Dear Brody and Friends:

While a stove or oven produces heat, a fridge can’t actually produce something called “cold.” So, how does a refrigerator manage to keep all your food cool?

We have to engineer it. We must design a system that can carefully remove heat and put it somewhere else. That’s what I found out when I visited my friend Jake Leachman, an engineer at Washington State University.

Our world is made up of all kinds of matter in different forms. Think of a puddle on a hot day. This puddle is liquid water, but then it disappears. Of course, it doesn’t really disappear. The heat from the sun warms the liquid and turns it into a gas that’s invisible to the eye. The liquid evaporates.

Maybe you’ve experienced evaporation when you get out of the shower or bath. For the water to evaporate, it needed some of your body heat. In the process, maybe you felt cold.

A refrigerator uses a particular substance, called a refrigerant, to help move heat around. It’s made up of particular molecules that can change forms—it can be a gas or liquid. Let’s follow where it goes.

The refrigerant will pass through a few different parts and small pipes—some are inside the refrigerator and some are on the outside. Since it will make this journey over and over again, we could really start anywhere. But let’s start inside the back of the fridge near the bottom where there’s a little unit called a compressor.

Here, the refrigerant starts as a vapor, kind of like a mist. In the compressor, all the molecules that make up the vapor get compressed and squished together. When this happens, the vapor turns into a hot, high-pressure gas. This gas gets pushed up through a small tube, called the condenser coil, on the back of the fridge.

But as it moves up through the tube, the gas starts to lose some heat. Where does it go? Remember, heat moves from something with a higher temperature to a lower temperature. The kitchen air is cooler than the very hot gas. The heat from inside the pipe transfers out into the kitchen.

In fact, the gas in the coil loses so much heat that its molecules slow down and it changes, or condenses, into a liquid. Next, the high-pressure liquid flows into a valve, also known as a throttle.

As it goes through the throttle, the liquid relaxes to a lower pressure and very cold temperature. This liquid moves through another tube that leads into the fridge to a part called the evaporator coil.

The liquid that flows in the evaporator is the coldest thing in the fridge, even colder than the freezer compartment.

Since the liquid refrigerant is now colder than the inside of the fridge, the very cold liquid will take as much heat as it can out of the inside compartments. As the energy flows into the refrigerant, the liquid evaporates. Then, as a vapor, it flows back into the compressor and completes the cycle again and again. It’s a whole lot of science and engineering happening right in your kitchen.

Dr. Universe