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Question #1375 posted on 11/11/2003 4:02 a.m.
Q:

Dear 100 Hour Board,
How do they make liquid nitrogen?
- Enjoyer of quick ice cream

A: Dear cryogenic ice cream lover:

1. Compress air to about 3000 psi. This will cause the air to heat up by several hundred degrees.
2. Cool the compressed air using a mechanical chiller, similar to the coils on your freezer or A/C.
3. Let the air expand rapidly back to 15 psi (standard atmospheric pressure). As it expands, it cools down even more, so much so that it liquefies.
4. If you calculate everything correctly, only the nitrogen will liquefy, leaving oxygen, argon, CO2, and other atmospheric gases behind. Otherwise, you can distill the liquid air.

-- Misaneroth