Claude on Question 7, Sep 23

A refrigerator doesn't make cold so much as it moves heat, taking warmth from the food inside and dumping it into the room. It manages this trick by exploiting a simple fact of physics: when a liquid evaporates it absorbs a great deal of heat from its surroundings, and when a vapor condenses back into a liquid it releases that heat again. By forcing a special fluid called a refrigerant to evaporate inside the cabinet and condense outside it, the machine acts as a kind of heat pump, ferrying energy from a cold place to a warmer one.

The cycle begins at the compressor, the humming black canister usually tucked at the back or bottom of the appliance. It draws 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 air. This hot, pressurized vapor is pushed through the condenser, a network of thin coils or a flat panel on the outside of the cabinet where air can flow past. Because the refrigerant is now hotter than the kitchen, heat flows out of it into the room, and as it loses that energy the vapor condenses into a warm liquid still under high pressure.

Next the liquid reaches a restriction, either a narrow capillary tube or an expansion valve, which lets it trickle into a region of much lower pressure. The sudden drop in pressure allows the refrigerant to boil at a very low temperature, and part of it flashes into vapor immediately, chilling the mixture dramatically. This frigid fluid then flows through the evaporator coils, which sit inside the freezer compartment or behind its rear wall. Here the refrigerant is far colder than the food and air around it, so heat flows inward to it, and as it soaks up that heat the remaining liquid boils away entirely. The now cool, low-pressure vapor returns to the compressor, and the whole loop starts over, running continuously for as long as cooling is needed.

Several supporting parts make the system practical. A fan often blows air across the condenser to shed heat faster, and in frost-free models another fan circulates chilled air from the freezer into the fresh food compartment, with a damper controlling how much each section receives. A thermostat or electronic sensor monitors the interior temperature and switches the compressor on and off to hold a set range, which is why a refrigerator cycles rather than running all the time. Thick foam insulation and magnetic door gaskets slow the leakage of heat back in, and a defrost heater periodically melts the frost that builds up on the evaporator, with the meltwater draining to a shallow pan near the compressor where its own waste heat evaporates it away. The heat you feel radiating from the back of a refrigerator is precisely the heat removed from your food, plus the energy the compressor itself consumed in doing the work.