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

[Doppler]

Brain bleach, anyone?

 

[RJP]

but as long as we know that no heat is added and no work is done on the system, then we know that the system energy is unchanged.

So the premise of this thought is essentially "...if we know that nothing has changed, then we can conclude nothing has changed."

As long as energy is unchanged, all we need are the final pressures and volumes to determine the final temperatures.

Or a thermometer...

 

[jimmyw]

The heart of the issue continues to be that you really have no idea what's going on here. Seriously. No one can explain "why they disagree" with you, other than to point out that you are wrong. As a logic professor in college used to say "Error has no defense."

No way I'm reading everything you just posted above closely, because it starts out wrong from the get go...

Consider this: Put an evacuated/empty receiving tank inside the donor tank. Open the valve of the receiving tank to let in the gas of the donor tank. Can you predict what will happen?

FIRST: The air coming out of the compressor is heated because is has been compressed.

We agree on this. To achieve compression, the compressor had to do work.

Has nothing to do with taking heat from the electricity coming out of the wall.

Agreed. It has to do with taking energy out of the wall, using that energy to do mechanical work to run the compressor (push the piston of the compressor down) to add energy to the gas. Some of the stored energy shows up as pressure and some as temperature. Since the process is reversible we can take back the energy, and if we do, the gas cools off by expansion and we have the energy we extracted. If we just let the gas suddenly expand (free expansion or Joule expansion) then that energy is left in the gas and appears as heat, so the gas doesn't cool off.

I could run the compressor in a walk-in freezer hooked up to an exercise bike and the air that is compressed would still be warmer than the ambient temperature at which it started.

Agreed.

Go grab a bicycle pump, hook it up to a tire, give it one compression. Feel the wall of the pump. It's warm. No electricity, no engine, no nothing.

There's something - You had to do work to push down the pump. You added energy. Again, some was stored as pressure, some as heat. (As an aside - notice how similar heat is to pressure. Heat results from the average motion of the molecules, while pressure also results from the molecular motion. The difference is that heat energy is also stored in vibrations and rotations of the molecules, while pressure only comes from the translational motion (XYZ motion) of the molecules. )

Just compression. Yes, energy from your arms was expended. But the heat is created from the compression of the gas, not the warmth of your muscles.

Agreed. No heat is added in any of the processes we are talking about (filling with compressor or filling with another tank). They are all "adiabatic processes" which just means no heat was added or subtracted.

SECOND: The air does NOT NOT NOT NOT NOT "expand into the receiving" tank.

You are wrong. Think about the case of the tank placed inside the donor tank. The gas is expanding into the empty space of the donor tank regardless of where the donor tank is placed. I agree this is the source of our disagreement. The gas starts out in the donor tank and ends up in the donor tank plus the receiving tank. I don't see any way to call that compression. It's not. It's expansion, and it's what has bugged me for years whenever I tried to get this resolved. Until we agree on something as fundamental as the starting and final volume of the gas, we won't get anywhere.

The air is COMPRESSED into the receiving tank. What causes this compression? The pressure of the gas remaining in the donor tank pushes the gas into receiving tank until the two tanks equalize.

We agree that during the process, some gas arrives in the receiving tank, and that as has expanded. We agree that after that expansion, it get recompressed. The problem is that it always gets compressed less than it expanded. It has to be the case if the final volume is going to be greater.

Again, if this was not the case, you would not be able to transfill the tank, because you would not be able to get more than 1atm/14psi into the tank without compressing the gas.

Again, we agree that when the receiving tank has reached 1 atm, the gas in that tank is getting compressed. But that gas started at the pressure of the donor tank before it decompressed to 1 atm. When it ends up, it will at higher pressure than 1 atm, and lower pressure than the donor tank. Since the gas started in the donor tank, the entire process is expansion and decompression.

I just ran this concept past my 8yr old son. His response? "Duh..."

Ask him what happens to compressed air when you open the valve of a tank. It expands and decompresses. That happens even if it ends up in another tank.

Because the air is compressed into the receiving tank it warms up. Just that same as if it came from a compressor, a bike pump, or any other manner you can find to get more than 1atm of air into the tank. In your words above, the pressure gradient of the gas in the donor tank "adds energy" to the gas in the receiving tank.

I'm not sure if we agreed - but I'm convinced that the donor tank is storing energy in the compressed gas (yes, I've seen a tank rupture) and when we let some of that gas out into the receiving tank, some of that energy is carried into the receiving tank. I don't claim that any energy is added to the entire system (donor plus receiving tanks)

In fact, the 3,000psi of energy stored in the donor tank came from a compressor at some point. That "compressor energy" is stored their as 3,000psi of pressure. When you open the valve that energy - that was initially transferred to the donor tank from a compressor - is then transferred to the receiving tank.

