Claude on Question 7, Sep 26

A refrigerator doesn't create cold; it moves heat from inside the insulated cabinet to the room outside. That sounds backwards, since heat naturally flows from hot things to cold ones, so the machine has to be driven by an outside source of energy, which is why your refrigerator is plugged into the wall. The work supplied by the motor is what allows heat to be pumped uphill, from the cool interior to the warmer kitchen.

The trick is accomplished with a fluid called a refrigerant, which circulates endlessly through a closed loop of tubing and changes back and forth between liquid and vapor. The key physical fact being exploited is that a liquid absorbs a large amount of heat when it boils into a gas, and releases that same heat when it condenses back into a liquid. Whether the refrigerant boils or condenses at a given temperature depends on its pressure, so by controlling pressure in different parts of the loop, the machine can make the refrigerant absorb heat where you want cooling and release it where you want to dump the heat.

Follow a bit of refrigerant around the circuit. It arrives at the evaporator, a set of coils inside or behind the walls of the food compartment, as a cold low pressure mixture of liquid and vapor. Because the pressure there is low, it boils at a temperature below that of the food compartment, so heat flows from the air and the food into the refrigerant, chilling the interior and turning the liquid entirely into vapor. That cool vapor is then drawn into the compressor, usually a small sealed pump at the bottom of the appliance, which squeezes it to a much higher pressure. Compression also heats the vapor, so it leaves the compressor hot, hotter than the room.

Next the hot high pressure vapor passes through the condenser, the black coils or grille on the back or underside of the cabinet. Because this gas is hotter than the surrounding air, heat flows out of it into the kitchen, often helped along by a fan, and the refrigerant condenses into a warm liquid. This is why the back of a refrigerator feels warm and why running one with the door open heats a room rather than cooling it: everything the appliance removes from inside, plus the energy consumed by the motor, ends up in the room.

Finally the warm high pressure liquid squeezes through a restriction, either a narrow capillary tube or an expansion valve, into the low pressure region again. Its pressure drops abruptly, part of it flashes into vapor, and the whole mixture becomes very cold. In that state it enters the evaporator to begin the cycle over, and the process repeats as long as the compressor runs.

Everything else in the appliance supports this basic cycle. Thick foam insulation and door gaskets slow the leakage of heat back in, a thermostat senses the interior temperature and switches the compressor on and off to hold it near the setting you choose, and in frost free models a small heater periodically melts ice off the evaporator coils so that the frost does not act as an insulating blanket. The freezer and fresh food compartments are typically cooled by the same evaporator, with a fan and adjustable vents distributing more or less cold air to each.