What is Absorption Cooling System?
Absorption cooling system; It is based on the principle of increasing the pressure of the refrigerant with a thermal mechanism instead of the compressor used to increase the pressure of the refrigerant in vapor compression refrigeration systems. This mechanism used in absorption cooling systems is called “thermal compression”. Generally, the principle of absorption is that substances with mutual solubility are less soluble at high temperatures and more soluble at low temperatures. Freon gases are used as refrigerant in compressor systems. In absorption systems, a solution consisting of two fluid pairs, lithium bromide + water and ammonia + water solutions are used. One of them is lithium bromide-water couple, lithium bromide is the absorbent and water is the refrigerant. The other is the suammonia couple, the water absorbent ammonia refrigerant.
In absorption cooling, the absorbent is liquid, and the other substance (refrigerant) is in the gas phase when absorbed. If the absorbent rate is high and the refrigerant rate is low, it is called a concentrated or strong melt, and if the refrigerant rate with a low absorber rate is high, it is called a weak melt.
Working Principle of the System
Figure 1 shows a single-stage absorption cooling system working with a LiBr/Water fluid couple.
In the system, the LiBr/Water mixture decomposes as water vapor and rich LiBr/Water solution with the heat given in the generator. The refrigerant (water vapor) enters the condenser from the generator and condenses here by giving off heat. The liquid that comes out of the condenser in liquid phase and at high pressure is reduced in pressure by passing through the throttle valve and enters the evaporator in liquid phase and at low pressure. It evaporates by taking heat in the evaporator and enters the absorber. LiBr/Water solution returning from the generator as a rich mixture enters the heat exchanger and cools down a bit and enters the throttling valve. The rich mixture, whose pressure is reduced, enters the absorber and mixes with the water vapor coming from the evaporator. In the meantime, the heat released is thrown out for better absorption process. The lean LiBr/Water mixture coming out of the absorber is pressurized by the pump and enters the heat exchanger and enters the generator from there.
Absorption Cooler System Elements
Melt pump: It is to transmit the rich melt from the absorber to the heat exchanger. Generally, closed type pumps are chosen.
Generator: Also called a boiler. With the heat given in the generator, the refrigerant is separated from the rich melt.
Condenser: They can be water or air cooled in accordance with the design of the system. It reduces the temperature of the refrigerant vapor coming from the generator and provides condensation.
Evaporator: It is the part where the liquid refrigerant evaporates by taking the ambient temperature. In the absorption cooling system, it is generally designed as countercurrent.
Absorber: Although it resembles a condenser in appearance, its interior consists of one or more sheets on top of each other to provide a good mixture.
Heat exchanger: Two heat exchangers can be used as pre-heater and pre-cooler. They are usually countercurrent.
Expansion valve: One is used to reduce the pressure of the refrigerant melt and the other is used to reduce the pressure of the weak melt.
Refrigerant pairs: Different working fluids are used in absorption cooling systems. These are Ammonia-Calcium Chloride, Ammonia-Water, Lithium Bromide-Water, Ammonia-Lithium Nitrate, R21-Glycoleter, Ammonia-Sodium Thicynate.
Features of Absorption Cooling System
1- It needs minimum electrical energy.
2- Small capacity ones are without pump. They are silent. It is used in hotels and offices.
3- There is no compressor. Thermal compression is available.
4- The risk of failure is low, the maintenance cost is low.
5- It is environmentally friendly. CFCs are not used.
6- It is efficient in variable cooling loads as in full load.
7- It is economical when resources such as solar, geothermal and waste heat are used.
1-Performance coefficients are low. (06<COP<1.5)
2-They take up a lot of physical space.
3-The initial investment cost is high.
4-There is a crystallization problem in the LiBr-Water couple.
5-Low vacuum pressure creates a problem.
The absorption cooling system with a small volume and pumped fluid is used as a pressure reduction H as the 3rd fluid.