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

[halocline]

I'd say it's more -

No hookers for you...come back, one year!

 

[windapp]

To save you some time: ...

[/FONT][/COLOR]

You seem to be fixed on one component of the whole equation, which by the way, is an extremely small component. You do realize that if you compress helium, it will get hotter, not colder.

 

[Bob DBF]

And ya'll keep making sense and jimmyw just isn't biteing. Clueless, troll, or getting his PhD on the dynamics of a thread being played by a pro, I just stopped to see how it's going.

I owe jimmy a beer when I see him, but he will have to listen to one of my sea stories.


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

A. Because they do. All the rest is trying to explain the phenomenon, it keeps scientests in business, but dosen't change the facts.



Bob
-----------------------------------------
On the Internet you can choose to be anything you want. It's strange that so many people choose to be stupid.

 

[JohnN]

You seem to be fixed on one component of the whole equation, which by the way, is an extremely small component. You do realize that if you compress helium, it will get hotter, not colder.

The JT-coefficient predicts an adiabatic compression of He would result in cooling, not heating. So for the question that started us down this rathole, filling a tank from a bank of He, you would expect the tank being filled to become cooler.

If you would like to re-work these observations (I believe Joule died in the late 1890's), please have at it.

 

[windapp]

The JT-coefficient predicts an adiabatic compression of He would result in cooling, not heating. So for the question that started us down this rathole, filling a tank from a bank of He, you would expect the tank being filled to become cooler.

If you would like to re-work these observations (I believe Joule died in the late 1890's), please have at it.

Read the article more closely. JT predicts the change in temperature for a free expansion where no work is done. For an ideal gas, this temperature change is zero as the temperature is constant for an ideal gas when the enthalpy remains unchanged. For a real gas, there is a very small change (larger for really big pressure changes during the expansion). The specific heat is not constant for a real gas, and therefore its temperature as a function of enthalpy is not constant over a large range of pressures.

There is no such thing as a free compression of a gas. You cannot compress a gas without doing work on it. In the case where you fill the tank with Helium, the JT effect predicts that the helium will get warmer as it expands into the tank, and the ideal gas law predicts that the gas already in the tank will also get warmer as it is compressed. The only cooling that occurs is the cooling of the gas in the bank as the gas expands. This cooling is predicted by the ideal gas law, and, as banks are usually very large, and this expansion is therefore very small, the effect of starting the helium out cooler before it goes through the valve is generally very small compared to the heating of the mix already in the tank. The work being done in this case is the work required to push the air through the valve.

 

[LAKelpDiver]

I read three posts into page two, then jumped to the end.

It just does Jimmy!

[video=youtube;1E6IfdUJn6s]http://www.youtube.com/watch?v=1E6IfdUJn6s[/video]

Sorry if this has already been posted in this thread, there was no way I could go through it all.

 

[rakpix]

This is sort of a geeky physics/gas law question, but I thought someone here might be able to answer it for me or at least point me in the right direction. I've been diving for over 40 years, but I've never really understood why tanks get hot when you fill them from other tanks. I signed up here just to ask.

I understand why tanks get hot when filled directly from a compressor, or at least I think I do. You start with a lot of air out in the atmosphere - a large volume V1. You compress that air with a compressor and it gets hot as you stuff it into the small tank - a smaller volume V2. I looked up "adiabatic heating" and I understand it. I even went through the equation for an ideal gas (PV=nRT). That equation has two unknowns if I stick in the volume change. I was able to figure out that a second equation comes from the effect of the number of degrees of freedom of nitrogen N2 and oxygen O2 diatomic molecules (PV**gamma is constant). Those two equations allows one to solve for temperature T and pressure P given the change in volume V. Gamma is derived from the number of degrees of freedom of the molecules and is also equal to the ratio of heat capacity at constant pressure to heat capacity for constant volume - roughly 1.4 for air.

Another way of looking at why the tank gets hot (when filled directly from the compressor) is just to notice that you are running a pump and putting energy into the system by compressing the air. At least it makes sense that adding energy makes it hot. You could get that energy out again by running a pneumatic motor from the scuba tank. I'm pretty sure that one could (at least in theory if we imagined perfect compressors, no frictions, no heat gain/loss, etc.) let the hot filled tank expand out into the atmosphere and get back to where you started with all the gas at room temp and atmospheric pressure.

