Claude on Question 7, Oct 3

A refrigerator doesn't actually make cold; it moves heat from inside the insulated box to the room around it. This seems to run against the ordinary tendency of heat to flow from hot things to cold things, so the machine has to do work to force heat in the opposite direction, which is why a refrigerator needs to be plugged in and why the air near it feels warm.

The trick is accomplished with a fluid called a refrigerant, which circulates in a closed loop of tubing and is chosen because it boils and condenses at convenient temperatures when its pressure is manipulated. The key physical fact is that a liquid absorbs a great deal of heat when it evaporates and releases that same heat when it condenses back into a liquid, much as sweat cools your skin as it dries. A refrigerator simply arranges for the refrigerant to evaporate inside the cold compartment and condense outside it.

Follow the loop starting at the compressor, the humming pump usually tucked at the back or bottom of the cabinet. It takes in low-pressure refrigerant vapor and squeezes it into a small volume, which raises both its pressure and its temperature so that the vapor becomes hotter than the kitchen air. This hot, compressed gas then travels through the condenser, the coil of black tubing on the back or underside of the appliance, where the surrounding air carries heat away. Having given up that heat, the refrigerant condenses into a warm, high-pressure liquid.

Next the liquid passes through a restriction, either a narrow capillary tube or an expansion valve, which separates the high-pressure side of the system from the low-pressure side. Emerging into the wider, low-pressure tubing beyond it, the liquid suddenly finds itself free to expand and begins to boil, dropping to a temperature well below that of the food compartment. These low-pressure tubes, the evaporator, run through the freezer or along the back wall of the cabinet, and as the refrigerant evaporates there it soaks up heat from the air, the walls, and the food inside. A small fan usually blows air across the evaporator and circulates it through the compartments so the cooling is spread evenly. The now-warmed vapor returns to the compressor, and the cycle repeats indefinitely.

Several supporting features make the arrangement practical. Thick foam insulation and magnetic door gaskets slow the leakage of heat back into the box, so the compressor only has to run intermittently. A thermostat senses the interior temperature and switches the compressor on and off to hold it near the setting you choose, typically a few degrees above freezing in the fresh-food section and well below freezing in the freezer, which is colder because it sits closest to the evaporator or receives a larger share of the chilled airflow. Because moisture from the air and from food condenses and freezes on the cold evaporator, most modern units include a small electric heater that melts the accumulated frost periodically, and the resulting water drains into a shallow pan near the compressor where the warmth of the machinery evaporates it away.