Electrolux Refrigeration Manual do Utilizador descarregar pdf (Página 2)

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Principle. The principle involved makes use of the properties of gas-vapor mixtures. If a liquid is

exposed to an inert atmosphere, it will evaporate until the atmosphere is saturated with the vapor of the

liquid. This evaporation requires heat which is taken from the surroundings in which the evaporation

takes place. A cooling effect is thus produced. The partial pressures of the refrigerant vapor (in this case

ammonia) must be low in the evaporator, and higher in the condenser. The total pressure throughout the

circuit must be constant so that the only movement of the working fluid is by convection currents. The

partial pressure of ammonia is kept low in requisite parts of the circuit by concentrating hydrogen in

those parts.

Working. The ammonia liquid leaving the condenser enters the evaporator and evaporates into the

hydrogen at the low temperature corresponding to its low partial pressure. The mixture of ammonia and

hydrogen passes to the absorber into which is also admitted water from the separator. The water

absorbs the ammonia and the hydrogen returns to the evaporator. In the absorber the ammonia therefore

passes from the ammonia circuit into water circuit as ammonia in water solution. This strong solution

passes to the generator where it is heated and the vapor given off rises to the separator. The water with

the vapor is separated out and a weak solution of ammonia is passed back to the absorber, thus

completing the water circuit. The ammonia vapor rises from the separator to the condenser where it is

condensed and then returned to the evaporator.

The actual plant includes refinements and practical modifications (which are not included here). The

following points are worth noting:

 The complete cycle is carried out entirely by gravity flow of the refrigerant.

 The hydrogen gas circulates only from the absorber to the evaporator and back.

 With this type of machine efficiency is not important since the energy input is small.

 It has not been used for industrial applications as the C.O.P. of the system is very low.

Role of Hydrogen. By the presence of hydrogen it is possible to maintain uniform total pressure

throughout the system and at the same time permit the refrigerant to evaporate at low temperature in the

evaporator corresponding to its partial pressure. Thus the condenser and evaporator pressures of the

refrigerant are maintained as below:

(i) In the condenser only ammonia is present, and the total pressure is the condensing pressure.

(ii) In the evaporator hydrogen and ammonia are present; their relative masses are adjusted such that

the partial pressure of ammonia is the required evaporator pressure.

These are achieved without the use of pumps or valves.

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