But that's as far as I got. Why does the scuba tank also get hot when filled from a higher pressure tank (or a bank array of higher pressure tanks? This has always bugged me. When the array of tanks was filled by the compressor, they got hot. At that point it's just like the first case of directly filling the scuba tank from the compressor. At least in theory it could expand out and the whole thing would return to the starting temp/pressure/volume.

So when you let the air out of a high pressure tank to fill a scuba tank, you are going from the initial high pressure small volume in the fill tank to lower pressure, larger volume, in the volume making up the fill tank plus the scuba tank. You aren't adding any energy like you did with the compressor in the first case. In fact, since the fill tank was allowed to cool after it was filled, it makes sense that it cools off below room temp. But why in heck does the scuba tank get hot? You're doing the opposite of the first case - you are now going from high pressure small volume to lower pressure larger volume. Shouldn't the air cool off? Why does the scuba tank get hot in both cases when you are doing the opposite thing in those two cases?

I've looked on the web and found some stuff, including some questions just like the one I asked, but I'm pretty sure the answers are wrong.

I know it's not because the remaining air in the scuba tank being filled gets compressed. The tank gets hot even if it was at a a vacuum when filled.

Thanks, in advance, for any help on this.

You touch on an interesting point. An important concept to realize is that temperature is not a measure of energy; rather, heat is a measure of energy. Temperature is a measure of the rate-of-transfer of energy (heat, light, etc) between two media. In the example of transfilling gas from a high-pressure vessel at ambient temperature to a low-pressure vessel at ambient pressure, you would expect the final state to be two intermediate-pressure vessel at ambient temperature ...

... after all, no work is being done on the system ....

Why then, does the high-pressure vessel drop in temperature, while the low-pressure vessel rises in temperature? The answer is not Boyle's law, or Charles law, or Carnot's law, or Gay-Lussac's Law, or Avagodro's Law, or Godwin's Law ... or any law, for that matter.

... after all, no work is being done on the system ....

The answer is pretty simple: gasses move very fast and make very poor conductors. When you open the valve, gas travels down the whip at approximately the speed of sound, and slows, slows, slows down as it encounters more, more, and more gas coming from the opposite direction. When the rate of of gas entering the low-pressure vessel approaches the rate of gas leaving the low-pressure vessel, we call that equilibrium.

... this transfer of energy - in the form of matter - happens very fast, in a matter of seconds....

When we opened the valve, gas particles traveled from the interior surface of the high-pressure vessel (where they absorbed energy) to the interior surface of the low-pressure vessel (where they deposit energy.) Now, lets imagine that the outside surfaces of both vessels are insulated from the atmosphere with foam insulation, so that the atmosphere is not able to heat or cool the vessels. After equilibrium is reached, gas continues to flow in both directions along the whip. However, the metals of the previously-low-pressure vessel have more energy than the metals of the previously-high-pressure vessel, and both metals are in contact with the gas. Gas particles travel from the interior surface of the previously-low-pressure vessel (where they absorbed heat) to the interior surface of the previously-high-pressure vessel (where they deposit heat) until the metals of both vessels are in equilibrium

... this transfer of energy - in the form of heat - happens very slowly, in a matter of hours ....

The reason we never get to see this redistribution of heat between the vessels is twofold. First, divers are very impatient people. Every hour we wait at the fill station is an hour we could spend here on ScubaBoa ... I mean, every hour we wait at the fill station is an hour we could spend diving. We usually disconnect the whip before significant heat transfer can take place. Second, our cylinders are not coated with insulating foam. Even if we left the whip connected, the atmosphere or the Sun will quickly warm the high-pressure bank to ambient temperature; and the cylinder will quickly radiate excess heat away.

... in summary, its kind of like when you try to return an item you bought with a Visa card at your LDS. When you want to purchase the item, the money travels very fast, in a matter of microseconds. But god help you when you want to return the item, because the refund travels very slowly, in a matter of hours, or days, or weeks .... those Nazis!

 

[JohnN]

You touch on an interesting point. An important concept to realize is that temperature is not a measure of energy; rather, heat is a measure of energy. Temperature is a measure of the rate-of-transfer of energy (heat, light, etc) between two media. In the example of transfilling gas from a high-pressure vessel at ambient temperature to a low-pressure vessel at ambient pressure, you would expect the final state to be two intermediate-pressure vessel at ambient temperature ...

