Why do tanks get hot when you fill them from higher pressure tanks?

[lowviz]

jimmyw,

I have an extra credit problem for you. What is the CURRENT world ranking of the US in math?

 

[Guba]

"1) There is no volume of gas in the original high pressure tank - no single cc parcel of gas that does anything other than expand - whether it stays in the original tank or leaves the original tank. All the gas goes from an initial compressed state to a final more expanded state."


(I feel the hook in my mouth but, like a trout, I simply Cannot resist...)

I'm sticking with conceptual models, not math...Okay, let's look at that "empty" tank. It has no molecules of ANYTHING in it, so no kinetic energy in its content. Then that first molecule of gas arrives and, yippee, it carries with it some energy. Now a single gas molecule can't "expand" in that void...it can only bounce around inside the tank. It can't "speed up", because to do that would require energy and there is no place to get that. It's KE is already finite. However, what was once only "space" is now occupied by a single particle, so there is indeed a "pressure", though it's a very low one. Now here comes that second molecule. It's much like the first, but wait! Someone is already in the tank, so now the population of the tank is twice what it once was, so that first molecule registers that things are getting more crowded. Since it bounces off the newcomer once in a while, its movements are somewhat restricted. That makes it lose some KE...in the form of heat. It will do the same with EVERY ADDITIONAL molecule of gas that enters the tank, and the same will happen to each one of those, too. For that reason, the gas entering the tank is not EXPANDING into it. It is simply occupying the space, and every additional molecule of gas that enters compresses that gas, thus forcing it to give up energy that takes the form of heat.

Voila'.

 

[JohnN]

"1) There is no volume of gas in the original high pressure tank - no single cc parcel of gas that does anything other than expand - whether it stays in the original tank or leaves the original tank. All the gas goes from an initial compressed state to a final more expanded state."


(I feel the hook in my mouth but, like a trout, I simply Cannot resist...)

I'm sticking with conceptual models, not math...Okay, let's look at that "empty" tank. It has no molecules of ANYTHING in it, so no kinetic energy in its content. Then that first molecule of gas arrives and, yippee, it carries with it some energy. Now a single gas molecule can't "expand" in that void...it can only bounce around inside the tank. It can't "speed up", because to do that would require energy and there is no place to get that. It's KE is already finite. However, what was once only "space" is now occupied by a single particle, so there is indeed a "pressure", though it's a very low one. Now here comes that second molecule. It's much like the first, but wait! Someone is already in the tank, so now the population of the tank is twice what it once was, so that first molecule registers that things are getting more crowded. Since it bounces off the newcomer once in a while, its movements are somewhat restricted. That makes it lose some KE...in the form of heat. It will do the same with EVERY ADDITIONAL molecule of gas that enters the tank, and the same will happen to each one of those, too. For that reason, the gas entering the tank is not EXPANDING into it. It is simply occupying the space, and every additional molecule of gas that enters compresses that gas, thus forcing it to give up energy that takes the form of heat.

Voila'.

I seriously question my own sanity for jumping back into this snake pit. But let me try again.

Jimmyw's analysis is flawed because all of the equations and formula's used are based on the behavior of an Ideal Gas.

Guba's analysis is flawed because it doesn't explain why filling identical cylinders with air, N2, and O2 will produce different amount of heating in the tank being filled. Nor does it explain why filling an identical cylinder with He would result in slight cooling


TRYING AGAIN: Gas(s) are not Ideal. Most heat on compression and cool on expansion (this is why air-conditioners and refigerators do what they do). Some cool on compression and heat on expansion (He and H).

Before you continue to debate the numer of angels dancing on the head of this particular pin, go to your LDS and watch them fill a tank with air and fill a tank with O2.

Can we please kill this topic now?

 

[lowviz]

A careful re-read of this thread confirms my suspicion that the community is being "played" for jimmyw's own entertainment. He passed by three big opportunities to deliver confounding observations. I can only believe that this comes from fundamental lack of understanding, not his sense of conviviality or desire to belong to the community.

Crating puppies works because puppies are intelligent enough to not piss where they live.

