The Joule expansion is an irreversible process in thermodynamics in which a volume of gas is kept in one side of a thermally isolated container (via a small partition), with the other side of the container being evacuated. The partition between the two parts of the container is then opened, and the gas fills the whole container.
Joule performed his experiment with air at room temperature which was expanded from a pressure of about 22 bar [319psi - expanding to 160psi]. ... With our present knowledge of the thermodynamic properties of air  we can calculate that the temperature of the air should drop by about 3 degrees Celsius...
We used them in industrial settings to cool remotely located equipment cabinets. No moving parts, just apply pressurized air and voila ! out one port comes hot air (burn you hot) and out the other port comes cold (really cold) air. Pipe the cold air into the cabinet, done.
That's the cooling system used by miners to keep cool...
November 14 2012, 7:42 PM
The air is compressed thereby increasing the sensible heat, which is cooled down by a heat exchanger. Then the high pressure air is delivered to the miner's suit where it is metered thtough an oriface to low pressure. The resultant pressure drop causes a drop in temperature,thus cooling the miner. Very clever these bald apes.
No. The article explains that Joule's 19th century calorimetry...
November 14 2012, 11:52 PM
...instrumentation was insufficiently sensitive and/or fast enough to detect the temperature change produced after the process of expansion was complete. They didn't have very good thermistors back then, ya' know?
But probably a contributing factor to Joule not detecting the temperature jump was that he wasn't expecting to see one in the first place. The only reason it happens at all is because air isn't (quite) an ideal gas.
Very sarcastic, but wrongly applied to the different process.
Steve's explanation related to the shooting part, where sudden valve opening let some air expansion to the transfer port and the breach area behind the pellet. In this process, no heat generated and very small temperature drop absorbed by surrounding metal parts. It has much more pronounced effect with CO2 in the semi-auto guns. You see chilling effect of the same nature and actually can freeze the valve and surrounds to very noticeable level while shooting quickly.
Overheating pressure reservoir while filling described by the different process of compressing gas.
This message has been edited by Boris_LA on Nov 14, 2012 2:06 PM
'just a bit miffed at the expressions directed my way in an earlier thread associated with the term reversibility.
So yes, I read the words and applied the response to my meaning.
Certainly we do not expect the Joule-Thompson effect to work by some other mechanism than Joule expansion. Both stipulate conditions of environmental isolation. The J-T effect is the application of Joule expansion. Time has no bearing, as the cited Wiki reference elucidates.
Technically, I'm just trying to say, that movement of a real gas from one real volume to another, under real conditions involves additional considerations associated with turbulence, viscosity,friction,and segregation, both spatial and temporal.
Equilibrium is only satisfied "in theory". There is a good joke about "close enough for all practical purposes", but let me not digress
The example of the CO2 guns is also associated with the heats of transformation, so an amplified case.. And into a constant pressure as opposed to an increasing pressure. Those aspects are not important here, even if very interesting.
I've cooled more than enough barrels to the point of condensation forming (even on temperate/cool days) to have seen the proof of net cooling.
The "but" pertains to having induced detonation in the "firing chamber" of a PCP. The cooling is net "but" localized heating can and does occur.
I really wish I'd taken video of the event as I was videoing many of the shots that day "but" I didn't realize it was such a big deal back then.
Having shot enough rounds to get the shrouded barrel completly fogged with condensation, I cleaned the barrel by swabbing it with a 3 in 1 oil soked patch. The next couple shots produced curls of smoke from the breech when reloading. And yes, I can tell smoke from water vapor!
The way I read that quote Steve ... leads me to believe there was
November 14 2012, 6:51 PM
no air in the other (RH?) side of the apparatus. It had been "evacuated" and therefore was a vacuum I guess. All the air came from one side, (there was none in the right side as I think you imply). Then the pressure halved when the volume was doubled. Same air just half as many collisions.
To be fair this is not the same situation as if there had been air in the second container, though the net result in terms of cooling would be similar (figures different) after stabilizing. ... There is an old adage about changing one thing and generalizations are up for grabs? ..... Kind regards, Harry.
Given this reminder, please remind me, what was your point again?
