chart h o h A b) Sensible heating (Process O-B): During this process, the moisture content of air remains constant and its temperature increases as it flows over a heating coil. The heat transfer rate during this process is given by: ) T T ( c m ) h h ( m Q O B pm a O B a h − = − = ( 2 8 . 2 ) is the humid specific heat ( ≈ 1.0216 kJ/kg dry air) and m pm a is the mass flow rate of dry air (kg/s). Figure 28.2 shows the sensible heating process on a psychrometric chart....
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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 28 - Lesson 28 Psychrometric Processes...

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