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
Ice House
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
Yakhchal of Yazd
province
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
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 fridge
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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