How 3-Way Fridges Work

A 3-way fridge is also known as an absorption fridge, as it uses absorption technology. It uses heat to provide energy needed to drive the cooling system.

The 3 sources of power used are:

  • LP Gas
  • 12V DC power (from battery / solar system)
  • Mains power (240V AC)

The absorption cooling cycle has 3 phases:

  • Evaporation - A liquid refrigerant evaporates in a low partial pressure environment, thus extracting heat from its surroundings – the refrigerator.
  • Absorption - The gaseous refrigerant is absorbed – dissolved into another liquid - reducing its partial pressure in the evaporator and allowing more liquid to evaporate.
  • Regeneration - The refrigerant-laden liquid is heated, causing the refrigerant to evaporate out. It is then condensed through a heat exchanger to replenish the supply of liquid refrigerant in the evaporator.

There are 3 substances used

  • Ammonia
    • A gas with a boiling point of -33°C at room temperature, but is pressurised to a point where it is liquid
  • Hydrogen gas
  • Water

These refrigerators use ammonia as their refrigerant, assisted by water and hydrogen to continuously cycle the ammonia. The refrigerator has five main components, refer to the drawing:

  • Generator - generates ammonia gas
  • Separator - separates ammonia gas from water
  • Condenser - where hot ammonia gas is cooled and condensed to create liquid ammonia
  • Evaporator - where liquid ammonia evaporates removing heat from inside the cabinet.
  • Absorber - where water absorbs the ammonia gas.

The cooling cycle starts with liquefied ammonia entering the evaporator at room temperature. The ammonia is mixed in the evaporator with hydrogen. The partial pressure of the hydrogen is used to regulate the total pressure, which in turn regulates the vapour pressure and thus the boiling point of the ammonia. The ammonia boils in the evaporator, providing the cooling required.

 

The next three steps exist to separate the gaseous ammonia and the hydrogen. First, in the absorber, the mixture of gasses enters the bottom of an uphill series of tubes, into which water is added at the top. The ammonia dissolves in the water, producing a mixture of ammonia solution and hydrogen. The hydrogen is collected at the top of the absorber, with the ammonia solution collected at the bottom.

 

The second step is to separate the ammonia and water. In the generator, heat is applied to the solution, to distill the ammonia from the water. Some water remains with the ammonia, in the form of vapour and bubbles. This is dried in the final separation step, called the separator, by passing it through an uphill series of twisted pipes with minor obstacles to pop the bubbles, allowing the collected water to drain back to the generator.

 

Finally the pure ammonia gas enters the condenser. In this heat exchanger, the hot ammonia gas is cooled to room temperature and hence condenses to a liquid, allowing the cycle to restart.

 

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