Thermodynamic Question What am I Missing

pharmer

pharmer

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I have a $60 Harbor Freight 8 gal air compressor. I just did inflation or several items, the tank running up to its' operating 150 psi. When done I need to bleed the tank completely so it doesn't rust from the condensation. It's 80* out and after the bleed, on the driveway beside the usual small puddle of liquid, a small spire of ice formed over an inch high (melted immediately). I understand the principle of a blanket covered canteen getting wet to cool it through evaporation but I missed something in Science class evidently. I'm sure somebody here knows exactly what's happening (I'd brush it off if'n it was below 32*F out). Joe
 
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Back in college, I put some beer in the freezer to chill and when I popped the cap off the bottle, the beer froze instantly. I was taking thermogodamics at the time so I asked my professor about it.

The sudden drop in pressure causes a similar dramatic drop in temperature which causes the freezing since the liquid was near freezing to begin with. Of course, that was over forty years ago, so my memory may be fuzzy.
 
It's called the Joule-Thomson effect, an expanding gas cools according to ideal gas laws. It's handy for some applications like cooling thermal optic sensor chips in smart bombs and Stinger missiles, because it doesn't need any machinery, just a vessel of compressed gas expanding through an orifice. It is not the process that makes air conditioners and heat pumps work as asserted in Post #4. Those work on a reverse Carnot thermodynamic cycle, a closed continuous cycle, as opposed to the J-T effect where the working fluid is lost.
 
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When matter changes state from solid to liquid to gas, or vice versa, it absorbs or releases energy in the form of heat, depending on direction. That is how evaporative cooling works, That is also the principle behind mechanical refrigeration and steam engines. Look up “Latent Heat of Fusion” and “Latent Heat of Vaporization” for more detailed information. It’s just high school physics. Or used to be.
 
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ggibson511960 said:
Ha ha. Don't dive too deep. The math and physics will hurt your head just as it did mine when I was forced to understand this stuff in college.
Click to expand...

Well, I was terrible in math, and the only two things I ever learned about physics were that the Starship Enterprise runs on dilithium crystals, and that it can do Warp 9.9... :)
 
Latent Heat of Vaporization- the same principal used to cool the body. Sweat evaporates and causing a cooling effect as the liquid turns to vapor. Seventh grade science iirc.

I also worked at a petroleum refinery in Billings MT. One unit (called the cryo unit) was there to liquefy the last remaining amounts of propane before the overhead gases from the refining process (methane and ethane, C1 and C2) went into the fuel gas line. That unit ran the compressed gas (around 170-180 psi iirc) across a valve that allowed the pressure to drop to 30 psi immediately. The result was process temperatures around -70 degrees F which liquefied the remaining propane. The unit was iced up year-round.

Same effect in the high pressure to low pressure side in your auto a/c.
 
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DWalt said:
When matter changes state from solid to liquid to gas, or vice versa, it absorbs or releases energy in the form of heat, depending on direction. That is how evaporative cooling works, That is also the principle behind mechanical refrigeration and steam engines. Look up “Latent Heat of Fusion” and “Latent Heat of Vaporization” for more detailed information. It’s just high school physics. Or used to be.
Click to expand...

now a days "Only in High Schools that stream the bright students". Dave_n
 
Math?? Math iss EASY. Now the English(Merican) language...ummm. I do have problems with nouns or pronouns...or whatever they is. An spelling I usta spell real guid. Now I cannt type them words for diddly squat.

I think its that spellin chicken on the compuyer
 
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Well, the Joule Thomson effect explains it well, although I am still surprised by ice forming just by throttling compressed air (before and after) at 80* ambient. The Carnot Cycle (heat into work) didn't hit it as well for me. The Hampson Linde Cycle that sjbrdn describes is a practical application of JT but definitely not part of the High School Science curriculum in any High School I've attended or taught. Thanks to all who taught me something new today, I am trying to avoid "cranial atrophy" in my retirement. Joe
 
This sounds like the Coudersport Ice Mine outside of Coudersport in northcentral PA. It has ice formations in the spring and summer which melt away in the winter. I remember hearing about it when I was living up there in 1976.
 
Perhaps the most striking example of the cooling effect of gas expansion is all around us. The cosmic microwave background radiation is at a temperature of 2.7 degrees Kelvin, or about -455 degrees Fahrenheit. It permeates the entire universe and is the result of the expansion of the the universe from a very hot, dense state over 13 billion years ago.
 
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