Actually, we agree on this! The source of the heat we see in the receiving tank is from the stored energy in the donor tank.

You're so worried about entropy and enthalpy and formulas that you're completely missing the obvious, fundamental flaw in your thinking: You believe that the gas going into the receiving tank is expanding. It is not. It is being compressed.

It's frustrating to have this simple point be the sticking issue. The gas that ends up in the receiving tank started at a higher pressure and ended up at lower pressure. I'd say it's simple, but obviously it's not . The gas that stayed in the donor tank also ended up at lower pressure, but the two gases changed temperature in opposite directions. My question was "why?"

I'm willing to consider the process in two steps - first the gas expands into the receiving tank, then it gets compressed in the receiving tank, but when we do that, we still end up with net expansion for all gas, not compression. The expansion is always greater than the subsequent compression. The heating from the compression is always less than the cooling from the original expansion. The heat comes from the release of energy by the donor tank.

---------- Post added February 24th, 2013 at 12:04 PM ----------

So the premise of this thought {energy is unchanged} is essentially "...if we know that nothing has changed, then we can conclude nothing has changed."

No. If we know that energy has not changed, we can figure out the effect on pressure and temperature if the volume changes. It's actually simpler than adiabatic heating and cooling (where energy does change). If energy is unchanged, the temperature is unchanged and PV is constant.

One problem is that this process is not reversible. You can let the gas freely expand (Joule expansion=free expansion), but you can't compress the gas without adding more energy, which heats up the gas.

 

[RJP]

I'm willing to consider the process in two steps - first the gas expands into the receiving tank, then it gets compressed in the receiving tank, but when we do that, we still end up with net expansion for all gas, not compression.

Dude - the gas does NOT expand into the receiving tank. I do not understand how this is not clear to you. The receiving tank is NOT empty. It has 1atm/14psi in it at sea level. If you put one more molecule of gas into it... you must compress that molecule and every other molecule already in the tank. So your premise that there is a "net expansion" for all gas is not correct. There is a decrease in pressure in the donor tank, and an increase in pressure in the receiving tank. If the receiving tank were a rubber balloon capable of expanding to 40cf, then yes, you could drain 1500psi from the donor tank into the balloon and then you would have a net expansion for all the gas. But the receiving tank is a rigid container. To put more gas in it than 14psi requires that any additional gas be compressed.

What would happen if you used a hypothetical "evacuated tank" as the receiving tank is irrelevant. But, just to play along with that nonsense, as soon as the "evacuated tank" reaches 1atm/14psi... any other gas that goes into that tank MUST MUST MUST MUST MUST be compressed. There is no other way around this fact.

 

[redacted]

We agree that different parcels of gas go through different PVT changes during the process, but as long as we know that no heat is added and no work is done on the system, then we know that the system energy is unchanged. As long as energy is unchanged, all we need are the final pressures and volumes to determine the final temperatures.

Of course the laws of conservation of energy and matter apply. In the bank fill operation, the expanding gas absorbs energy (heat) from the matter in and around the doner tank, primarily the controlling valve and that part of the system gets cool. When the gas is again compressed into the receiving tank it sheds that excess energy (heat) and the receiving tank gets warm. By law, the heat gain and losses are equal.

 

[halocline]

Yes, it's as simple as that. Except.....

Okay, then please answer this question with one word, yes or no. Do you accept the fact that air gets warm when you compress it?

 

[jimmyw]

Dude - the receiving tank is NOT empty. It has 1atm/14psi in it at sea level.

Years ago this was the answer I kept getting. It seems reasonable, but the first thing that made me question it was that I watched an oxygen tank being filled. It was required by law to be evacuated to make sure there wasn't an explosive gas in there that would react with the high pressure oxy. The tank still got hot, even when started at vacuum empty. Later I worked through the math. Adding some room temp air to the tank at the start does add some heat to the process due to its compression, but not near enough to account for the total heating.

If you put one more molecule of gas into it... you must compress that molecule and every other molecule already in the tank. What would happen if you used a hypothetical "evacuated tank" as the receiving tank is irrelevant.

It's not irrelevant since the same process is at work. If the evacuated tank gets hot (and it does - I've felt it) then the non-evacuated tank also gets hot.

