# How a refrigerator keeps its cool

## Jake Port explains the physics at work behind this kitchen workhorse.

The refrigerator has revolutionised the way we store food, keeping perishables fresh longer. It works by taking advantage of some clever principles of physics to whisk heat out of an enclosed space.

Imagine a hot oven. Your job is to remove the heat in the oven, but you can’t open the door and can’t have any openings to allow the heat to escape. Sounds impossible right? Well, by simply compressing and expanding some gas, it actually isn’t that difficult.

If you have ever had to pump something, you would have noticed that the pump heated the longer you used it. This is because gas, under pressure, reacts by increasing in temperature. You are, in essence, forcing more and more atoms of gas (in this case, air) into a smaller space.

Each individual atom of gas carries with it a little bit of energy, so when trillions of atoms are crammed into a smaller space, their collective energy is focused, raising the temperature inside the pump.

The opposite can be seen with compressed gas cans. Anyone who has used spray deodorant has probably noticed that the gas coming out is always cold. This is because the gas, which was forced into the tight confines of the can, is now being allowed to escape and expand. The energy of each atom is now spread out over a much larger space, cooling that area.

With these two elements in mind, let’s get back to understanding refrigeration.

The basic machinery consists of a pipe containing a gas that exists as a liquid at low temperature and a gas at higher temperatures. Known as the coolant, it spirals up and down each side of the insulation.

The journey starts inside the compressor, an electrically driven pump that squeezes air in much the same way as a bicycle pump.

When compressed, the coolant is at a high temperature and begins to travel inside a length of pipe that weaves its way up the back of the refrigerator. As it does so, it releases a lot of the heat that was built up as it was compressed.

At the top of the fridge, the gas reaches the expansion valve. This resembles a cone, with a tight opening that expands to a wider pipe that sits on the other side of an insulating wall.

This sudden opening allows the gas to expand, rapidly dropping the pressure and the temperature.

The gas is now at its coldest and so this is generally where the freezer portion of the refrigerator is placed, with the cooling pipe sitting just behind a cover.

The gas continues to flow down the pipe, weaving its way down behind a plastic panel and pulling the heat from inside the fridge as it goes.

At the bottom, the pipe returns through the insulating wall to the outside of the fridge and directs the semi-cool air back to the compressor, where the whole process begins again.

The compressor does not need to run constantly. Thanks to the insulation of the refrigerator door, cold air is trapped inside and hot air kept out.

Jake Port contributes to the Cosmos explainer series.
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