PS: My understanding of the J-T effect is that it's proportional to the change in pressure (i.e., units of oK/bar). Therefore whatever the initial pressure in the right side of the apparatus might have been - call it X - the same temperature change would have been obtained if the initial pressure in the left side was set to 22bar + X. The reason being because then the pressure change caused by uniting the halves would have been ths same: 11bar.
This message has been edited by pneuguy on Nov 14, 2012 7:09 PM This message has been edited by pneuguy on Nov 14, 2012 7:03 PM This message has been edited by pneuguy on Nov 14, 2012 7:00 PM
The right side of the apparatus was quoted as being evacuated. OK so Joule may not have reached absolute but I bet he had the best available pump at the time which means the air left would be something about 0.15% of a Bar. Not much air there, but OK "none" is absolute and therefore wrong. You can chase down the state of vaccuum pumps by 1845.
" ...[319psi - expanding to 160psi]..." ??
The rest of my response: says what it says ... Kind regards, Harry.
Not that there's really a problem, Harry, but your post seemed to be saying...
November 14 2012, 9:02 PM
...that initial (partial) evacuation of one side of the Joule Expansion apparatus invalidates it as an example and predictor of the physics of air transfer in our airguns.
So I'm simply curious if you would assign a threshold pressure to that invalidation - i.e., what minimum initial pressure would be required to transform the physics of air transfer, and reverse cooling to heating?
Or did I misconstrue your comment?
Either way, my apologies if the questions are unwelcome.
Sorry I broke off there Steve, an old mate called in for lunch and a yarn.
November 14 2012, 11:35 PM
No, I'm not saying that. My points were:
Firstly I was pointing out that the right side had no air (change that to virtually no air as I doubt that weency bit was significant). Experimentally that could be seen as a difference from if there was already a chamber full of air which seemed to be implied here..." Note that temperature drop applies to all the air in both sides of the container, in the right side as well as left, even though the pressure on the right side increases by 11bar".... The mere fact that the pressure dropped from 22 bar to 11 bar indicated that.
Secondly, I indicated that (regardless of the RH air volume)..." the net result in terms of cooling would be similar (figures different) after stabilizing". ... which I assume you would have agreed with. The figures would obviously be different if there was a significant amount of air already in the right hand chamber ( which obviously there wasn't ).
I have checked the status of vacuum technology at your prompt. In 1850 it was already down to 1.0 Torr so my guess is Joule was able to get close to that figure. ... Sure would make breathing difficult in that chamber.
Kind regards, Harry.
Good catch, Cal. Thank you - footnote #3 nails it.
November 14 2012, 11:56 PM
3. Note that the fact that the gas expands in a vacuum and thus against zero pressure is irrelevant. The work done by the system would also be zero if the right hand side of the chamber were not evacuated, but is instead filled with a gas at a lower pressure. While the expanding gas would then do work against the gas in the right-hand side of the container, the whole system doesn't do any work against the environment
I am impressed that the temperature drop was so small
November 15 2012, 12:23 AM
-3 c in modern times by calulation? Not even measured in that time. Zip, Nada, ziltch. Joules must have been disappointed.
Would "friction" in our air guns result in a measurable increase in temperature? May be...or....not?
Consider a real discharge volume and transfer geometry rather than a lab experiment. Though I do like controlled conditions, I just can't seem to control them all, or enough.
(I remember a disaster with an alcohol bath and dry ice...oh! let me forget.. -40 c .. Night time on Mars.. such a hard number to hold...how to keep the ice from forming on the alcohol bucket....) Sorry , memories come ;} (LN2 and a little spatial separation was the solution )
I do know Hydraulic systems heat up very quickly depending on flow and restrictions. Last I checked, compressed gasses followed some of the same cause and effect rules as non compressible flow. but not all for sure.
Amazing what a few undetermined degrees will do even the best estimate
November 15 2012, 11:33 AM
For most of my purposes. just knowing if it's a "very big number", or a "very small" number. Fills the bill.
The "strings of zeros" being the message in the original post
The degree of rarification associated with "a good vacuum" may be subjective, depending on the application requirements. Though if trying to match adatoms with energetic electron energies, a factor of 10^3 might not return the results desired.
This message has been edited by CalG on Nov 15, 2012 11:45 AM
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