Claude on Question 7, Sep 11

A refrigerator does not create cold; it moves heat from inside the cabinet to the room outside. The trick that makes this possible is that a liquid absorbs a great deal of heat when it boils into vapor, and releases that heat again when it condenses back into liquid. A refrigerator simply arranges for a special fluid, called a refrigerant, to boil inside the food compartment and condense outside it, over and over, in a closed loop of tubing.

The cycle has four main stages. It begins at the compressor, the humming motor-driven pump usually mounted at the back or bottom of the appliance. The compressor takes in low-pressure refrigerant vapor and squeezes it into a much smaller volume, which raises both its pressure and its temperature, so the gas leaves the compressor hot. This hot gas then travels through the condenser, the black coils or flat panel on the back or underside of the refrigerator. Because the gas is now hotter than the kitchen air, heat flows naturally from the coils into the room, often helped along by a small fan. As it gives up that heat, the refrigerant condenses into a warm high-pressure liquid.

Next the liquid is forced through a restriction, either a narrow capillary tube or an expansion valve, which drops its pressure sharply. At this low pressure the refrigerant's boiling point falls far below the temperature inside the cabinet, so when it enters the evaporator coils, hidden behind the interior walls or in the freezer compartment, it immediately begins to boil. Boiling requires energy, and that energy is drawn out of the air, the walls, and the food inside, which is precisely why the interior gets cold. The refrigerant emerges as a cool low-pressure vapor and returns to the compressor, where the whole sequence begins again.

Several supporting parts keep the process practical. A thermostat or electronic sensor monitors the interior temperature and switches the compressor on and off to hold it near the desired setting, which is why a refrigerator runs intermittently rather than continuously. Thick foam insulation and magnetic door gaskets slow the leakage of heat back in, reducing how often the compressor must run. In most modern units a fan circulates air between the freezer and the fresh food compartment, and a small heater periodically melts frost off the evaporator coils, the meltwater draining into a shallow pan near the compressor where the waste heat evaporates it away.

The net effect is that electrical energy is used to pump heat uphill, from a cold place to a warmer one, something heat will not do on its own. Because the energy consumed by the compressor also ends up as heat, a refrigerator always releases slightly more heat into the kitchen than it removes from its contents, which is why leaving the door open never cools a room.