Claude on Question 7, Sep 14

A refrigerator doesn't actually make cold; it moves heat from the inside of the insulated box to the room around it. That sounds like it should be impossible, since heat naturally flows from hot things to cold things, but by doing mechanical work on a special fluid and manipulating its pressure, a refrigerator can persuade heat to travel uphill, from a cold interior to a warmer kitchen.

The trick relies on a substance called a refrigerant, chosen because it boils and condenses at convenient temperatures when its pressure is adjusted. The refrigerant circulates continuously through a sealed loop of tubing, and at different points in that loop it exists as a liquid, a vapor, or a mixture of both. The key physical fact is that a liquid absorbs a large amount of heat when it evaporates, and releases that same heat when it condenses back into a liquid. The refrigerator arranges for evaporation to happen inside the cabinet and condensation to happen outside it.

Following the loop, the cycle begins at the compressor, the humming pump 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 until it is hotter than the surrounding room air. This hot, high-pressure gas then travels through the condenser, a network of thin coils or a flat plate on the outside of the cabinet where the refrigerant can shed heat to the room. As it gives up that heat, the vapor condenses into a warm, high-pressure liquid. If you feel the back of a running refrigerator and notice it is warm, that is the heat from your food and drinks being dumped into your kitchen, along with the extra heat added by the compressor's work.

The liquid next passes through a restriction, either a narrow capillary tube or an expansion valve, which is the heart of the whole arrangement. On the far side of that restriction the pressure is much lower, so the liquid suddenly finds itself able to boil at a very low temperature, and part of it flashes into vapor, chilling the whole stream dramatically. This frigid mixture flows into the evaporator coils, which sit inside the freezer compartment or behind an interior panel. There the refrigerant is colder than the air and contents of the cabinet, so heat flows into it, and the remaining liquid boils away completely. Having absorbed that heat, the refrigerant returns as a cool low-pressure vapor to the compressor, and the cycle repeats indefinitely.

Several supporting parts make the system practical. Thick foam insulation and magnetic door gaskets slow the leakage of heat back in, so the compressor does not have to run constantly. A thermostat or electronic sensor monitors the interior temperature and switches the compressor on and off to hold it near the desired setting. A small fan usually blows air across the evaporator and distributes the chilled air through ducts to the freezer and fresh food compartments, and another fan may help cool the condenser. Because the cold coils also condense moisture out of the air as frost, most modern units periodically warm the evaporator with a small heater and let the meltwater drain into a shallow pan, where the condenser's waste heat evaporates it away. The overall result is a machine that runs on electricity, consumes a modest amount of work at the compressor, and uses it to shuttle a much larger quantity of heat out of your food and into the room.