R&AC Lecture 22 - Lesson 22 Condensers Evaporators...

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Lesson 22 Condensers & Evaporators Version 1 ME, IIT Kharagpur 1
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The specific objectives of this lesson are to: 1. Discuss general aspects of evaporators and condensers used in refrigeration systems ( Section 22.1 ) 2. Introduce refrigerant condensers ( Section 22.2 ) 3. Classify refrigerant condensers based on the external fluid used, based on constructional details etc. ( Section 22.3 ) 4. Compare air cooled condensers with water cooled condensers ( Section 22.3.4 ) 5. Present analysis and design aspects of refrigerant condensers, estimation of heat transfer coefficients on external fluid side on refrigerant side for different configurations ( Section 22.4 ) 6. Discuss briefly the effect of presence of air and other non-condensible gases in refrigerant condensers ( Section 22.5 ) 7. Discuss briefly the concept of optimum condensing pressure for lowest running cost of a refrigeration system ( Section 22.6 ) At the end of the lecture, the student should be able to: 1. Classify and describe refrigerant condensers based on the external fluid used, based on the external fluid flow and based on constructional aspects 2. Compare air-cooled condensers with water-cooled condensers 3. Perform condenser design calculations using various correlations presented for estimating heat transfer coefficients on external fluid and refrigerant side and estimate the required condenser area for a given refrigeration system 4. Explain the effect of presence of non-condensible gases on condenser performance 5. Explain the concept of optimum condenser pressure 22.1. Introduction: Condensers and evaporators are basically heat exchangers in which the refrigerant undergoes a phase change. Next to compressors, proper design and selection of condensers and evaporators is very important for satisfactory performance of any refrigeration system. Since both condensers and evaporators are essentially heat exchangers, they have many things in common as far as the design of these components is concerned. However, differences exists as far as the heat transfer phenomena is concerned. In condensers the refrigerant vapour condenses by rejecting heat to an external fluid, which acts as a heat sink. Normally, the external fluid does not undergo any phase change, except in some special cases such as in cascade condensers, where the external fluid (another refrigerant) evaporates. In evaporators, the liquid refrigerant evaporates by extracting heat from an external fluid (low temperature heat source). The external fluid may not undergo phase change, for example if the system is used for sensibly cooling water, air or some other fluid. There are many refrigeration and Version 1 ME, IIT Kharagpur 2
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air conditioning applications, where the external fluid also undergoes phase change. For example, in a typical summer air conditioning system, the moist air is dehumidified by condensing water vapour and then, removing the condensed liquid water. In many low temperature refrigeration applications freezing or
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R&AC Lecture 22 - Lesson 22 Condensers Evaporators...

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