Before the development of the vapor compression system of refrigeration, vapor absorption cycle was very widely used. The vapor compression system replaced vapor absorption system because it has high coefficient of performance (COP). The vapor absorption system requires very less amount of electricity but large amount of heat; hence it can be used very effectively in industries where very large stocks of excessive steam are available. In such cases there is not only effective utilization of steam, but also lots of savings in electricity costs. Of late the vapor absorption systems are being employed by a number of industries to save on their electric bills. However, the vapor absorption system is useful only where large scale refrigeration in excess of at least 20 tons is required.
The various processes of the vapor absorption cycle are similar to the one in vapor compression cycle, only the method of compression of the refrigerant is different. In vapor absorption system ammonia is used as the refrigerant, which has very high affinity to dissolve in water. Here are various processes of vapor absorption cycle:
1) Compression or absorption of the refrigerant: In vapor absorption system there is no traditional compressor, instead there is absorber. The absorber consists of water, called as absorbent, in which the refrigerant, ammonia, dissolves. This mixture of water and ammonia is then pumped and heated thus increase in temperature and pressure of the ammonia occurs. Ammonia leaves the absorber at high pressure and high temperature. Some work has to be provided to the pump and heating is carried out by the steam. The amount of electricity required by the pump is much lesser than that required by the compressor hence there is lots of saving of electricity, however, the additional source of heat in the form of steam has to be provided.
2) Condensation: The refrigerant at pressure and temperature then enters condenser where it is cooled by water and its temperature and pressure reduces.
3) Expansion: Thereafter the expansion of refrigerant occurs in throttling valve due to which the temperature and pressure of the ammonia refrigerant reduces drastically and suddenly.
4) Evaporation: Finally the refrigerant enters the evaporator where it produces the cooling effect. It leaves the evaporator in vapor state and then enters absorber, where it is absorbed by absorbent, water and compressed by the pump. This process repeats again and cycle continues.
There are different types absorbents like water and lithium bromide that can be used with refrigerant ammonia. These systems are called water absorption system or lithium bromide absorption system.