Vapour absorption refrigeration system (VARS) is mainly used where high power is not easily available.
It is different from vapour compression refrigeration system because in this system compressor is replaced with an absorber, pump, and generator.
Vapour absorption refrigeration system uses heat energy for refrigeration while the Vapour Compression System uses work energy for refigeration which is much more expensive to produce.
Vapour absorption refrigeration system is best for places where heat energy is available easily at a low price.
This process is best for Steam Power Plants. Steam Power Plants can easily run this refrigeration system using the waste heat produced in the power plant.
Why Vapour absorption refrigeration system (VARS) was needed:
Vapour absorption refrigeration system (VARS) was mainly needed because Vapour compression refrigeration system (VCRS) consumed very high power and was not practical use or affordable where power was not easily available or was available at very high price.
Components in Vapour Absorption Refrigeration System:
The main function of the evaporator is to provide cooling to the area with which it is in contact.
The chilled liquid will enter inside this evaporator and will receive heat from the evaporator and will convert into vapour. This vapour will be at low pressure.
From this evaporator, the ammonia vapour comes out at low pressure and will go towards absorber.
The absorber is used to absorb the refrigerant. At the absorber, there will be a weak solution of water and ammonia. When the ammonia vapour from evaporator will reach the absorber, water present in the absorber will absorb it. As water will absorb ammonia, strong solution of ammonia and water will start getting created.
When the water will absorb ammonia water will liberate heat and the absorbing capacity of water reduces. So, cool water is supplied in the absorber so that the absorbing capacity stay high so that it continuously absobs the ammonia vapour.
The pump will pump the strong solution of ammonia and water from the absorber to the generator.
Ammonia and water solution is used inside this system. Ammonia is used as a refrigerant and water is used as an absorbent.
A solution of these two is made because water has a strong affinity towards ammonia. Water plus ammonia solution is present inside the absorber.
The generator is provided with auxiliary heat from outside. For providing this auxiliary heat steam or hot water or any type of heater can be used. The heat is provided so that the ammonia and water solution converts to vapour.
Analyzer is placed at the top of the generator. Ammonia will convert into vapour before the water but some water particles convert into vapour with ammonia. This analyzer is used to separate the water particles from ammonia vapour.
If the water particles move forward from the generator, it will decrease the efficiency of the whole system. If the water particle passes forward in large quantity, it can damage the system also.
So the analyzer condenses the water particles but the ammonia vapour will pass through the analyzer and will go forward in the system. Some ammonia vapour will also condense in the analyzer but most of the ammonia vapour will pass through the analyzer.
5 Pressure Reducing Valve:
After the ammonia vapour passes through the analyzer, the weak solution present in the generator will pass through the pressure reducing valve and will reach the absorber again.
Condenser is used to convert the ammonia vapour into liquid phase. This condenser can be either water-cooled or air-cooled.
6 Expansion Valve:
Its main function is to convert the liquid into chilled liquid and give it to the evaporator.
At the expansion valve, the ammonia liquid will come from the condenser and the temperature and pressure of this liquid ammonia will reduce and this ammonia liquid will become chilled liquid ammonia temperature of which will be very low.
- Very cold liquid ammonia will come out of the expansion valve enters into the evaporator coils through the connected pipe. The main cooling always occurs in the evaporator.
- When the liquid ammonia will enter the evaporator coil, it will absorb all the heat present on the surface of the evaporator coils by absorbing all the heat from the surrounding region of the evaporator coils.
- The cold liquid ammonia will convert into ammonia vapour inside these coils and the surrounding surface of the evaporator will become cold by losing the heat to the liquid, thus cooling effect or refrigeration effect has occurred inside the evaporator. After that this low pressure ammonia vapour will leave the evaporator and enter the absorber through the connecting pipe.
- The absorber already have a weak solution of ammonia and water inside it and when this low pressure ammonia vapour enters the absorber, the water present in the weak solution of this absorber will start absorbing this ammonia vapour and the weak solution will gradually convert into a strong ammonia-water solution. The more ammonia vapour coming from the evaporator is absorbed by water from this weak solution, the stronger the solution will become but when water absorbs ammonia vapour it also liberates heat. When water will absorb the incoming ammonia vapour, it will produce heat which will increase the temperature of the solution and when the solution becomes hot gradually the capacity of water to absorb ammonia decreases.
- To keep the temperature of the solution to an optimal level, a supply of cool water is provided through this pipe so that this cool water carries away heat from the solution and thus water regains the capacity to continuously absorb incoming ammonia vapour.
- Beside the absorber there is a pump, now when power is provided then this pump starts working. The strong solution of ammonia and water will be pumped from the absorber to the generator using this pump.
- Auxiliary heat or external heat is provided to this generator either by using steam or hot water or any heater, gas burner.
- So when the solution of ammonia and water reaches the generator and heat is applied to the solution from the external source both ammonia and water from the ammonia water solution turns into vapour inside this generator. Actually ammonia turns into vapour faster than water and water takes comparatively more time to completely turn into vapour. But eventually both ammonia and water will turn into vapour when heat is provided. Now here we also have an analyzer on the top of the generator. When the vapour of ammonia and water tries to pass through this analyzer only ammonia is allowed to pass through.
- The analyzer continuously condenses the water vapour and sends water back to the generator. This is because if water vapour enters the system it can decrease the efficiency of the refrigeration system or if larger quantity of water vapour enters the system, then the system could be damaged, thus the analyzer separates water particles from ammonia vapour and only lets ammonia to pass through to the next component through this pipe for further process and the analyzer sends the water particles back to the generator.
- From this generator, the residue weak solution of water and ammonia accumulated here will be passed back to the absorber again passing through the pressure reducing valve.
- So the high pressure high temperature pure ammonia vapour coming out of the generator will now enter into this condenser through this connected pipe.
- We have a condenser, when high pressure high temperature ammonia vapour enters the cold condenser, then the condenser absorbs the heat from the ammonia vapour and completely converts it into liquid.
- This condenser can be either water cooled or air cooled. It will liberate the latent heat of the vapour coming into the condenser and thus condensing keeps happening.
- Now we have an expansion valve. After condensation, liquid ammonia will leave the condenser and pass through this expansion valve. Now this high pressure liquid ammonia coming from the condenser will be expanded inside this expansion valve. We know that when expansion occurs, the pressure between the molecules decreases considerably. Thus the temperature falls so this high pressure liquid ammonia will be expanded into low pressure low temperature liquid ammonia thus we get very cold chilled low temperature liquid ammonia coming out of the expansion valve. After that this liquid ammonia will be passed over to the evaporator through the connected pipe where by absorbing all the heat from the surrounding region of the evaporator coils, the cold chilled liquid ammonia will again turn into low pressure ammonia vapour inside the coils and the surrounding region of the evaporator will become cold by loosing the heat to this liquid this liquid. Then this low pressure ammonia vapour will leave the evaporator and enter the absorber through this connecting pipe.
- This whole cycle will be repeated again and again. Hence, refrigeration will occur continuously in the evaporator region.