How does a refrigerator move heat?

The inside of a refrigerator is cold, yet its outside coils or panels can feel warm. Both observations come from the same process. A refrigerator carries thermal energy out of its food compartment and releases that energy into the room. Electricity supplies the work needed to keep this transfer going against its usual direction.

Left alone, energy transfers as heat from warmer surroundings into a colder compartment. Insulation slows that leakage but cannot reverse it. A typical household refrigerator uses a circulating substance, called refrigerant, whose temperature can be changed at different points along a sealed loop. The key is that a substance's boiling temperature depends on its pressure. Boiling does not always mean reaching the temperature of a saucepan of boiling water.

Inside the evaporator, pressure is low enough for the refrigerant to boil while colder than the food compartment. Energy passes from the warmer interior into the refrigerant. Much of that energy changes liquid into vapor rather than simply making the liquid hotter. The evaporating refrigerant takes energy away from the compartment, just as evaporation can cool a wet surface.

The compressor then squeezes the vapor, using electrical work to raise its pressure and temperature. The hot vapor flows into the condenser on the room side. Now the direction of the temperature difference has changed: the refrigerant is hotter than the surrounding air. It releases energy to the room and condenses into liquid. The compressor has created the conditions for heat to leave at the outside coil.

The liquid next passes through a narrow pressure-reducing device. Its pressure drops, some of it evaporates, and a colder mixture of liquid and vapor returns to the evaporator. The same refrigerant can make the journey again. The pipes carry refrigerant around the loop; heat crosses the pipe walls at the two coils. Food, room air and refrigerant do not need to mix.

An energy account reveals why the warm side matters. If one example cycle removes 30 joules from the compartment and receives 10 joules of work, it releases 40 joules to the room. The extra 10 joules did not vanish. This is also why an open refrigerator cannot cool the whole kitchen: it would take energy from the room at one coil and return that energy plus the compressor's work at the other. The same principle lets a heat pump warm a home by collecting thermal energy outdoors.