We use refrigerators everyday to keep our foods fresh and drinks cool. However, did you ever stop and think about how refrigerators work? (⁎⁍̴̛ᴗ⁍̴̛⁎) The principle of refrigeration is not magic! 
Imagine your chore for today is to get water from a nearby river. The best thing to do is to use buckets. Moving something you can see is easy.
But now, let’s give you a harder chore: to move the heat from the inside of a refrigerator to the outside to keep your food fresh. Tricky problem, right? However, it’s not impossible – at least not if you understand some basic science of liquid and gas. (3/26)
Let’s step away a moment and look at the physics of gases. If you have ever pumped the tires on a bicycle, you would know that the pump gets warm pretty quick. That’s because gases heat up when being compressed. (4/26)
As you squeeze the air, you have to work hard with the pump. The energy you spend is eventually being converted into potential energy of the compressed gas, and also makes it heat up. (5/26)
Going back to the original question, if you can inflate the tire in one place and deflate it in another place, you are basically moving heat with work! (6/26)
It's a clumsy idea though, and we can't really move much heat like that. We could, however, move a lot of heat by letting a gas contract and expand much more so it converts into a liquid and back to gas again. (7/26)
Evaporation of a liquid takes away lots of heat; that’s why it feels cold when rubbing alcohol evaporates from your skin – it turns into a gas by stealing heat from your body. It’s also how sweating works to cool your body on hot days. (8/26)
In other words, changing a liquid into a gas is a way to remove the energy (heat) from stuff. Similarly, changing a gas back into a liquid is a way to release that energy again. This is essentially how refrigerators move heat from the cooling cabinet to the outside air. (9/26)
Suppose we made a pipe that’s partly inside the fridge, and partly outside it. The pipe is sealed such that it looks like a continuous loop. (10/26)
It needs to be filled with a carefully chosen chemical called refrigerant so that it can easily change back and forth between liquid and gas states. (11/26)
The liquid refrigerant boils and turns into a gas at around -18 ℃ (or 0 ℉) inside the food compartment (to pick up heat from the food). In contrast, water boils at 100 ℃ (212 ℉) at atmospheric pressure. (12/26)
It is then being pumped to the outside and compressed back into a liquid again at about 50 ℃ (or 122 ℉) to release the heat to the surrounding air. (13/26)
And, surprise surprise, this is almost exactly how a refrigerator works. (14/26)
Besides the two heat-exchanging processes described above, there are some extra details worth noting. (15/26)
Refrigerants in the circular pipe won’t start to flow and pump heat out all by itself. A compressor is needed to put work into the system, much like a bicycle pump except that it is driven by electricity. This is also the thing that's humming inside your refrigerator. (16/26)
Pressurized by the compressor, the gas refrigerant becomes even hotter; it then enters a grill-like device called a condenser (thin radiator behind the fridge, like the one in your car) that expels the unwanted heat and cool down the refrigerant, making it go back to liquid. (17)
The liquid refrigerant then goes through the expansion valve, where it releases the pressure, turns partly into a gas and cools dramatically. (18/26)
Inside the fridge, it then evaporates in another grill-like pipe called an evaporator and turns into a gas, taking heat away from the food. (19/26)
This bit of science is sometimes known as the Joule-Thomson effect for the physicists who discovered it, James Joule (1818–1889) and William Thomson (Lord Kelvin, 1824–1907). (20/26)
Food preservation is as old as mankind. Humans have always been interested in the methods of preserving food. Being able to preserve food to use at a later time has allowed humans to form communities. (21/26)
The first method is sun-drying, and it dates back to 12000 B.C. However, such a method requires strong sunlight and dry climate. It also dehydrates the food so that it loses its original shape, color, and taste. (22/26)
Therefore, humans started to preserve food by storing it in a cold space like an icehouse. This allows foods to be preserved at lower temperatures, but it is strongly dependent on appropriate climates and can only be applied to limited areas in the world. (23/26)
The basis for modern refrigerators was created in 1755 AD, when Scottish professor William Cullen designed a small refrigerating machine. This machine had a pump and a container of ether. (24/26)
Pump made vacuum in the container which lowered the boiling point of the ether. Boiling ether absorbed the heat from the surrounding air. However, it was not efficient enough to be used in practical applications. (25/26)
In 1851 AD, the first practical vapor compression refrigeration system was built in by James Harrison. The core principle of a refrigerator actually remains unchanged ever since then. (26/26)
The end. Refrigerator is your best friend to help you stay inside.
