The compressor in vapor absorption system consumes a lot of energy. To avoid this, vapor absorption refrigeration system has been developed. The absorption system uses heat energy supplied by a flame or some other source of heat.
Michel Faraday experiment on liquefaction of gas:
vapor absorption refrigeration system is based on the principle of experiment developed by Michel Faraday. He sealed in a bent test tube a compound of silver chloride (a white powder) and dry ammonia gas which have been absorbed by the powder. He heated the end of tube containing the powder and at the same time, cooled the opposite end of the tube with water. Ammonia vapor was released during the application of heat and was condensed to cool end of the tube. When the flame under the powder was extinguished, vapor that had not been liquefied was reabsorbed by the powder reducing the pressure on liquid ammonia. The liquid ammonia begins to boil, change back to vapor and was reabsorbed by the powder. The end of tube containing boiling liquid was intensely cold. The evaporating ammonia having drawn heat from the nearest substance, the test tube itself.
The vapor absorption refrigeration system has following components
1. Generator: The generator receives the strong solution of aqua- ammonia from the absorber and heats it. Because of its heating, the aqua –ammonia solution gets separated into ammonia vapor at high pressure and hot weak ammonia solution which contains mostly water.
2. Condenser: The condenser converts high pressure ammonia vapor received form generator into high pressure ammonia liquid. This condensation is done by means of circulating water.
3. Expansion valve: This valve is otherwise called throttling valve, since the expansion which takes place here is throttling. While passing through this valve, liquid ammonia gets expanded into low pressure and low temperature ammonia.
4. Evaporator: The evaporator is otherwise called cold chamber. Here, the refrigerant absorbs heat from material which is to be cooled and gets evaporated. It has many coils made of copper.
5. Absorber: Absorber receives low pressure ammonia vapor from evaporator and weak ammonia solution from the generator and mixes them well to form a strong solution of aqua-ammonia.
The working fluid in vapor absorption refrigeration system is normally ammonia. The ammonia vapor and water are mixed to form a strong solution of aqua- ammonia in the absorber. This aqua ammonia solution is then pumped into generator where it is evaporated by application of heat at high pressure and leaves behind weak ammonia solution which mostly contains water.
The high pressure ammonia vapor produced by generator is condensed in condenser and becomes liquid ammonia. This high pressure liquid ammonia is allowed to pass through expansion valve or throttling valve where it expands and become low pressure and low temperature ammonia which mostly contains liquid ammonia and little vapor ammonia.
The driving force for absorbent system is heat. Here, heater and generator takes the place of compressor. The generator contains an absorbent, a liquid or solid which has absorbed the refrigerant. Water passes through coil to cool the absorbent and are permitted to reabsorb the refrigerant at the top of the cycle. Here, the tank serves as liquid receiver or cooling unit. Check valve guarantee the movement of refrigerant in only one direction.
The cycle begins when heat is applied to the generator, releasing refrigerant as a gas from the absorbent. The gaseous refrigerant travels upward to the condenser which is cooled by air or water. Here, it is liquefied, its heat is dispelled and it flows through expansion valve and by gravity to the cooling unit.
As the liquid refrigerant accumulates, the float rises until it reaches to upper electrical contact point, setting off the device which automatically turn off the flame of the burner. The same device also sends the stream of water through the coil in the generator. The absorbent which surrounds the coil is thus cooled. Here, two processes cause refrigerant to evaporate
1. Transfer heat to the refrigerant from the food to near the cooling unit.
2. Absorbent when cooled, reabsorb the refrigerant.
When liquid level drops sufficiently to reach the lower electrical contact point, the burner is automatically lit. Heat is once more supplied to the absorbent and cycle is renewed. In this way, intermittent absorption cycle is produced through alternate application and withdrawal of heat.
Name : Pratiksha Shrestha
Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.