RAC Lecture 17 - Lesson 17 Vapour Absorption Refrigeration...

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Version 1 ME, IIT Kharagpur 1 Lesson 17 Vapour Absorption Refrigeration Systems Based On Ammonia- Water Pair
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Version 1 ME, IIT Kharagpur 2 The specific objectives of this lesson are to: 1. Introduce ammonia-water systems ( Section 17.1 ) 2. Explain the working principle of vapour absorption refrigeration systems based on ammonia-water ( Section 17.2 ) 3. Explain the principle of rectification column and dephlegmator ( Section 17.3 ) 4. Present the steady flow analysis of ammonia-water systems ( Section 17.4 ) 5. Discuss the working principle of pumpless absorption refrigeration systems ( Section 17.5 ) 6. Discuss briefly solar energy based sorption refrigeration systems ( Section 17.6 ) 7. Compare compression systems with absorption systems ( Section 17.7 ) At the end of the lecture, the student should be able to: 1. Draw the schematic of a ammonia-water based vapour absorption refrigeration system and explain its working principle 2. Explain the principle of rectification column and dephlegmator using temperature-concentration diagrams 3. Carry out steady flow analysis of absorption systems based on ammonia- water 4. Explain the working principle of Platen-Munter’s system 5. List solar energy driven sorption refrigeration systems 6. Compare vapour compression systems with vapour absorption systems 17.1. Introduction Vapour absorption refrigeration system based on ammonia-water is one of the oldest refrigeration systems. As mentioned earlier, in this system ammonia is used as refrigerant and water is used as absorbent. Since the boiling point temperature difference between ammonia and water is not very high, both ammonia and water are generated from the solution in the generator. Since presence of large amount of water in refrigerant circuit is detrimental to system performance, rectification of the generated vapour is carried out using a rectification column and a dephlegmator. Since ammonia is used as the refrigerant, these systems can be used for both refrigeration and air conditioning applications. They are available in very small (as pumpless systems) to large refrigeration capacities in applications ranging from domestic refrigerators to large cold storages. Since ammonia is not compatible with materials such as copper or brass, normally the entire system is fabricated out of steel. Another important difference between this system and water-lithium bromide systems is in the operating pressures. While water-lithium bromide systems operate under very low (high vacuum) pressures, the ammonia-water system is operated at pressures much higher than atmospheric. As a result, problem of air leakage into the system is eliminated. Also this system does not suffer from the problem of crystallization encountered in water-lithium bromide systems. However, unlike water, ammonia is both toxic and flammable. Hence, these systems need safety precautions.
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Version 1 ME, IIT Kharagpur 3 17.2. Working principle Figure 17.1 shows the schematic of an ammonia-water absorption refrigeration system. Compared to water-lithium bromide systems, this system uses
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RAC Lecture 17 - Lesson 17 Vapour Absorption Refrigeration...

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