Claude on Question 7, Sep 13

A refrigerator does not create cold; it moves heat from inside the insulated cabinet to the room outside. This may seem to violate common sense, since heat naturally flows from hot places to cold ones, but the trick is that the machine uses electrical energy to force the transfer in the opposite direction, much as a pump can push water uphill.

The work is done by a substance called a refrigerant, a fluid chosen because it boils and condenses at convenient temperatures and pressures. It circulates continuously through a sealed loop of tubing, changing back and forth between liquid and vapor, and it is these phase changes that carry the heat. When a liquid evaporates it absorbs a surprisingly large amount of energy from its surroundings, and when the vapor condenses again it releases that same energy. A refrigerator arranges for the evaporation to happen inside the cold compartment and the condensation to happen outside it.

Following the cycle around, the refrigerant enters the evaporator coils near or inside the food compartment as a cold, low pressure mixture of liquid and vapor. Because it is colder than the air and food around it, heat flows into it, and this heat boils the liquid away into vapor. The now slightly warmed vapor is drawn into the compressor, usually a small motor driven pump at the back or bottom of the appliance, which squeezes it into a much smaller volume. Compression raises both its pressure and its temperature, so the vapor leaves the compressor hot, hotter than the kitchen air.

That hot gas then passes through the condenser, the black coils or plate on the back or underside of the cabinet, where it gives up heat to the room, often helped by a fan or simply by natural air currents. Losing this heat makes the vapor condense into a warm high pressure liquid. The liquid next passes through a narrow restriction, either a thin capillary tube or an expansion valve, which throttles its flow and drops its pressure sharply. With less pressure holding it together, some of the liquid flashes into vapor immediately and the mixture becomes very cold, whereupon it enters the evaporator again and the cycle repeats indefinitely.

Around this core cycle sit the supporting parts. Thick foam insulation and magnetic door gaskets slow the leakage of heat back into the cabinet so the compressor need not run constantly. A thermostat senses the internal temperature and switches the compressor on and off to hold it near the chosen setting. Most modern units also run a fan to circulate air inside, and a defrost heater that periodically melts frost from the evaporator, the meltwater draining to a shallow tray near the warm condenser where it simply evaporates away. The faint hum you hear is the compressor, and the warmth you feel behind the appliance is the heat that once belonged to your food, together with the energy the motor consumed in moving it there.