How Does a Refrigerator Work?

On this website, I discuss specific steps to improve refrigerator efficiency, tips for buying a new energy efficient refrigerator, and potential benefits of buying refurbished refrigerators. However, if you find yourself confused about exactly why all of these metrics relate to efficiency, it may help to understand a little about how a refrigerator works.


Standard Refrigeration Cycle

Standard Refrigeration Cycle

The most basic explanation is that it transfers heat from inside the refrigerator cabinet to outside; it does not “create” cold air. The way it performs this transfer is through the flow of a refrigerant. You may think of a refrigerant as a cold fluid, but that is not entirely true. It is cold when it runs through the coils inside the fridge but quite warm when running through coils outside the fridge. To understand exactly how it does this, you need to get familiar with the basic refrigerator parts.

Heat Exchanging Coils

You can see these on the back of smaller refrigerators and also maybe inside, perhaps next to the ice tray. The important thing to know is that if you could follow a single strand of the coil through all of its twists and turns, it would make a full circle through the inside to the outside of the fridge. All of that convoluted piping is really just one big loop, as shown in the diagram above.

Which raises the question, if it is just one big loop why does it look so complicated? Most simply, all of that zig zagging is intended to increase the piping’s exposure to air. When the refrigerant runs through the outside portion of the refrigerator (stage 1), heat is conducted to the air. The refrigerant is simply warmer than the air, and it loses its heat in exactly the same way you lose body heat when out in the cold. If it just made a short peak out of the back of the fridge and headed back in, there wouldn’t be much time for heat transfer to occur.

Compressor

So how does it get warmer than room temperature? Isn’t it strange that it could be warm right after leaving the inside of the fridge? Consider a second question. Why do some mountains have snow at their peaks but not at their bases? If you can answer either one of these questions, you can answer the other.

Mt. Fiji contains the same gaseous states as a refrigeration cycle.

Mt. Fiji contains the same gaseous states as a refrigeration cycle.

The answer is the Ideal Gas Law. The Ideal Gas Law finds that a gas will decrease in temperature when it decreases in pressure. At the top of Mt. Fiji, the air is very thin and low pressure because there is much less air sitting on top and compressing it than at sea level.

Likewise, after hanging out inside the fridge for a bit the refrigerant has only warmed to near fridge temperature; maybe 35 to 37 degrees (stage 3). But then it enters the compressor (stage 4). The compressor does exactly as it sounds; it puts a great deal of pressure on the refrigerant. This is like pushing that thin, cold air at the top of Mt. Fiji down to ground level. Or, think of it as the squeezing of a sponge. It “squeezes” the refrigerant, making it temporarily warm so that it loses heat to the surroundings. Now, when it is “unsqueezed” it will be able to “soak up” more heat.

Expansion Valve

The next logical question is: what “unsqueezes” it? This is the work of the expansion valve (stage 2). In more exact terms, the expansion valve converts the high pressure, low velocity refrigerant into a low pressure, high velocity mode. This lowers the temperature as the ideal gas law describes.

If that seems complicated, try this. Put your hand in front of your mouth, make a small circle with your lips, and blow. Notice that the airstream feels cool. The air in your lungs is like the high pressure region of the refrigerant, and the airstream hitting your hand is like the low pressure region. Your lips are like the throttle valve. The throttle valve itself is nothing but a small opening.

Since the refrigerant has cooled to almost room temperature just before passing through the throttle valve, when it passes through it loses a great deal of pressure and becomes very cold. It then can cool the interior of the refrigerator again(back to stage 3).

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