To achieve a cooling effect the fridge relies on the simple notion of evaporation, absorbing heat when a liquid changes its state. This evaporation is the central principle of the refrigeration cycle, a perpetual loop in which a refrigerant is forced to change state in order to invoke heat absorption.
The cycle begins with the refrigerant in a vapour state, which is then pressurised in an internal compressor. This compression forces the refrigerant to heat up before being sent outside of the fridge into a condenser and expelled into the surrounding area, cooling the refrigerant vapour in the process and condensing it into a highly pressurised liquid state. This liquid is then sucked through an expansion valve and back into the low-pressure fridge compartment causing the refrigerant to boil, vaporise and drop in temperature, cooling the compartment in the process. The cycle then begins again, with the low-pressure refrigerant vapour being sucked up into the compressor.
Fridges through the ages
1700 BCE – Ice houses
Zimri-Lim, the king of Mari in Syria, ordered the construction of an ice house, which no previous king had ever built. Ice was collected from nearby mountains and stored in pits in the ground so it would remain cool. Ice houses were still used in the UK and USA right up until the 20th century.
1400 BCE – Evaporative coolers
Without access to ice, ancient Egyptians stored wine in earthenware jars called amphorae. They would leave the amphora outside during the cool nights, and slaves would sprinkle them with water. The cold wind caused the water to evaporate, slowly cooling the wine inside.
400 BCE – Yakhchal
To store ice in the desert, Persians built mud brick domes. In winter, water was led into channels underground and left to freeze. The ice was moved into the yakhchal, which had two parts: the dome and a pit. Warm air rose, leaving cold air underground to chill the ice.
1805 – Ice box
The ice harvesting industry took off in the 19th century and it became common for people to have an ice box in their home. They were made of wood, lined with metal and insulated with straw or cork. Ice was delivered every few days and placed inside to keep food from spoiling.
1748 – Artificial refrigeration
Artificial refrigeration was first demonstrated by Scottish chemist William Cullen, but in 1834 US inventor Jacob Perkins built the first refrigerating machine. However, early fridges were expensive and used toxic gases as refrigerants, making any faulty leaks deadly.
1920s – Domestic fridges
Early domestic fridges were still dangerous and cost more than a car, but soon a much safer refrigerant chemical called Freon was developed and the fridge soon became a common feature of most kitchens. Over the next few decades, they became even cheaper and more eco-friendly.
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