This type of refrigerator takes advantage of the fact that the liquid ammonia evaporators very rapidly in hydrogen. For this refrigerator, no pump is required as the circulation is maintained by gravity and the heat is supplied by a small gas jet or electric heater.
Figure 36.36 shows a diagrammatic view of Electrolux refrigerating system. This system consist of an absorber A which contains a strong solution of ammonia dissolved in distilled water. When a gas burner B is lighted the circulation of the system commences because of the warming of ammonia solution in the boiler or heater D.
The strong ammonia solution now flows from A through the intercharger C, where it is heated by the hot work solution returning from D. It then passes around the heater D, which causes the ammonia vapour to be driven out of the solution. Now the released ammonia vapour passes through a rectifier, after which it enters the condenser E.
The object of the rectifier is to drain off any particles of water coming with the ammonia vapours. The hot weak solution left behind D drains back into the absorber A. In doing so it passes through the intercharger C, where it gives its heat to the strong solution flowing from A.
After condensation in the condenser, ammonia vapours forms the liquid and this ammonia liquid flows by gravity into the evaporator G. This is full of hydrogen into which the liquid ammonia evaporates. Liquid ammonia absorbs latent heat producing intense cold around G, which is situated in the food cabinet or any space which is to be refrigerated and thus the cabinet or space is cooled and the objects place in the cabinet or space are cooled.
The mixture of hydrogen and ammonia vapour now flows through the gas heat exchanger F, where it cools the fresh supply of hydrogen flowing into G. From F the NH3 ammonia vapour and hydrogen gas flows into the absorber A, where the hydrogen rises to the top and flows back to the evaporator G. Thus, the cycle is complete.
The main advantage of this Electrolux refrigerator is that there is no moving part like compressor, pump or fan and therefore there is no noise and no machinery to give mechanical troubles.
The coefficient of performance of this refrigerator is given by the equation-
Total pressure in the system is about 15 bar. H2 gas is present in the evaporator at about 11-12 bar pressure. The quantity of H2 is so proportioned that the practical pressure of NH3 within evaporator will be only a smaller portion of the total pressure for the pressure of NH3 vapours in the evaporator will only 4-3 bar. This low pressure causes NH3 to boil or evaporator at low temperatures. Here the Dalton’s law of partial pressure is applied.
Other substances that can be used in vapour absorption system are:
(i) Water as the refrigerant and lithium bromide salt solution as absorbent.
(ii) Water as the refrigerant and lithium chloride brine as the absorbent.
(iii) Methylene chloride as refrigerant and dimethyl ether of tatraethylene glycol as absorbent.
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