A refrigerator does not make cold. There is no such thing as cold to make. What it does is move heat out of the box and dump it into your kitchen, and it does that using a fluid that is deliberately chosen for how easily it changes between liquid and gas.
Once you see it as a heat pump rather than a cold generator, several things that seem odd about fridges stop being odd: why the back is warm, why the door being open heats the room, why it works harder in summer, and why you should not put a hot pan straight in.
The principle: evaporation takes heat with it
Step out of a swimming pool on a warm day and you feel cold. The air has not changed. Water evaporating off your skin needs energy to turn from liquid to vapour, and it takes that energy from the nearest available source, which is you. This is latent heat, and a fridge is a machine built to do that trick on a loop.
A liquid evaporating absorbs a substantial amount of heat. The same substance condensing back to liquid releases it again. If you can make the evaporation happen inside the fridge and the condensation happen outside it, you have moved heat from one place to the other. Everything else is plumbing.
The four parts of the loop
- The compressor. The motor you hear switching on. It takes low-pressure refrigerant gas and compresses it, which raises both its pressure and its temperature sharply. It is the only part that consumes significant electricity, and it is the pump that drives the whole cycle.
- The condenser. The black coils on the back or underneath. The hot, high-pressure gas flows through them, and because it is now hotter than your kitchen, it loses heat to the room. As it loses heat it condenses into a liquid, releasing its latent heat as it goes. This is why the back of a fridge is warm.
- The expansion valve. A deliberate narrow restriction. The high-pressure liquid is forced through it into a low-pressure region on the far side, and the sudden drop in pressure makes it very cold, partly flashing to vapour as it goes.
- The evaporator. The coils inside the cabinet, usually behind the back wall or around the freezer compartment. The cold liquid refrigerant is now colder than the food, so heat flows from the food into the refrigerant, and the refrigerant boils into a gas. That absorbed heat is the whole point.
The gas returns to the compressor, and the loop begins again. Nothing is used up. The refrigerant is a courier, carrying heat from inside to outside, over and over.
What this explains
- Leaving the door open heats the kitchen. The fridge is a net heat producer: it moves the heat from inside the box into the room, and adds the electrical energy the compressor consumed on top. An open fridge is a small heater with extra steps.
- It runs harder in summer. The condenser sheds heat by being hotter than the room. A hotter room means a smaller temperature difference, slower heat transfer, and a compressor that has to run longer.
- Do not block the back. Push a fridge tight against a wall and you choke the condenser. Most manufacturers ask for several centimetres of clearance, and ignoring it costs real electricity.
- Do not put hot food straight in. That is a large amount of heat introduced at once, which the compressor must then remove, while the released steam raises the humidity and encourages frost. Cool it on the counter first, but not for so long that it sits in the bacterial danger zone.
- Dusty coils cost money. A layer of dust on the condenser insulates it. Vacuuming it once or twice a year is the single easiest efficiency gain available.
- A full fridge is more efficient than an empty one. Cold food and drink hold their temperature, so opening the door loses less. This is why half-empty fridges are worth filling with bottles of water.
The freezer is the same machine, colder
A freezer uses the same cycle with the refrigerant expanded to a lower pressure, so it boils at a lower temperature. Many fridge-freezers run one compressor and one circuit for both compartments, which is why the fridge section can warm up when the freezer is working hard, and why a fault often affects both.
A note on refrigerants
The fluid has changed several times, and each change was driven by a problem with the last one. Early domestic fridges used ammonia and sulphur dioxide, which were dangerous when they leaked. CFCs replaced them and were safe indoors, but were found to destroy stratospheric ozone and were phased out under the Montreal Protocol from 1987. HFCs replaced those and do not harm ozone, but are potent greenhouse gases, and are themselves now being phased down. Most new domestic fridges use isobutane, which has a very low warming potential and is mildly flammable, which is why the charge in a domestic unit is deliberately small.
The rest of the kitchen
For a direct consequence of fridge temperature, see why bread goes stale faster in the fridge. For what the labels on the food inside actually mean, use by, best before, and the difference that matters. For heat travelling the other way, why a room feels colder than the thermostat says.
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