Claude on Question 7, Sep 17

A refrigerator doesn't create cold so much as it moves heat from inside the insulated box out into the room, and it does this by exploiting the fact that a liquid absorbs heat when it evaporates and releases heat when it condenses. The working substance, called the refrigerant, circulates continuously through a closed loop of tubing driven by an electric compressor.

Starting at the compressor, which usually sits at the bottom or back of the appliance, the refrigerant arrives as a cool low-pressure gas. The compressor squeezes it into a much smaller volume, which raises both its pressure and its temperature, so it leaves as a hot high-pressure gas. This gas then travels through the condenser, the black coils or flat panel on the back or underneath the unit. Because the refrigerant there is hotter than the surrounding kitchen air, heat flows out of the tubing into the room, often helped along by a small fan. As it sheds that heat, the refrigerant condenses into a warm high-pressure liquid.

Next the liquid is forced through a narrow constriction, either a capillary tube or an expansion valve. On the far side of this restriction the pressure is much lower, so the refrigerant expands and partly flashes into vapor, and its temperature plummets to well below the temperature inside the cabinet. This very cold mixture then passes through the evaporator coils, which line the freezer compartment or are hidden behind a panel there. Heat from the food and air inside the box flows into these cold coils, which is precisely what makes the interior cold, and that absorbed heat boils the remaining liquid refrigerant into a gas. A fan typically blows air over the evaporator and distributes the chilled air through ducts to the freezer and fresh food compartments. The now slightly warmed low-pressure gas returns to the compressor and the cycle begins again.

A thermostat or electronic sensor switches the compressor on and off to hold the interior near the set temperature, which is why you hear a refrigerator cycle rather than run constantly. Thick foam insulation and magnetic door gaskets slow the leakage of heat back in, reducing how often the compressor must run. Moisture from the air and from food condenses and freezes on the evaporator, so most modern units periodically run a small heater to melt that frost, letting the meltwater drip into a shallow pan near the compressor where the waste heat evaporates it away. The refrigerants themselves have changed over time, from ammonia and sulfur dioxide in early machines, to chlorofluorocarbons like Freon, and now to substances such as isobutane or tetrafluoroethane chosen because they are less damaging to the ozone layer and the climate.