Claude on Question 7, Sep 12

A refrigerator doesn't actually make cold; it moves heat from the inside of the cabinet to the room around it, which is why the back or bottom of the appliance feels warm when it's running. The trick that makes this possible is a working fluid called a refrigerant, a substance chosen because it boils and condenses at convenient temperatures and pressures. By pushing this fluid around a closed loop and changing its pressure at the right moments, the machine forces it to absorb heat where you want things cold and release that heat somewhere else.

The cycle has four essential stages. Inside the compartment, or in the walls surrounding the freezer, sits a set of tubes called the evaporator. The refrigerant arrives there as a cold, low-pressure mixture of liquid and vapor, colder than the food and air around it, so heat naturally flows from the contents of the fridge into the tubes. That heat makes the liquid refrigerant boil into a gas, and because evaporation soaks up a great deal of energy without the fluid's temperature rising much, this step removes heat very efficiently. The now-warmed gas is drawn away to the compressor, usually a sealed pump driven by an electric motor at the back of the appliance. The compressor squeezes the gas into a much smaller volume, which raises both its pressure and its temperature considerably, and it is this input of electrical work that lets the whole arrangement push heat "uphill" from a cold place to a warmer one.

From the compressor the hot, high-pressure gas flows into the condenser, the black grille of tubing on the back or underneath the unit, sometimes helped along by a small fan. Because the gas is now hotter than the kitchen air, heat flows out of it into the room, and as the refrigerant loses that heat it condenses back into a liquid, still under high pressure. The liquid then passes through a restriction, either a narrow capillary tube or an expansion valve, which lets it squirt into the low-pressure side of the loop. The sudden drop in pressure lets some of the liquid flash into vapor, and that expansion and partial evaporation chill the mixture dramatically, so it enters the evaporator cold once again and the cycle repeats.

Everything else in the appliance supports this basic loop. Thick foam insulation and magnetic door gaskets slow the leakage of heat back in, so the compressor only has to run intermittently. A thermostat or electronic sensor watches the interior temperature and switches the compressor on and off to hold it near the setting you chose. In a combined fridge and freezer, a fan often circulates air from the coldest evaporator area into the fresh food compartment through adjustable vents. Frost-free models periodically warm the evaporator with a small heater to melt accumulated ice, and the resulting water drips into a shallow pan near the compressor, where the waste heat evaporates it away. Household air conditioners and heat pumps work on exactly the same principle, differing mainly in which side of the loop faces the space you care about.