The fridge
How does a fridge make cold?
The question
A fridge keeps your food cold all day with the door shut. Where does the cold come from?
Have a guess, then show the answer
It doesn't make cold. It moves heat out. A fridge is an insulated box plus a heat pump. A refrigerant goes round a closed loop, soaking up heat inside the box and giving it away through the coils on the back, into your kitchen.
Play
Peel the fridge
Lift the layers off one at a time, then watch the refrigerant go round the loop. Red is hot gas after the compressor, orange is cooling in the coils, blue is ice cold after the valve. Tap any part to see what it does, and slide the compressor work to see where the heat goes.
Tap any part of the fridge to see what it does.
- Heat taken from inside
- 160 W
- Work from the compressor
- + 80 W
- Heat into the kitchen
- = 240 W
Peel every layer and run the loop to watch the refrigerant carry the heat out.
Illustration only: the watts are made-up example numbers with an example COP of 2, not figures for a real fridge. The balance Q_hot = Q_cold + W always holds.
What's happening
- 1
The evaporator. Behind the inside back wall, cold liquid refrigerant at low pressure runs through pipes. It boils into a gas, and boiling soaks up heat from the air and the food inside.
- 2
The compressor. At the bottom of the back, a pump squeezes that gas into a much smaller space. Squeezing a gas makes it hot, hotter than your kitchen.
- 3
The condenser. The hot gas runs through the coils on the back. Because it is hotter than the room, it gives its heat to the kitchen air, and as it cools it turns back into a liquid.
- 4
The expansion valve. The liquid is forced through a tiny opening. On the other side the pressure drops, part of it flashes to vapour, and what is left goes ice cold.
- 5
The loop repeats. The cold liquid goes back into the evaporator to soak up more heat. The foam walls slow down the heat leaking back in, so the pump does not have to run all the time.
The maths
Energy is never made or destroyed. Each second, the heat that leaves out of the back is the heat taken from inside plus the work the compressor puts in. How well a fridge does is the heat it moves per unit of work, its coefficient of performance (COP).
So the back of a fridge always gives off more heat than it took from your food. If a fridge moved 2 J of heat out for every 1 J of work (COP 2, an example), it would put 3 J into the kitchen.
Simplified, and sources
The short and the widget are a simplified toy model, not a real fridge and not to scale. The COP of 2 used in the widget and the maths is a round example number to show the balance, not a measured figure for any fridge. The watt numbers in the widget are illustration only. Real fridges often hide the condenser under the floor or in the side walls, use a thin capillary tube instead of a separate valve, and switch the compressor on and off with a thermostat. A fan, a defrost heater and the door seals are left out.