Claude on Question 7, Sep 29

A refrigerator doesn't actually create cold; it moves heat from the inside of the cabinet to the room around it. The whole machine is essentially a heat pump, and it works by exploiting a simple fact about liquids: when a liquid evaporates it absorbs heat from its surroundings, and when a vapor condenses back into a liquid it releases that heat again. By forcing a substance to evaporate inside the fridge and condense outside it, the appliance ferries warmth out of the food compartment and dumps it into your kitchen.

The substance that does this shuttling is called a refrigerant, a fluid chosen because it boils at a conveniently low temperature. It circulates endlessly through a closed loop of tubing that passes through four main components. The cycle begins at the compressor, the humming black pump usually bolted to the bottom rear of the cabinet. The compressor takes in low-pressure refrigerant vapor and squeezes it, which raises both its pressure and its temperature, so the gas leaving the compressor is hotter than the surrounding air.

That hot, pressurized gas then travels through the condenser, the network of thin coils or the flat black panel on the back or underside of the fridge. Because the gas is hotter than the room, heat flows out of it into the air, sometimes helped along by a small fan. As it sheds that heat, the refrigerant condenses into a warm liquid, still under high pressure. This is why the back of a refrigerator feels warm and why the machine slightly heats the room it stands in.

Next the liquid reaches a deliberate bottleneck, either a narrow capillary tube or an expansion valve. Passing through this restriction into a region of much lower pressure causes the refrigerant to expand and its temperature to plummet, so that it emerges as a cold, misty mixture of liquid and vapor. This chilled refrigerant then flows through the evaporator coils, which line the freezer compartment or sit behind an inner wall. Here the refrigerant is colder than the food and air inside, so heat flows into it, boiling it back into a gas and cooling the compartment in the process. In most modern units a fan blows air across these coils and circulates it through the fresh food section as well. The resulting low-pressure vapor returns to the compressor, and the cycle begins again.

Everything else in the appliance supports this basic loop. Thick foam insulation and magnetic door gaskets slow the leak of outside warmth 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 set point. Because the evaporator runs below freezing, moisture from the air condenses and freezes on it, so a defrost heater periodically melts that frost, and the meltwater drains into a shallow pan near the compressor where the warmth of the machine evaporates it away.