... after all, no work is being done on the system ....

Why then, does the high-pressure vessel drop in temperature, while the low-pressure vessel rises in temperature? The answer is not Boyle's law, or Charles law, or Carnot's law, or Gay-Lussac's Law, or Avagodro's Law, or Godwin's Law ... or any law, for that matter.

... after all, no work is being done on the system ....

The answer is pretty simple: gasses move very fast and make very poor conductors. When you open the valve, gas travels down the whip at approximately the speed of sound, and slows, slows, slows down as it encounters more, more, and more gas coming from the opposite direction. When the rate of of gas entering the low-pressure vessel approaches the rate of gas leaving the low-pressure vessel, we call that equilibrium.

... this transfer of energy - in the form of matter - happens very fast, in a matter of seconds....

When we opened the valve, gas particles traveled from the interior surface of the high-pressure vessel (where they absorbed energy) to the interior surface of the low-pressure vessel (where they deposit energy.) Now, lets imagine that the outside surfaces of both vessels are insulated from the atmosphere with foam insulation, so that the atmosphere is not able to heat or cool the vessels. After equilibrium is reached, gas continues to flow in both directions along the whip. However, the metals of the previously-low-pressure vessel have more energy than the metals of the previously-high-pressure vessel, and both metals are in contact with the gas. Gas particles travel from the interior surface of the previously-low-pressure vessel (where they absorbed heat) to the interior surface of the previously-high-pressure vessel (where they deposit heat) until the metals of both vessels are in equilibrium

... this transfer of energy - in the form of heat - happens very slowly, in a matter of hours ....

The reason we never get to see this redistribution of heat between the vessels is twofold. First, divers are very impatient people. Every hour we wait at the fill station is an hour we could spend here on ScubaBoa ... I mean, every hour we wait at the fill station is an hour we could spend diving. We usually disconnect the whip before significant heat transfer can take place. Second, our cylinders are not coated with insulating foam. Even if we left the whip connected, the atmosphere or the Sun will quickly warm the high-pressure bank to ambient temperature; and the cylinder will quickly radiate excess heat away.

... in summary, its kind of like when you try to return an item you bought with a Visa card at your LDS. When you want to purchase the item, the money travels very fast, in a matter of microseconds. But god help you when you want to return the item, because the refund travels very slowly, in a matter of hours, or days, or weeks .... those Nazis!

Reproduced without any permission. . .

Steve walks warily down the street,
With the brim pulled way down low
Ain't no sound but the sound of his feet,
Machine guns ready to go

Are you ready,
Are you ready for this
Are you hanging on the edge of your seat
Out of the doorway the bullets rip
To the sound of the beat

[Chorus]
Another one bites the dust
Another one bites the dust
And another one gone, and another one gone
Another one bites the dust
Hey, I'm gonna get you too
Another one bites the dust

How do you think I'm going to get along,
Without you, when you're gone
You took me for everything that I had,
And kicked me out on my own

Are you happy, are you satisfied
How long can you stand the heat
Out of the doorway the bullets rip
To the sound of the beat

[Chorus]

Another one bites the dust
Another one bites the dust
Another one bites the dust
Another one bites the dust
There are plenty of ways you can hurt a man
And bring him to the ground
You can beat him
You can cheat him
You can treat him bad and leave him
When he's down
But I'm ready, yes I'm ready for you
I'm standing on my own two feet
Out of the doorway the bullets rip
Repeating the sound of the beat

 

[rakpix]

I'm confused.

 

[Chilly_Dipper]

I'm confused.

I remember feeling the exact same way when I took thermodynamics... oddly enough it does come together EVENTUALLY

---------- Post added March 8th, 2013 at 01:09 AM ----------

You touch on an interesting point. An important concept to realize is that temperature is not a measure of energy; rather, heat is a measure of energy. Temperature is a measure of the rate-of-transfer of energy (heat, light, etc) between two media. In the example of transfilling gas from a high-pressure vessel at ambient ..blah blah blah

I think your misunderstanding the basic concept of potential energy vs kinetic energy in your very long winded explanation, and the fact there is no such thing as a 100% efficient conversion of one form to the other. But hey that could just be the rum talking