How do hospitals get their oxygen supply?
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The oxygen can be bought commercially in liquid form. Manufacturers have multi-step refrigerators that can go to very low temperatures. Basically you put one refrigerator inside another to get colder temperatures. The last step is usually a sterling cycle cooler which can make oxygen go liquid. Air from the outside is first filtered and dried in order to remove most of its impurities. Then it is cooled down so it condenses into liquid air. The problem with this is that air is mostly nitrogen with some oxygen and a few other gasses. But nitrogen have a higher boiling point then oxygen which is used to separate these gasses. When the air is cold enough for nitrogen to condense but not cold enough for oxygen to condense you get oxygen gas and liquid nitrogen. So the providers usually sell both liquid oxygen and liquid nitrogen as well as argon which is separated in a similar way. Any excess gasses that is produced is just vented. Hospitals can then buy the liquid oxygen which is delivered in specially insulated tanker trucks and stored in special tanks outside the hospital. As it boils the oxygen is collected into pipes through regulators and heaters to all the hospital beds.
There is also some mobile oxygen separators which are sometimes used. Especially when moving patients around it can be easier then using a gas bottle. These use a molecular sieve to filter out oxygen from the nitrogen. Basically the oxygen molecules are slightly bigger then the nitrogen molecules so you can make a sieve that allows nitrogen to pass but not oxygen.
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