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Household refrigerators, which first appeared around 1913, are very common today. Although not a purely electrical technology, most depend on an electric motor to run, and in many people’s homes, the refrigerator is one of the heaviest users of electricity. However, the process the refrigerator uses to keep things cool is not necessarily dependent on electricity. Steam-powered industrial refrigerators were once available, and some types of refrigerators have no motor at all.
The key to refrigeration is the fact that an evaporating liquid tends to draw away heat from its surroundings as it evaporates. You can feel the effect of this on your skin if you wet it and then stand in a breeze. The wind accelerates the evaporation of the water, so your skin will feel cooler than it would if it were not wet. A refrigerator takes advantage of this principle, too.
Hidden inside the walls of the refrigerator is a network of tubes and reservoirs containing a special refrigerant liquid which evaporates at a very low temperature. If you removed it from the sealed system, it would disappear before your eyes as it changed from liquid to gas. A motor-driven compressor pumps the liquid from one area, called the condenser (usually located underneath or in back), to another area called the evaporator. The room inside the evaporator is larger than that in the condenser, and the refrigerant instantly evaporates because it has lots of space to expand. As it evaporates, it sucks in heat from inside the refrigerator, making the inside of the refrigerator cool. Then the gas is forced into the condenser under pressure, where it once again becomes a liquid. As it does so, it expels the heat that it collected, usually into the surrounding air. A fan blows air on the condenser to draw the heat away and into the room. Then the cycle begins again until a thermostat inside the refrigerator switches off the motor.
In some kinds of refrigerators, there is no motor or compressor. From the 1930s to the late 1950s, the “gas” refrigerator provided an alternative. In a gas refrigerator, the refrigerant circulates from the condenser to the evaporator and back all on its own. Inside, a tiny plume of burning propane, natural gas, kerosene, or other fuel heats the liquid refrigerant, causing it to evaporate even more quickly and forcing it into the evaporator under high pressure. This provides the same thing as the pumping action of a compressor.
In the 1980s, refrigerators became available which do not need a refrigerant at all. They use a curious feature of certain electronic devices called the Peltier effect. A Peltier device is somewhat like a specialized kind of transistor, but instead of being an amplifier, it has the odd property of getting cold when a current passes through it. But neither the gas nor the electronic refrigerators are often used today.
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