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Q & A: cooling gases with pressure?

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Most recent answer: 01/03/2013
Why is it that in a Diesel engine the pressure increase in the cylinder causes heat which leads to combustion, but when you pressurize propane or oxygen etc. it gets colder?
- Austin (age 20)
Kansas, usa
I'm not sure why you say that  compressing propane or oxygen cools them. The basic thermodynamics for their compression closely resembles that of the nitrogen-oxygen-hydrocarbon mixture in the diesel cylinder.  Perhaps what you've noticed is that if you have a compressed gas tank, when you open a valve you get some cold gas coming out. That's actually the same effect as the warming on compression, just in reverse. The gas heated up when it was compressed. As it sat around, the temperature gradually became the same as the surrounding temperature. Then on expansion, the gas cools down.

There are tow reasons for that cooling on expansion:

1. The gas does work on the surrounding atmosphere, and so must lose some of its energy, since energy is conserved.

2. Even on expansion into a vacuum, where (1) doesn't happen, the gas will cool. That's because, in the typical range of pressures, there's some negative potential energy of interactions between the molecules of the compressed gas. In other words, they sort of stick to each other. So it takes some energy for them to pull apart. Supplying that energy cools the gas. (This is called the Joule-Thomson effect.) In extreme cases, at high pressure the gas turns to liquid, with a lot lower potential energy. Then when the pressure is lowered, the liquid boils, which gives a lot of cooling.

Run in reverse, both reasons give heating on compression.

Mike W.

(published on 01/03/2013)

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