The refrigeration cycle uses a low boiling point coolant to produce coldness, preventing bacteria from spoiling food. The cycle has four stages: compression, heat release, expansion, and absorption. Understanding phase change is crucial. Ammonia was traditionally used but is toxic, while newer compounds like HFCs are non-toxic and effective. Refrigeration is a vital invention for food preservation.
The refrigeration cycle is a heat engine that works in reverse, known as a phase change heat pump. Using coolant that boils at a low temperature produces relative coldness, lowering the temperature of the refrigerator to a level that prevents bacteria from multiplying and spoiling food. A refrigeration cycle works on essentially the same principle that your hand feels cold when water evaporates from it. Other liquids, including some known as refrigerants, produce even lower temperatures when they evaporate.
The refrigeration cycle is a closed gas circuit which undergoes four stages. The first stage is the compressor, which compresses the refrigerant to raise its temperature. The gas is then routed through heat sink coils which release heat outside the refrigerator. As it dissipates heat, the refrigerant cools and condenses back into a liquid. This liquid then passes through a high/low pressure threshold, called an expansion valve, which causes it to expand and change phase into a gas. The cold gas circulates back into the refrigerator, absorbing heat from within, before being fed back into the compressor. The purpose of the freeze cycle is to take heat from inside the refrigerator and transfer it outside.
To better understand the refrigeration cycle, you need to know the idea of a phase change. An evaporating liquid remains at a constant temperature until it transforms the phase into a gas. Boiling water, for example, always remains at the temperature of its boiling point, 212°F (100°C). If a refrigerator could keep food fresh by making sure it stayed below 300°F (148°C), for example, instead of 40°F (4°C), water could be used as a refrigerant because the freezing point boiling water would be cold relative to the refrigeration threshold of 300°F.
The traditional refrigerant used in a refrigeration cycle is ammonia, but it is toxic to humans. Ammonia was replaced in the 1930s by chlorofluorocarbons (CFCs), but this compound was found to damage the ozone layer and was phased out in the 1970s. Newer compounds, such as hydrofluorocarbons (HFCs), cool effectively, are non-toxic and do not damage the ozone layer. The invention of refrigeration is probably one of the most important inventions of the 20th century, allowing food to be transported long distances or stored for long periods without spoiling.
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