Claude on Question 7, Sep 16

A refrigerator doesn't create cold so much as it moves heat from one place to another, pumping warmth out of the insulated box and dumping it into the surrounding room. The whole thing depends on a simple physical fact: when a liquid evaporates it absorbs a great deal of heat from its surroundings, and when a vapor condenses back into liquid it releases that heat again. By arranging for a substance to evaporate inside the cabinet and condense outside it, you can carry heat across the wall of the box continuously.

The substance doing this work is called a refrigerant, a fluid chosen because it boils at a conveniently low temperature. It circulates endlessly through a closed loop of tubing driven by a compressor, the humming motor usually tucked into the back or bottom of the appliance. The compressor takes in low-pressure refrigerant vapor and squeezes it, which raises both its pressure and its temperature so that it becomes hotter than the kitchen air. This hot gas then flows through the condenser, the black coils or flat plate on the back or underside of the unit, where the room air carries heat away. As it loses that heat, the refrigerant condenses into a warm high-pressure liquid.

Next the liquid passes through a narrow restriction, either a thin capillary tube or an expansion valve, which is the pivotal moment in the cycle. On the far side of that restriction the pressure is much lower, and when the liquid is suddenly allowed to expand into that low-pressure region its boiling point drops dramatically, so it begins to flash into vapor and becomes intensely cold. This frigid mixture travels through the evaporator coils, which line the freezer compartment or sit behind an interior panel. There it is colder than the food and air around it, so heat flows naturally into it, and as the refrigerant absorbs that heat it finishes boiling into a gas. A fan often blows air across these coils and circulates it through the compartments, and moisture from that air freezes onto the coils as frost, which is why self-defrosting models periodically warm them with a small heating element and drain the meltwater into a pan near the compressor, where it evaporates.

The now cool low-pressure vapor returns to the compressor and the cycle begins again. A thermostat or electronic sensor watches the interior temperature and switches the compressor on and off to hold it near the set point, which is why a refrigerator runs intermittently rather than constantly. Thick foam insulation in the walls and magnetic gaskets around the doors slow the leakage of heat back in, reducing how often the compressor must run. Because the machine must be paid for in electricity, and because the energy consumed by the compressor also ends up as heat, a refrigerator releases more warmth into the room than it removes from its contents, which is why a kitchen gets slightly warmer rather than cooler when the appliance is working hard.