R&AC Lecture 23 - Lesson 23 Condensers &...

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Lesson 23 Condensers & Evaporators Version 1 ME, IIT Kharagpur 1
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The specific objectives of this lesson are to: 1. Classify refrigerant evaporators as natural convection or forced convection type, flooded or dry type, refrigerant flow inside the tubes or outside the tubes ( Section 23.1 ) 2. Discuss salient features of natural convection coils ( Section 23.2 ) 3. Discuss salient features of flooded evaporators ( Section 23.3 ) 4. Discuss salient features of shell-and-tube type evaporators ( Section 23.4 ) 5. Discuss salient features of shell-and-coil evaporator ( Section 23.5 ) 6. Discuss salient features of double pipe evaporators ( Section 23.6 ) 7. Discuss salient features of Baudelot evaporators ( Section 23.7 ) 8. Discuss salient features of direct expansion fin-and-tube type evaporators ( Section 23.8 ) 9. Discuss salient features of plate surface evaporators ( Section 23.9 ) 10. Discuss salient features of plate type evaporators ( Section 23.10 ) 11. Discuss thermal design aspects of refrigerant evaporators ( Section 23.11 ) 12. Discuss enhancement of boiling heat transfer ( Section 23.12 ) 13. Discuss the concept of Wilson’s plot ( Section 23.13 ) At the end of the lecture, the student should be able to: 1. Classify refrigerant evaporators and discuss the salient features of different types of evaporators 2. Perform thermal design calculations on refrigerant evaporators using various heat transfer correlations presented in the lecture 3. Use Wilson’s plots and determine external and internal heat transfer coefficients from given experimental data and specifications of evaporators and condensers Introduction: An evaporator, like condenser is also a heat exchanger. In an evaporator, the refrigerant boils or evaporates and in doing so absorbs heat from the substance being refrigerated. The name evaporator refers to the evaporation process occurring in the heat exchanger. 23.1. Classification There are several ways of classifying the evaporators depending upon the heat transfer process or refrigerant flow or condition of heat transfer surface. 23.1.1. Natural and Forced Convection Type The evaporator may be classified as natural convection type or forced convection type. In forced convection type, a fan or a pump is used to circulate Version 1 ME, IIT Kharagpur 2
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the fluid being refrigerated and make it flow over the heat transfer surface, which is cooled by evaporation of refrigerant. In natural convection type, the fluid being cooled flows due to natural convection currents arising out of density difference caused by temperature difference. The refrigerant boils inside tubes and evaporator is located at the top. The temperature of fluid, which is cooled by it, decreases and its density increases. It moves downwards due to its higher density and the warm fluid rises up to replace it. 23.1.2. Refrigerant Flow Inside or Outside Tubes
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R&AC Lecture 23 - Lesson 23 Condensers &...

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