Phase transition. Evaporation. Condensation. And how heat transfer made refrigerators indispensable.
Ice cream. Cold soda. Fresh meats. We take these for granted at home and in restaurants, thanks in no small way to the refrigerator. The refrigerator has become an indispensable part of many homes, kitchens and restaurants, be it to freeze food, keep food fresh, or to prevent food from spoiling.
The idea behind the refrigerator is a simple one: transfer the heat from one place and put it somewhere else so that cooling can take place.
First start with a liquid that evaporates easily. Call it a refrigerant. The liquid refrigerant moves around in the evaporator coils inside the refrigerator. As it does so, it draws in energy in the form of heat from different compartments of the refrigerator. This causes it to boil and evaporate into a gas. Thus what we have is a cool refrigerator but a hot gas.
The compressor then pushes the hot gas out into the condenser coils outside the refrigerator. As the temperature outside is lower than the temperature of the hot gas, the gas refrigerant condenses back into a liquid. As it condenses, it loses heat to the surroundings (if you stand near these external coils, you can feel it is warmer).
The now-liquid refrigerant re-enters the evaporator coils. It captures heat as it moves past the different compartments of the fridge again, cooling them down before exiting as a hot gas. The cycle thus repeats itself, making it possible for the fridge to keep all varieties of food cold.
The change of a liquid into a gas, and vice-versa, is known as a phase transition. During a phase transition, be it from gas to liquid or liquid to gas, the heat is absorbed and lost without a change in temperature (this is called latent heat).
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(photos: in order – depositphotos/pitrs10; edesignua; belchonock