What is the vapour-compression cycle?
Vapour-compression is the cycle almost every fridge, freezer and air conditioner runs on. A refrigerant is sealed in a closed loop of pipes and pushed round and round by a compressor, changing between liquid and gas to carry heat out of the cold space and dump it into the room.
Why does it need a compressor?
Heat only flows by itself from hot to cold, so moving heat the other way can never happen on its own. The compressor forces the refrigerant to a higher pressure so it becomes hotter than the room; only then will its heat flow out. Without that push, no heat would leave the cold space.
1. The evaporator absorbs heat
Inside the cold space the evaporator holds liquid refrigerant at a low pressure, so it boils at a very low temperature. As it boils it takes in heat from the food and air around it and turns into a cold, low-pressure vapour.
2. The compressor raises the pressure
The compressor pulls that vapour in and squeezes it. Squeezing a gas heats it and raises its pressure, so the vapour leaves the compressor as a hot, high-pressure gas — hotter than the room it is about to meet.
3. The condenser releases heat
The hot gas flows through the condenser coils outside, where moving air carries the heat away. Losing that heat makes the refrigerant condense back into a warm liquid.
4. The expansion valve drops the pressure
The warm liquid is forced through the expansion valve, a narrow restriction. As it squeezes through, its pressure plunges, so it flash-cools and partly boils, becoming a cold, low-pressure mixture.
5. The cycle repeats
That cold mixture re-enters the evaporator and the loop begins again. The compressor does the work and the refrigerant keeps circulating, continually moving heat from inside the cold space to the room outside.
Does a fridge make cold?
No. A fridge cannot create cold; there is no such thing as adding cold. It moves heat from inside out into the room, so the back of a fridge is always warm. Cold is simply what is left behind once the heat has been carried away.
Why does the back of a fridge feel warm?
That is the condenser doing its job. It is the part where the refrigerant gives up the heat it picked up inside, and that heat spreads into the room. A fridge actually warms the room slightly overall.
What does the compressor do?
The compressor raises the refrigerant's pressure, which also raises its temperature. That is essential: heat will only leave the refrigerant to the room once the refrigerant is hotter than the room.
What is the expansion valve for?
It is the pressure step. It forces the liquid refrigerant through a narrow opening so its pressure drops sharply. The sudden drop makes it flash-cool, and the cold mixture is ready to absorb heat again.
Can you have refrigeration without a compressor?
Yes — absorption fridges use heat instead of a compressor to drive the cycle, which is why they are silent. But the everyday fridge and air conditioner use the vapour-compression cycle because a compressor is efficient and compact.