A classic experiment showing sudden transfers of air cause cooling - not heating.November 14 2012 at 12:50 PM
Steve in NC (Login pneuguy)
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...
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.
They knew centuries ago that the sudden movements of air driven by pressure changes always produces net cooling - not heating - and our PCPs confirm that result with every shot.
|This message has been edited by pneuguy on Nov 14, 2012 1:01 PM|
|November 14 2012, 1:14 PM |
And that, gentle reader, irreversibly explains why we must be so careful not to overheat our reservoirs, cylinders and tanks while filling.
Though there may be other factors and phenomenon at play also.....
Steve in NC
Please note the keyword: Sudden. That means too quickly for...
|November 14 2012, 1:49 PM |
|Mike in Maryville|
Ever heard of a vortex cooler ?
|November 14 2012, 2:52 PM |
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.
I know of vortex tubes
|November 14 2012, 4:24 PM |
so there's a real world example of expanding air reaching higher than initial temperature in one part and lower in another.
Steve in NC
Actually, I have, but haven't a clue how they work. A few things...
|November 14 2012, 5:02 PM |
...about them seem obvious, however.
1. Something about the sonic-speed spinning of the vortex is essential to separating the inner and outer longitudinal flows into different temperatures.
2. It takes a significant amount of time for the heat exchange to occur, as reflected in the minimum length the coolers have to have.
3. Whatever the details of the process that makes them work may be, it doesn't seem likely to occur in any normal airgun firing cycle.
|This message has been edited by pneuguy on Nov 14, 2012 5:03 PM|
Fertile Ground for the future!
|November 14 2012, 10:42 PM |
I can see the name in lights!
that name is taken......
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.
Your link explains that the experiment "was not adiabatic"
|November 14 2012, 10:54 PM |
In practice so the tanks were thermally isolated to preserve heat in the system. Evidently heat could be exchanged between the copper and the air durring the process. And did.
PS Or was the tank so insulated/isolated as to create the principals of a calorimeter only? I'm Confused.
|This message has been edited by oo7fuzz on Nov 14, 2012 11:41 PM|
Steve in NC
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.
|November 14 2012, 2:04 PM |
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|
Yes cynical , My apologies to the form.
|November 14 2012, 2:52 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.
you really need to spend more time just shooting....
|November 14 2012, 5:31 PM |
Steve in NC
Awwwww - why don't you go and...
|November 14 2012, 8:00 PM |
J/K, of course. Good suggestion re: shooting, though. I fit in a few minutes today with the li'l VIP Izzy. Not enough, but something.
|This message has been edited by pneuguy on Nov 14, 2012 8:04 PM|
And I taking the advice before even hearing it
|November 14 2012, 10:39 PM |
Pumped up and shot down an AR6 in both .17 and .22 along with a bunch of shots at a blistering 495 fps from the "multi shot" 392.
So IT works, this air gun stuff is more than fun, It's self induce hypnosia!
My appreciation, all!
Re: you really need to spend more time just shooting....
|November 14 2012, 9:39 PM |
Cal, these systems you propose need....
|November 14 2012, 8:29 PM |
Ron, It so happens that the words "Maxwell's Demon" are written
|November 14 2012, 10:47 PM |
On the top page of my current note pad here on my desk!
Just over the sketch of rise and run consideration for the little car park roof I have started to build out front (before winter hits!)
" Great minds...." awe forget it..... ;-]
|November 14 2012, 6:04 PM |
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!
Steve in NC
PCP + 3-in-1 oil = detonation? Sounds like an easy enough experiment...
|November 14 2012, 6:35 PM |
...to replicate, although I'm prettysure what the outcome will be.