The basic principle used for refrigeration is: Expansion of a gas, reduces its temperature.
» This concept is explained by the Joule-Thomson effect.
The second principle is: When two surfaces of different temperatures come in contact with each other, the surface that is at higher temperature cools and the surface at lower temperature warms.
» This concept is explained by the second law of thermodynamics.
The mechanism used to maintain a low temperature inside the refrigerator involves several components that perform variable functions. The various parts along with their functions have been listed below:
Refrigerant: This is the substance that performs the cooling function in the refrigerator through the changes in its temperature and pressure. The refrigerant commonly used is ammonia, sulfur dioxide, or hydrocarbons. Earlier, freon was used as a refrigerant whose use has been eliminated due to its adverse effect on the ozone layer (leads to ozone depletion). The liquid used as a refrigerant evaporates at a very low temperature, thus, resulting into a freezing temperature inside the refrigerator.
Evaporator: It is that part of the system that absorbs the heat inside the refrigerator with the help of the evaporating liquid refrigerant. Also called heat exchanger, this component draws heat impinging it or passing through it.
Heat-exchanging Pipes: The entire unit of the refrigerator is surrounded by a series of heat-exchanging coils. These coils carry the refrigerant from one part of the refrigerator to the other. Most of the heat-exchanging pipes are placed behind the compressor.
Compressor: It is a heavy metallic device that is powered by a motor and compresses the refrigerant. The main function of the compressor is to raise the pressure and thus, the temperature of the refrigerant gas.
Condenser: It is a part of the refrigerator that condenses the vaporized refrigerant (converts into liquid form) and reduces its temperature.
Expansion Valve: This component reduces the pressure on the liquid refrigerant. Usually the expansion valve is made up of thin copper coil.
The working of the refrigerator is a cyclic process that comprises changes in the temperature and pressure of the refrigerant. Given below is a step-by-step explanation of its working:
- The refrigerant gas is passed through a compressor. Here, the gas is compressed and thus, the pressure on it increases. The temperature of the refrigerant gas, in turn, increases as a result of the increased pressure, which then turns into superheated vapor.
- The refrigerant then passes through the heat-exchanging pipes and releases heat to the surroundings. Thus, it cools down due to the loss of heat to the surroundings.
- As the refrigerant passes through the condenser, its temperature reduces. However, the pressure on it remains the same. Due to the reduction in its temperature, it gets converted into its liquid form.
- The expansion valve causes a sudden reduction in the pressure on the refrigerant. Some of the refrigerant evaporates and expands. This expansion results in the lowering of the temperature of the refrigerant.
- During the evaporation of the liquid refrigerant, it is present in the evaporator, which absorbs the heat from the foodstuffs kept in the refrigerator and thus, cools them. The second law of thermodynamics is being used here.
- The refrigerant again heats up to form a gas. This refrigerant, which is now in the gaseous form, again enters the compressor and the cycle repeats.