What is a refrigerant?

Put your hand on the side of a running fridge. It often feels warm! That warmth is heat taken out of the food inside. It was carried out by a special fluid that flows round and round in the fridge’s pipes: a refrigerant.

A liquid that boils in the cold

Water boils at 100 °C. A refrigerant is chosen because it boils at a much lower temperature, usually far below 0 °C. Propane, one refrigerant, boils at about −42 °C. Every liquid has its own boiling point, and refrigerants have very low ones.

Why is that useful? When a liquid evaporates or boils, it soaks up heat from its surroundings. A refrigerant boils even when it is freezing cold. So it can soak up heat from cold things: food in a fridge, or even cold winter air.

A refrigerant is sometimes a liquid and sometimes a gas. A word for both is fluid: anything that can flow.

The refrigerant’s round trip

A refrigerant travels in a closed loop of pipes. It never gets used up, so it never needs refilling, unless a pipe leaks. On every trip around the loop, it goes through four steps.

1. It evaporates and soaks up heat. In a coil of thin pipes called the evaporator, the refrigerant is at low pressure. There it boils and turns into a gas. It is colder than the air around the coil, so heat flows into it.
2. It gets squeezed and heats up. A pump called the compressor squeezes the gas into a much smaller space. Squeezing a gas makes it hot. You can feel a little of this when you pump up a bike tyre: the pump gets warm.
3. It condenses and gives the heat away. The hot gas flows into another coil, the condenser. It is hotter than the air around it, so it gives its heat away. Under the high pressure, it condenses back into a liquid while it is still warm.
4. It expands and gets cold. The warm liquid squeezes through a tiny opening called the expansion valve. Behind it, the pressure drops, and part of the refrigerant boils at once. This makes the rest icy cold, ready to start the trip again.

In a fridge, the evaporator is inside, and the condenser is outside, at the back or in the sides. So heat moves from the food into the kitchen. An air conditioner moves heat from a room to the outdoors. A heat pump that heats a home does the opposite: it moves heat from the outdoor air into the house.

Which refrigerants are used?

Engineers give refrigerants code names that start with R, for “refrigerant”. Here are a few common ones:

• R-290 is propane, the same gas people use in camping stoves.
• R-600a is isobutane, a close cousin of propane. Most new fridges in Europe use it.
• R-744 is carbon dioxide (CO₂), the gas that makes the bubbles in fizzy drinks.
• R-32 is a man-made gas used in many air conditioners and heat pumps.
• R-134a is an older man-made gas, found in many older fridges and car air conditioners.

How refrigerants harmed the sky

The first fridges, about a hundred years ago, used gases such as ammonia. They worked, but they were poisonous, and a leak could make people very ill. So around 1930, chemists invented new refrigerants called CFCs. They were not poisonous and did not burn, so soon they were everywhere: in fridges, air conditioners and spray cans.

Many years later, scientists found a problem. High in the sky there is a thin layer of a gas called ozone. This ozone layer works like sunscreen for the whole Earth: it blocks much of the Sun’s harmful ultraviolet (UV) light. CFCs that leaked out drifted up to it and slowly destroyed it.

By the 1980s, a huge ozone hole opened above Antarctica every year. So in 1987, countries around the world signed an agreement called the Montreal Protocol, promising to stop making CFCs. It worked. Today the ozone layer is slowly healing.

Refrigerants and a warming planet

The refrigerants that replaced CFCs, such as R-134a, do not harm the ozone layer. But they have another problem. When they leak into the air, they trap heat around the Earth, much more strongly than carbon dioxide does. This makes the climate warmer.

So engineers are now switching to refrigerants that warm the climate much less, such as propane, isobutane and carbon dioxide. Laws in Europe and in many other countries are speeding up this change.