# How does a fridge work?

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In: Engineering

If you have gas in a closed container and try to compress it (squeeze the container), the gas will also get hotter because the molecules of gas get closer together and rub one another more vigorously, generating heat. So you used energy to generate heat.

If, on the other hand, you expand the container, the gas will actually get colder. So you just used energy to “generate cold”.

Such a machine is in the refrigerator expanding the gas, “generating cold”, absorbing the heat from inside of the fridge, and taking it outside to cool to room temperature. Basically you transport heat energy from inside to the outside by cooling the gas in one place, letting the gas warm up a little bit and absorb the heat of the inside, and let it out somewhere else. And repeat.

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Fridges don’t make things cold, they move heat around.

They cool the main box by taking heat from the inside and putting it on the outside, specifically the back of the fridge. Fridges can get hot on the back.

HOW they move heat is the hard part. Essentially they use a trick that you might not have heard of or understand fully.

It takes energy to heat something, but it takes a very large amount of energy to break the threshold and turn a material from liquid to gas, from gas to liquid, etc.

This is why you cool down very fast when you sweat. The evaporation process requires very large amounts of heat, and that heat leaves your body with the evaporating liquid.

Fridges abuse this “trick”, and have a substance inside that is a liquid or a gas at just the right temperatures they want. They move it around the fridge and change the temperature and pressure so that it’s a liquid sometimes, and a gas at other times in the cycle. Hooking a system like this up correctly allows the substance to take heat from one part of the cycle and give it to the other part of the cycle. One half is hooked to the cold box, one to the outside of the fridge.

The exact process is somewhat complicated, so I encourage research on the topic. Technology connections has great YouTube videos on appliances such as this explaining them in more detail.

You would also further understand the topic with physics courses. A refrigerator is the opposite of an engine, and is a little like a steam engine in reverse.

Rehashing much of what was already said, but…. oh, please correct me if I am wrong anywhere.

The cooling system has three major components, evaporator, condenser, and compressor.

Using a substance that is gaseous at the desired cold temperature and low pressure but liquid at a higher pressure and “room temperature” makes it work.

The high pressure liquid expands through a valve in the evaporator and basically boils into a gas pulling in heat and getting cold. You now have a relatively low temperature low pressure gas.

This now slightly warmer gas (as it pulled in heat) is sent to a compressor that increases the temperature and pressure and sends this now relatively high pressure high temperature gas to a condenser where it dumps heat the the room and cools down to a liquid.

This liquid is now allowed to expand and get cold to repeat the process in the evaporator.

So works on the latent heat of evaporation and condensation. Also look into the gas laws of heat vs pressure.

(EDIT: u/nullrecord and u/KnowingestJD both have excellent and concise answers, but I’ll be damned if I typed all this just to delete it now!)

Wiki’s not always the best for introducing science and technology–[Here’s a nice page on it.](https://www.scienceabc.com/innovation/how-does-a-refrigerator-work-working-principle.html)

As others have said, the main trick behind it is to run something (the “refrigerant”) through coils of metal pipes that snake between the inside of the refrigerator and the outside, in a loop.

If you think of temperature as heat energy, the more space a given amount of energy is distributed in, the less concentrated it will be at any given point, so a widely-spread cloud of gas will be “cooler” that if it was densely packed.

Refrigerators have a device that expands the “refrigerant” gas travelling through the pipes just before sending it inside the refrigerator so that this gas becomes colder than the air within the fridge, and the heat-conducting metal pipes will naturally allow heat to flow through them from the warmer outside of the pipe (the refrigerator box) to the inside of the pipe (the gas that was just expanded in order to cool it enough to make this work) as the refrigerant passes through.

Having leached heat energy from the surrounding air in the refrigerator, the now warmer gas continues through the pipes until it reaches another device just before leaving the inside of the refrigerator box, and this device compresses the gas until it is hotter than the room-temperature air in your kitchen. Then it goes through another set of coils on the outside, only this time the now-hotter gas is bleeding head outwards to the cooler surrounding air rather than taking it in. Once it loses that unwanted heat, it returns back to the starting point where it is expanded again, and starts over.

It’s the air-pumping to expand and compress the gas that makes them such energy hogs; the refrigerant itself is naturally inclined to flow in the direction desired within the two coils: hotter gas naturally moves upwards, and cooler gas/vapor will sink downwards, so if you put the expander device at the bottom of the fridge and the compressor device at the top, the refrigerant itself is already inclined to move in the direction you want in both the inside and outside coils.

Have you ever used one of those compressed air cans? Did you notice that it got very cold if you sprayed it for any length of time?

That’s because when gas expands it gets cold, basically when it was made it got quite warm and transferred all the excess heat to the air around it so when you spray it the reverse happens.

A fridge (or an air conditioner use that principle in a loop. Gas is compressed, cooled, expanded, and warmed and then it repeats.

The warming step transfers heat from the inside of the fridge to the gas, and then the cooling transfers heat from the gas to your home (or in the case of the air conditioner from inside your home to outside your home).