Claude on Question 7, Sep 21

A refrigerator doesn't actually create cold; it moves heat from inside the insulated box to the room outside. That sounds backwards, since heat naturally flows from hot places to cold ones, so the machine has to do work to push it in the wrong direction. It accomplishes this with a fluid called a refrigerant, which circulates endlessly through a closed loop of tubing and changes back and forth between liquid and gas along the way.

The key to the whole arrangement is that a liquid absorbs a great deal of heat when it boils into vapor, and releases that same heat when it condenses back into liquid. Just as important, the temperature at which a fluid boils depends on its pressure: squeeze it and it will condense at a relatively high temperature, let it expand and it will boil at a very low one. By controlling the pressure in different parts of the loop, the refrigerator can make the refrigerant boil in the cold place and condense in the warm place, which is exactly what is needed to carry heat from one to the other.

Following the cycle, start at the compressor, the humming pump usually tucked at the bottom or back of the cabinet. It takes in low-pressure refrigerant vapor and squeezes it, which raises both its pressure and its temperature so that it is now hotter than the kitchen air. This hot gas travels through the condenser, the coils or black panel on the back or underside of the appliance, where air passing over the metal carries heat away. Losing that heat, the refrigerant condenses into a warm high-pressure liquid.

That liquid then passes through a narrow restriction, either a thin capillary tube or an expansion valve, which lets it escape into a region of much lower pressure. The sudden drop in pressure causes part of it to flash into vapor and the whole mixture to become extremely cold, colder than the inside of the freezer. It now flows through the evaporator, the coils hidden in the walls or behind a panel inside the compartment, where it soaks up heat from the food and air and boils completely into vapor. A fan usually blows air across these coils and through the compartments to spread the cooling around. The cool low-pressure vapor returns to the compressor, and the cycle begins again.

A thermostat or sensor watches the interior temperature and switches the compressor on and off to keep things in the desired range, which is why a refrigerator runs in bursts rather than continuously. 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 freezes onto the evaporator coils over time, so most modern units include a small heater that periodically melts this frost, sending the water down a tube to a shallow pan near the compressor where the warmth of the machinery evaporates it away. The heat released out the back is always more than the heat removed from inside, because the electrical energy used by the compressor ends up as heat too, which is why leaving the door open will never cool a room.