Vapor Absorption Refrigeration System

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1. Vapor Absorption Refrigeration System (VARS) 1 Faculty Name: Prof . Ajaypalsinh G. Barad Branch: Mechanical Semester : 6 th Name of Subject: Refrigeration and Air Conditioning

2. Outline Introduction Desirable characteristics of refrigerant Selection of pair, Practical H 2 O -NH 3 cycle, LiBr – H 2 O system and its working, Electrolux refrigeration system 2

3. Introduction: The vapour absorption refrigeration system is one of the oldest methods of producing refrigeration effect. An ammonia is the commonly used refrigerant in a vapour absorption system. The vapour absorption system uses heat energy, instead of mechanical energy as in vapour compression systems, in order to change the conditions of the refrigerant. This system may be used in both the domestic and large industrial refrigerating plants. 3

4. The compressor is replaced by an absorber, a pump, a generator and a pressure reducing valve. In this system, the vapour refrigerant from the evaporator is drawn into an absorber where it is absorbed by the weak solution of the refrigerant forming a strong solution. This strong solution is pumped to the generator where it is heated by some external sources. During the heating process, the vapour refrigerant is driven off by the solution and enters into the condenser where it is liquefied. The liquid refrigerant then flows into the evaporator and thus the cycle is completed. Introduction: 4

5. Introduction: 5

6. Solubility requirement: The refrigerant should have proper solubility in the absorbent so that a strong solution, highly rich in the refrigerant, is formed in the absorber by the absorption of the refrigerant vapour. Boiling point requirement: There should be a large difference, about 200 o C, in the boiling points of the two substances, thus absorbent free refrigerant is boiled off from the generator. Thermodynamic Requirement of Refrigerant – Absorbent Mixture: 6

7. Some other Properties required: The refrigerant should have high affinity for the absorber at low temp and less affinity at high temp. It should have low freezing point. It should have good thermal and chemical stability Irreversible chemical reactions of all kinds are to be avoided. Thermodynamic Requirement of Refrigerant – Absorbent Mixture: 7

8. Ammonia – Water Ammonia – Sodium Thiocynate Ammonia – Lithium Nitrate Ammonia – Calcium Chloride Water – Lithium Bromide Methylene Choride – Dimethyl Ether tetra ethylene glycol Selection of Pairs: 8

9. Simple Ammonia - Water VAR: 9

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11. Practical Ammonia - Water VAR: 11

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13. Advantages: Feasible to separate refrigerant from absorbent, as water is more volatile compared to Li-Br. Operating pressures are low so pumping cost is low. Wall thickness of the system less compared to VCR. Water as refrigerant is non toxic, so can be directly used for chilled coil. Can be built up to 1,00,000 TR capacity plants. Operation and maintenance cost is very low. Lithium Bromide Absorption System: 13

14. Disadvantages: Evaporation temperature must be kept above the freezing point of water and hence the temperature of chilled water cannot be less than 5 o C. Li-Br solution is corrosive. So inhibitors need to be added to the system to prevent the metal parts from corrosion. Lithium Bromide Absorption System: 14

15. Introduction: 15

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18. Silent operation Cost effective Balanced pressure No need of valve No lubrication problem Less maintenance Advantage of Three-Fluid 18

19. VAR uses heat energy, VCR uses mechanical energy Operation is simple in VAR as less moving parts Less wear, tear and noise. No bad effect of liquid refrigerant leaving the evaporator Law COP More component, bulky system Charging of refrigerant is difficult Comparison of VAR and VCR 19