What Is Vapour Absorption Refrigeration System?
The Vapour absorption refrigeration systems include all processes in a vapor compression refrigeration system, such as compression, condensation, expansion, and evaporation—the refrigerant used in Vapour absorption systems as ammonia, water, or lithium bromide.
The refrigerant condenses in the condenser & evaporates in evaporation. The refrigerants produce a cooling effect in the evaporator & release heat to the atmosphere through the condenser.
The majors difference between the two systems is the method of suction and compression of refrigerant in the refrigeration cycle. In a Vapour compression system, the compressor sucks the refrigerant from the evaporator and compresses it under high pressure.
The compressor also enables the flow of refrigerants through the entire refrigeration cycle. In the vapor absorption cycle, a process of suction and compression is carried out by two different devices called absorbers and generators.
Thus the absorber and generator replace the compressor in the vapor absorption cycle. The absorber enables the refrigerant to flow by absorbing from the absorber to the generator. Another major difference between vapor compression and vapor absorption cycles is the method by which energy input is given to the system.
The energy input to a vapor compression system is given as a mechanical function from an electric motor driven by an electric motor. Energy input in a vapor absorption system is given as heat. This may be from the heat process or from steam over hot water.
Heat can also be produced by other sources such as natural gas, kerosene, heaters, etc., although these sources are used only in small systems.
The Vapour absorption refrigeration system uses heat energy for refrigeration, while the vapor compression system uses work energy for refrigeration which is much more expensive to produce.
The Vapour absorption refrigeration systems are best for locations where heat energy is readily available at a low cost. This process is best for steams power plants. Steams power plants can easily run this refrigeration system using the waste heat produced in the power plant.
Working of Vapour Absorption Refrigeration System:
The next component and analyzer send water particles back to the generator through this pipe for further processing. With this generator, the diluted solution of water and ammonia residues deposited here will be sent back to the absorber again. The very cold liquid will exit the ammonia expansion valve that enters the evaporator coil through the connected pipe. The main cooling is always in the evaporator.
When liquid ammonia enters the evaporator coil, it will absorb all the heat present on the surface of the evaporator coil by absorbing all the heat from the area around the evaporative coil. The cooled liquid ammonia will convert to ammonia vapor inside these coils, and the surrounding surface of the evaporator will be cooled by losing heat to the liquid; thus, a cooling effect or refrigeration effect has occurred insides the evaporator. It will then release the low-pressure ammonia vapor evaporator and enter the absorber through the connecting pipe.
The absorber already has a weak solution of ammonia and the water inside it, and when it enters the low-pressure ammonia vapor absorber, the water present in the weak solution of this absorber will start absorbing this ammonia vapor, and A weak solution will gradually transform into a strong one—Ammonia-water solution.
The more ammonia Vapour from the evaporator is absorbed by the water of this weaker solution, the stronger the solution will form, but when the water absorbs the ammonia Vapour, it also releases it from heat. When the water absorbs the incoming ammonia vapor, it will produce heat that will increase the temperature of the solution, & when the solution is heated, the ability of the water to absorb the ammonia gradually decreases.
To keep the slurry temperature at an optimum level, cold water is supplied through this pipe so that this cold water keeps the heat away from the slurry, and thus the water gains the ability to absorb the incoming ammonia Vapour continuously.
There is a pump next to the absorber; now that power is provided, this pump starts working. A strong solution of ammonia & water will be pumped from the absorber to the generator using this pump.
An Auxiliary generator or external heat is provided to this generator using steam or hot water or any heater, gas burner. So when the ammonia and water solution reaches the generator and heat is applied to the slurry from an external source, the water from the ammonia-water solution both turn into vapor inside this generator.
In fact, ammonia turns into Vapour faster than water, and water completely turns into vapor. But eventually, both ammonia & water will turn into Vapour upon providing heat. Now here we also have analyzers on top of the generator. Only ammonia is allowed to pass when ammonia and water Vapour try to pass through this analyzer.
The analyzer continuously condenses the water Vapour & sends water back to the generator. This is because if waters Vapour enters the system, it may reduce the efficiency of the refrigeration system, or if a large amount of water vapor enters the system, the system may be damaged; Thus, the analyte separates the water particles from the ammonia Vapour and only allows the ammonia to pass through the pressure reducing valve.
Therefore the high pressure, high-temperature pure ammonia vapor coming out of the generator will now enter the condenser through this connected pipe. We have a condenser; When high pressure, high-temperature ammonia Vapour enters the cold condenser, the condenser absorbs heat from the ammonia vapor and converts it completely into a liquid.