But, just to play along with that nonsense, as soon as the "evacuated tank" reaches 1atm/14psi... any other gas that goes into that tank MUST MUST MUST MUST MUST be compressed. There is no other way around this fact.

To say that the other gas going into the tank "gets compressed" is to ignore the fact that the "other gas" started at higher pressure. Once the gas going into the evacuated tank reaches 1 atm, it has expanded from its original high pressure.

---------- Post added February 24th, 2013 at 12:32 PM ----------

Okay, then please answer this question with one word, yes or no. Do you accept the fact that air gets warm when you compress it?

Yes. I will happily write down the equations.
Let me ask one, yes or no: Do you accept the fact that air does not always get cold when allowed to expand - specifically, the temperature is unchanged when allowed to "freely expand" (Joule expansion) as described in the wiki article on Joule expansion?

---------- Post added February 24th, 2013 at 12:45 PM ----------

Of course the laws of conservation of energy and matter apply. In the bank fill operation, the expanding gas absorbs energy (heat) from the matter in and around the doner tank, primarily the controlling valve and that part of the system gets cool.

I wouldn't describe it quite like this. Even if the donor tank and all the valves are well insulated, the gas remaining in the donor tank itself will cool as it expands. I'd also say that the reason it cools is that it is doing work. What work is it doing? It's pushing the gas going into the receiving tank and getting it up to speed. We could have extracted the energy that was given to that gas by running the gas through a turbine generator or a pneumatic motor. If we had, the gas in both tanks would have cooled. Because we didn't, the gas in the receiving tank got more of the energy than the gas in the donor tank, even though they both expand and both are at the same final pressure.

When the gas is again compressed into the receiving tank it sheds that excess energy (heat) and the receiving tank gets warm. By law, the heat gain and losses are equal.

Again, I don't agree with this description. Thermodynamics laws don't allow the gas to extract energy from the room temperature tank and valve, so the gas never has any excess energy. The total energy is unchanged. However, the compressed gas of the donor tank has internal energy (in the form of both pressure which is derived from the XYZ motion of the gas and its internal vibrations and rotations). That internal energy can be released as kinetic energy (speeding up the molecules leaving the tank) and that kinetic energy can be transformed into heat as the molecules slam into the walls of the receiving tank.

Another way to look at this is that the donor tank gas expands reversibly - giving up energy and remaining at constant entropy (cooling as a result) and the receiving tank expands irreversibly - increasing its entropy and also receiving the energy released from the donor tank.

 

[JohnN]

. . . the first thing that made me question it was that I watched an oxygen tank being filled. It was required by law to be evacuated to make sure there wasn't an explosive gas in there that would react with the high pressure oxy. The tank still got hot

Close oh so close. . . so you've observed that filling a tank with O2 warms it more than filling it with air.

Ergo, something about air is different than pure O2.

If you were to observe a tank being filled with He, you'd observe slight cooling.

But wait ! This isn't accounted for by the Ideal Gas Law (or any of the equations you reference which are based on Ideal Gas(s) )

Simple. Gas(s) are not ideal. Why is this such a problem for you?


Since you're getting twisted around by the equations (you've stated this many time), take a trip to your local technical LDS and ask to watch them fill a tank with He and the same pressure of O2. Measure the temperature change and get back to us. Right now you're just indulging in mental masturbation and counting (and re-counting) how many Angels are dancing on that pin.

 

[RJP]

To say that the other gas going into the tank "gets compressed" is to ignore the fact that the "other gas" started at higher pressure. Once the gas going into the evacuated tank reaches 1 atm, it has expanded from its original high pressure.

Are you ****ing kidding me? Of course the gas started at a higher pressure! That cannot be ignored. Why? Because there's no other way to get it out of the donor tank and into the receiving tank other than to have a pressure gradient sufficient to overcome the pressure inside the receiving tank.

Please answer the two questions below:

After the tanks equalize, you close the valves and remove the fill whip. You put a reg and a BCD on the tank, open the valve, strap it to your back, and jump in the water. With the reg in your mouth, you attempt to draw a breath from the tank.

1.) What happens?
2.) Why?

 

[halocline]

Yes. I will happily write down the equations.

Well, I did ask for a one word answer.....

Anyhow, forget the freakin equations. So, you do agree that when gas gets compressed it warms up.

Next question, one word answer, yes or no. Do you accept the fact that the gas in the recipient tank is getting compressed?

It really is as simple as that. Consider this one of those glorious times in the universe when there's a super-easy-peezy answer.

Now put down the equations and go outside, have some fun!