Working Principle of an Absorption Machine
To understand the working principle of an absorption machine, it is good to know how a compression refrigeration machine works.
Compression Cycle:
In a compression cycle there are two pressure sections: In the low-pressure section a refrigerant (here ammonia) evaporates at a low temperature and cooles the fluid which has to be chilled. In the high-pressure section the refrigerant vapor is liquified (condensed) at a higher temperature, which depends on the temperature of the cooling water. When the liquefied refrigerant is expanded to the evaporator in the low-pressure section, its temperature decreases according to its vapor curve and can be evaporated again. A compressor driven by an electrical motor is used to transport the low-pressure vapor to the high-pressure section. The main driving energy for this process is the electricity for the compressor.
Absorption Cycle:
In an absorption cycle, this mechanical compressor is replaced by a thermal compressor. The main components of this are an absorber, a desorber and a pump. In the absorber, the low-pressure ammonia vapor that comes from the evaporator is mixed with an ammonia-water solution and absorbed. Absorption means that the vapor is liquefied and dissolved in the ammonia-water solution. This absorption process increases the ammonia concentration of the solution. There is more ammonia in the solution at the outlet of the absorber than at the inlet. For this reason, the inlet solution is called “Weak Solution” and the outlet is called “Strong Solution”.
The strong solution is pumped from the absorber to the desorber, which is in the high pressure section. Here it is heated until it starts to boil. The vapor that boils from the strong solution is high pressure ammonia vapor that flows to the condenser where it is liquefied. The ammonia concentration of the solution has decreased in the desorber (as the ammonia vapor has been separated) and flows back to the absorber as a weak solution.
The main driving energy for this process is the heat needed in the desorber for the separation of ammonia gas from the ammonia-water-solution and to a small extent the electricity for the solution pump.
For information about the efficiency of the absorption process look on our page: Operating Conditions of an Absorption Refrigeration Plant.