 

[windapp]

Most heat on compression and cool on expansion (this is why air-conditioners and refigerators do what they do).

Can we please kill this topic now?

All gasses heat on compression, and cool on expansion during a reversible process. The permanent change you refer to is known as the Joule Thompson effect. This is a change in the heat capacity of the gas as it changes pressure, and it is insignificant over relatively small pressure changes. Air-conditioners and refridgerators do not rely on this effect at all. Rather, they are designed to cool through a phase change in the refrigerant from liquid to gas. This is why cryogenics require staged refridgeration as, for reasonable pressures, the temperature at which a gas can also exist as a liquid varies by gas.

 

[JohnN]

All gasses heat on compression, and cool on expansion during a reversible process. The permanent change you refer to is known as the Joule Thompson effect. This is a change in the heat capacity of the gas as it changes pressure, and it is insignificant over relatively small pressure changes. Air-conditioners and refridgerators do not rely on this effect at all. Rather, they are designed to cool through a phase change in the refrigerant from liquid to gas. This is why cryogenics require staged refridgeration as, for reasonable pressures, the temperature at which a gas can also exist as a liquid varies by gas.

Sigh. . . Please research the JT-Coefficient for different gases. Here is a link to assist: Joule?Thomson effect - Wikipedia, the free encyclopedia

To save you some time:

In the Joule experiment, the gas expands in a vacuum and the temperature drop of the system is zero, if the gas were ideal.

The throttling process is of the highest technical importance. It is at the heart of thermal machines such as refrigerators, air conditioners, heat pumps, and liquefiers.^[7] Furthermore, throttling is a fundamentally irreversible process. The throttling due to the flow resistance in supply lines, heat exchangers, regenerators, and other components of (thermal) machines is a source of losses that limits the performance.

Helium and hydrogen are two gases whose Joule–Thomson inversion temperatures at a pressure of one atmosphere are very low (e.g., about 51 K (−222 °C) for helium). Thus, helium and hydrogen warm up when expanded at constant enthalpy at typical room temperatures. On the other hand nitrogen and oxygen, the two most abundant gases in air, have inversion temperatures of 621 K (348 °C) and 764 K (491 °C) respectively: these gases can be cooled from room temperature by the Joule–Thomson effect

 

[windapp]

Sigh. . . Please research the JT-Coefficient for different gases. Here is a link to assist: Joule?Thomson effect - Wikipedia, the free encyclopedia

To save you some time:





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Not sure how any of this contradicts anything I have said.

To clarify, my first statement was not to imply that all processess are reversible, only that these statements are true during a reversible process.

No refrigeration process in the world actually uses the JT effect. If you look at at Nitrogen as an example, for 300K it would take a 100 bar change in pressure to exact a 20 degree celcius change in temperature. This would be completely overshadowed by the frictional heating during the compression and throttling.

There seem to be a lot of Wikipedia trained Engineers in the world. Some of us actually have advanced degrees in it.

 

[Guba]

(with great indignation...)

Uh, humph! I take exception, John. My analysis is not "flawed"....just incomplete. It was not intended to provide comprehensive answers to ALL gases and situations, only a generalization that could be applied to the gases most likely to be filling the tank (I've never used tri-mix, so I tend to ignore helium). However, I appreciate your more complete answer. I was just trying to keep things simple (I'm used to working with high school students, remember.)

That said, can someone kill this zombie of a thread? It shares many of the same characteristics...it wanders through a virtual wasteland emanating moans and groans, it's short of sight, and just when you think it's gone it comes back alive and tries to grab you.

I shall now remove the hook and swim myself somewhere else.

 

[JohnN]

Not sure how any of this contradicts anything I have said.

So that was your evil twin that hijacked your account and wrote:

All gasses heat on compression, and cool on expansion during a reversible process.

Got it!!

 

[RJP]

That said, can someone kill this zombie of a thread? It shares many of the same characteristics...it wanders through a virtual wasteland emanating moans and groans, it's short of sight, and just when you think it's gone it comes back alive and tries to grab you.

And, it eats BRAINS!!!