A classic experiment showing sudden transfers of air cause cooling - not heating.

A classic experiment showing sudden transfers of air cause cooling - not heating.

Joined: May 12th, 2001, 1:29 pm

November 14th, 2012, 5:50 pm #1

Joule expansion



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 [6] we can calculate that the temperature of the air should drop by about 3 degrees Celsius...


http://en.wikipedia.org/wiki/Joule_expansion

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.

Steve
Last edited by pneuguy on November 14th, 2012, 6:01 pm, edited 1 time in total.
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Joined: June 7th, 2010, 12:14 am

November 14th, 2012, 6:14 pm #2

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.....
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Joined: May 12th, 2001, 1:29 pm

November 14th, 2012, 6:49 pm #3

...the air in transit from source to destination to absorb heat from its surroundings.

If you'll recall, we discussed the time factor (i.e., Step #2) at some (!!) length in an earlier thread. Was that effort all for naught?

http://www.network54.com/Forum/79537/me ... 27t+happen.

Steve
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Joined: March 27th, 2009, 3:51 am

November 14th, 2012, 7:04 pm #4

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.....
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.
Last edited by Boris_LA on November 14th, 2012, 7:06 pm, edited 1 time in total.
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Joined: December 25th, 2002, 12:34 am

November 14th, 2012, 7:52 pm #5

...the air in transit from source to destination to absorb heat from its surroundings.

If you'll recall, we discussed the time factor (i.e., Step #2) at some (!!) length in an earlier thread. Was that effort all for naught?

http://www.network54.com/Forum/79537/me ... 27t+happen.

Steve
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.
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Joined: June 7th, 2010, 12:14 am

November 14th, 2012, 7:52 pm #6

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.
'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.
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Joined: June 5th, 2006, 12:49 am

November 14th, 2012, 9:24 pm #7

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.
so there's a real world example of expanding air reaching higher than initial temperature in one part and lower in another.



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Joined: May 12th, 2001, 1:29 pm

November 14th, 2012, 10:02 pm #8

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.
...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.

Steve
Last edited by pneuguy on November 14th, 2012, 10:03 pm, edited 1 time in total.
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Joined: September 15th, 2000, 3:23 pm

November 14th, 2012, 10:31 pm #9

'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.
go out, shoot some pellets, and have fun...

Regards,
DaveG
See more picts @http://davegcustomstocks.com
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Joined: January 22nd, 2007, 10:28 pm

November 14th, 2012, 11:04 pm #10

Joule expansion



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 [6] we can calculate that the temperature of the air should drop by about 3 degrees Celsius...


http://en.wikipedia.org/wiki/Joule_expansion

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.

Steve
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!

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