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Ammonia-Water Vapor Absorption Refrigeration System
The working of ammonia-water absorption refrigeration system is based on the simple vapor absorption refrigeration system. In this system ammonia is used as the refrigerant and water is used as the absorbent. The ammonia-water absorption system is used in the domestic as well the commercial applications where the requirement of the temperature is above 32 degree F.
The major advantage of the ammonia-water solution is that water has strong affinity for ammonia and they are soluble with each other in wide operating conditions that occur in different refrigeration applications. Further, the ammonia-water solution is highly stable and works well with many materials except copper and its alloys that get corroded in the presence of ammonia. We have already seen that ammonia is considered to be the most suitable refrigerant, however, due to its toxicity its applications are limited and it is not used in the air conditioning applications.
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Parts of the Ammonia-Water Absorption Refrigeration System and its Working
The various parts of the ammonia-water vapor absorption refrigeration system and their working are explained below (please refer the figure above):
1) Evaporator: It is in the evaporator where the refrigerant pure ammonia (NH3) in liquid state produces the cooling effect. It absorbs the heat from the substance to be cooled and gets evaporated. From here, the ammonia passes to the absorber in the gaseous state.
2) Absorber: In the absorber the weak solution of ammonia-water is already present. The water, used as the absorbent in the solution, is unsaturated and it has the capacity to absorb more ammonia gas. As the ammonia from evaporator enters the absorber, it is readily absorbed by water and the strong solution of ammonia-water is formed. During the process of absorption heat is liberated which can reduce the ammonia absorption capacity of water; hence the absorber is cooled by the cooling water. Due to absorption of ammonia, strong solution of ammonia-water is formed in the absorber.
3) Pump: The strong solution of ammonia and water is pumped by the pump at high pressure to the generator.
4) Generator: The strong solution of ammonia refrigerant and water absorbent are heated by the external source of heat such as steam or hot water. It can also be heated by other sources like natural gas, electric heater, waste exhaust heat etc. Due to heating the refrigerant ammonia gets vaporized and it leaves the generator. However, since water has strong affinity for ammonia and its vaporization point is quite low some water particles also get carried away with ammonia refrigerant, so it is important to pass this refrigerant through analyzer.
5) Analyzer: One of the major disadvantages of the ammonia-water vapor absorption refrigeration system is that the water in the solution has quite low vaporizing temperature, hence when ammonia refrigerant gets vaporized in the generator some water also gets vaporized. Thus the ammonia refrigerant leaving the generator carries appreciable amount of water vapor. If this water vapor is allowed to be carried to the evaporator, the capacity of the refrigeration system would reduce. The water vapor from ammonia refrigerant is removed by analyzer and the rectifier.
The analyzer is a sort of the distillation column that is located at the top of the generator. The analyzer consists of number of plates positioned horizontally. When the ammonia refrigerant along with the water vapor particles enters the analyzer, the solution is cooled. Since water has higher saturation temperature, water vapor gets condensed into the water particles that drip down into the generator. The ammonia refrigerant in the gaseous state continues to rise up and it moves to the rectifier.
6) Rectifier or the reflex condenser: The rectifier is a sort of the heat exchanger cooled by the water, which is also used for cooling the condenser. Due to cooling the remaining water vapor mixed with the ammonia refrigerant also gets condensed along with some particles of ammonia. This weak solution of water and ammonia drains down to the analyzer and then to the generator.
7) Condenser and expansion valve: The pure ammonia refrigerant in the vapor state and at high pressure then enters the condenser where it is cooled by water. The refrigerant ammonia gets converted into the liquid state and it then passes through the expansion valve where its temperature and pressure falls down suddenly. Ammonia refrigerant finally enters the evaporator, where it produces the cooling effect. This cycle keeps on repeating continuously.
Meanwhile, when ammonia gets vaporized in the generator, weak solution of ammonia and water is left in it. This solution is expanded in the expansion valve and passed back to the absorber and its cycle repeats.
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Reference and Image Courtesy
Book: Principles of Refrigeration by Roy J. Dossat, fourth edition, Prentice Hall