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Ricky
SFN Die Hard
USA
4907 Posts |
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Valiant Dancer
Forum Goalie
USA
4826 Posts |
 Posted - 07/26/2004 : 10:59:00 [Permalink]
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quote:
Originally posted by Ricky
Just kind of a random question here. There is a can of air duster, says that it contains 100% Difluoroethane. When it comes out, it is extremely cold. I found that the boiling point of it is -25C, and it comes out as a liquid. Now it has liquid in the can (I can hear it when I shake it) and also compressed gas. I was wondering if anyone knew exactly how this works. My best guess is that the gas is compressed so much that the pressure keeps it at room temperature (molecules bouncing, creating the heat), but when released the pressure is lost so it gets cold. However, this would not explain the need for a liquid in the can (as you have to hold it upside down for the liquid to be at the top for it to work). Any ideas?
Just going on a lark here, but...
I've noticed that in quite a few air dusters I've used have pressure inside suffient to hold some of the Difluoroethane in liquid form. The can becoming cooler as you use it has to do with the liquid evaporating. Once the pressure in the can equalizes enough to where there is no liquid left (all of it evaporates), it stops getting cold when you use it.
Difluoroethane's other nome de plume is Freon 152.
The liquid isn't required. It's just the byproduct of manufacture. |
Cthulhu/Asmodeus when you're tired of voting for the lesser of two evils
Brother Cutlass of Reasoned Discussion |
| Edited by - Valiant Dancer on 07/26/2004 11:05:41 |
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Ricky
SFN Die Hard
USA
4907 Posts |
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R.Wreck
SFN Regular
USA
1191 Posts |
Posted - 07/26/2004 : 13:32:04 [Permalink]
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quote:
When it comes out, it is extremely cold.
The low temperature is due to isenthalpic throttling. As the gas passes through the valve releasing it from the can (at a relatively higher pressure) to the atmosphere (at a lower pressure), its enthalpy remains constant. Throttling is by definition a process in which enthalpy remains constant. You can verify this effect for steam with a set of steam tables. Saturated steam at temperature T1 and pressure P1 and enthalpy h1, throttled to a lower pressure P2 at a constant enthalpy (h1=h2) will wind up at a lower temperature T2. I don't have a set of tables for difluoroethane but the principal is the same. |
The foundation of morality is to . . . give up pretending to believe that for which there is no evidence, and repeating unintelligible propositions about things beyond the possibliities of knowledge.
T. H. Huxley
The Cattle Prod of Enlightened Compassion
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