It's obvious that the breakthroughs in
refrigeration technology in the early part of the
20th century are responsible for a good deal of our way of life. Refrigeration guarantees that we can essentially eat any type of
food, at any time during the year, anywhere.
Wow.
Now, I consider myself a science kid -- I know why an airplane wing works (good ol' Bernoulli). I know what the Lockheed SR-71 is made out of (Titanium) and why it leaks when it's on the ground (the fuel tanks only fit together and seal properly at high temperatures when the heat has caused the plane's parts to expand and snug up against each other). I've had what I consider to be a bare-bones primary science education, and I only have that because I was a reading kid as well as being a science kid. My knowlege is just barely enough for me to appreciate the marvels I use every day.
But I never really understood how a household refrigerator worked. I knew there was some kind of gas, and I knew it involved a compressor somehow. I knew it wasn't simply a matter of circulating the magical frost goo through the chassis of the good ol' Frigidaire, but I didn't realized exactly how it worked.
But I was playing with an old portable bicycle pump the other day and it came to me. I was holding my thumb over the nozzle of the thing and compressing the air down as hard as I could. I'd feel the end of the cylinder body turn warm very quickly, because the temperature of a gas varies as its pressure -- the aluminum body of the pump allowed me to feel changes in gas temperature very easily.
Then I simply leaned on the pump, keeping that little bit of compressed air in there for as long as I could. After a moment, I removed my thumb. A little puff of air jumped out, and it was cold.
So I repeated the idle fiddling than had suddenly become an experiment, and got the same results.
I realized that because I'd let the column of compressed air cool a bit before releasing it (that is, I let it radiate the heat that now differentiated it from its thermal environment), when I allowed it to decompress, its new temperature at the normal pressure would be lower than its temperature before I compressed it. This is why aerosol sprays are cold. This is why the bottom (or the top, in the case of my sulfer dioxide 1927 General Electric Refrigerator) of your refrigerator is warm while the inside remains nice and chilly.
It's the little epiphanies in life that make it all worthwhile.
So now I understand how a common household refrigerator works. Now, to get really low temperatures (the kind you need to condense helium into a liquid), some kind of electromagnetic cooling process (that I don't even pretend to understand) is used. Obviously, if you freeze the gas you're using as coolant, none of this works anymore.
Theoretically, you could have self-refrigerating soft drinks, if you had very strong cans and could achieve very high pressures. As I sit here waiting form my Dr Pepper to cool down in the freezer, that seems like a very nice idea.