This condenser can be either water-coolers or air-cooled. This will release the latent heat of the Vapour coming into the condenser, and thus condensation continues.
Now we have an expansion valve. After condensation, the liquid ammonia will release the condenser and pass-through this expansion valve. Now inside this expansion valve, the high-pressure liquid ammonia coming from the condenser will be expanded. We know that when the expansion occurs, the pressure between the molecules decreases significantly.
Thus as the temperature falls, this high-pressure liquid ammonia will be expanded into low-pressure, low-temperature liquid ammonia; Thus, we exit the very cold low-temperature liquid ammonia expansion valve.
After that, thises liquid ammonia will be passed through the connection pipe to the evaporator, absorbing all the heat from the area around the evaporator coil, the cooled cold liquid ammonia will again turn into low-pressure ammonia Vapour inside the coil, And the area around the evaporator will be cooled by losing heat to this liquid.
This low-pressure ammonia Vapour will then release the evaporator and enter the absorber through this connecting pipe. This entire cycle will be repeated again and again. Therefore, refrigeration will occur continuously in the evaporation zone.
Components in Vapour Absorption Refrigeration System:
The main function of the evaporators is to provide cooling to the area with which it is in contact. The cooled liquid will enter inside this evaporator and receive heat from the evaporator, and be converted into vapor. This Vapour will be at low pressure. With this evaporator, the ammonia Vapour comes out under low pressure and will go towards the absorber.
Absorbers are used to absorb refrigerants. In the absorber, there will be a weak solution of water and ammonia. When the ammonia Vapour from the evaporator reaches the absorber, the water present in the absorber will absorb it. As the water absorbs the ammonia, a strong ammonia solution and water will begin to form.
When the water absorbs ammonia, the water will liberate from the heat, and the absorptive capacity of the water will be reduced. So, cold water is supplied to the absorber so that the absorptive capacity is high so that it is continuously absent of ammonia vapor.
The pump will pump a strong solution of ammonia and water from the absorber to the generator.
Ammonia and water solutions are used inside this system. Ammonia is used as a refrigerant, & water is used as an absorbent.
A solution of these two is formed because water has a strong affinity for ammonia. Water plus ammonia solutions are present inside the absorber.
The generator is provided with auxiliaries heat from outside. This auxiliary can be used to provide heat, steam or hot water, or any type of heater. Heat is provided so that the solution of ammonia and water is converted into vapor.
The analyzers are placed on top of the generator. Ammonia will convert to Vapour before water, but some water particles convert to Vapour along with ammonia. This analyzer is used to separate water particles from ammonia vapor.
If water particles move past the generator, it will reduce the efficiency of the entire system. If the water particles move in large quantities, it can also damage the system.
So the analyzer condenses the water’s particles, but the ammonia Vapour will pass through the analyzer and go further into the system. Some ammonia will also condense in the vapor analyzer, but most of the ammonia will pass through the Vapour analyzer.
#6. Pressure-Reducing Valve
After the ammonia Vapour passes through the analyzers, the weak solution present in the generators will pass through the pressure reducing valve and reach the absorber again.
Condensers are used to convert ammonia Vapour into the liquid phase. These condensers can be either water-cooled or air-cooled.
#8. Expansion Valve
Its main functions are to convert the liquid to cold liquid and pass it to the evaporator.
At the expansion valve, the ammonia will come from the liquid condenser, and the temperature and pressure of this liquid ammonia will decrease, and this ammonia will become the temperature of the liquid-cooled liquid ammonia, whose temperature will be much lower.
Single-Effect and Double-Effect Cycles:
In Vapour absorption refrigeration systems, the temperature of the heat source plays an important role. The heat given by the source can be used in a single-phase or in more than one phase. Accordingly, the system is called a single-phase or single-effect cycle of operation, and if heat is supplied in two phases, the system is called a two-phase or dual-effect cycle of operations.
It is found that when the temperatures of the heat sources are up to 105 ° C, the heat is used in a single phase, and the cycle is a single effect cycle. When the temperature of the heat source exceeds 105 ° C, the heat is usually used in two stages, and the system becomes a double impact cycle.
#1. Single Effect Cycle
A simple and practicals absorption system using ammonia as an absorber and refrigerant, as described in previous articles, is an example of a single-effect cycle system for Vapour absorption refrigeration systems.
#2. Double Effect Cycle
The schematic sketch of the double-effect system is shown. The Vapour of refrigerants is generated in two stages, as shown. For this purpose, two heat exchanger is used.