Claude on Question 7, Sep 20

A refrigerator works by moving heat from inside the insulated box to the room around it, which sounds backward but is exactly what makes it possible to keep food cold. It does this using a circulating fluid called a refrigerant, chosen because it boils and condenses at convenient temperatures, and four main components arranged in a closed loop: a compressor, a condenser, an expansion device, and an evaporator.

The cycle begins at the compressor, usually the humming black canister at the back or bottom of the appliance. It takes in low-pressure refrigerant vapor and squeezes it, which raises both its pressure and its temperature so that the gas becomes hotter than the surrounding kitchen air. This hot, high-pressure gas then travels through the condenser, the coils or flat panel on the back or underneath, where the room air carries heat away. Having given up that heat, the refrigerant condenses into a warm liquid, still under high pressure.

Next the liquid passes through a restriction, either a narrow capillary tube or an expansion valve, which drops its pressure sharply. When pressure falls, the boiling point falls with it, so the refrigerant emerges as a very cold mixture of liquid and vapor. This cold fluid flows through the evaporator coils inside the freezer compartment, where it is now colder than the food and air around it. Heat therefore flows naturally from the compartment into the refrigerant, boiling the liquid into vapor and chilling the interior in the process. Many refrigerators blow air over these coils with a small fan and duct some of it into the fresh-food section, which is why the freezer and refrigerator temperatures are linked. The resulting low-pressure vapor returns to the compressor, and the whole sequence repeats.

Two other things make the system practical. 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 monitors the interior temperature and switches the compressor on and off to hold it near the setting you choose. Moisture from food and from air entering when you open the door freezes onto the evaporator, so most modern units include a small heater and timer that periodically melt that frost, letting the water drain into a shallow pan where warmth from the condenser and compressor evaporates it away.

The underlying principle is simply that a fluid absorbs heat when it evaporates and releases heat when it condenses. By using a compressor and a pressure drop to control where each of those changes happens, the refrigerator forces evaporation to occur inside the cold box and condensation to occur outside it. Energy from the electrical supply pays for this uphill transfer of heat, which is why the back of a refrigerator feels warm and why running one with the door open would warm your kitchen rather than cool it.