Science Puzzle
How Does a Fridge Make Things Cold?
Put your hand near the grille on the back of a fridge and it's warm. The inside, meanwhile, sits at about 4 °C in a kitchen at 21 °C. Heat normally flows from hot things to cold things, never the other way, yet a fridge keeps pushing heat out of its cold inside into the warmer room.
How does a fridge pump heat from a cold place to a warm one?
The Answer
It uses a fluid that boils inside the fridge and condenses again outside it, carrying heat out with it on every lap.
Boiling takes in heat. That's why a wet swimmer shivers: the water evaporating from their skin draws heat out of them. A fridge's pipes hold a special fluid, the refrigerant, that boils at a very low temperature when its pressure is low. After squeezing through the narrow valve, the refrigerant arrives in the pipes inside the fridge at low pressure and boils, soaking up heat from the food compartment as it turns to vapour.
The vapour then flows to the compressor, the part that hums. Squeezing a gas heats it; the compressor squeezes the vapour until it's hotter than the kitchen. Now heat can flow the normal way, from the hot grille on the back into the cooler room, and as the vapour gives up its heat it condenses back into a liquid. Through the narrow valve again, the pressure drops and the lap begins once more.
So a fridge doesn't make cold. It moves heat uphill, from cold to warm, and the electricity pays for the lift. The heat coming off the back is the heat taken out of your food plus the energy the compressor used. A heat pump for warming a house is the same machine turned round: it pulls heat out of the cold air outside and pumps it indoors.
The principle: Heat pumps. A liquid absorbs heat when it boils and gives heat out when it condenses, so by boiling a refrigerant inside the fridge and condensing it outside, using a compressor to do the work, heat can be moved from a colder place to a warmer one.