Two pressurized bottle with a valve is open on each

Two pressurized bottle with a valve is open on each


It depends on whether the gas is above or below the [Joule-Thomson](https://en.wikipedia.org/wiki/Joule%E2%80%93Thomson_effect) inversion temperature.


My thoughts as well. If it were Ghe it would actually get warmer as it expands. But not by much if I understand.


The gas exiting through the valve is affected by the Joule-Thomson effect, but the gas inside of the bottle will cool down regardless (even Helium bottles get cold when you vent them to atmosphere or transfer the contents from a high pressure bottle to a lower pressure bottle)


I believe non-insulated one will empty first. They will both cool down due to expansion which will decrease pressure (ideal gas), and therefore decrease mass flow (choked flow equation) compared to if it were isothermal. However, the one without insulation will be closer to isothermal since the heat from the surroundings will heat it up more. I'm kind of assuming here the cylinder doesn't start warmer than the surroundings.


I would personally go for adiabatic vs isothermal expansion here. If you assume that the bottles are draining slowly enough that the insulated one never changes temperature (infinite thermal mass of the surrounding medium), and the insulation is perfect such that no heat is exchanged with the environment.


Hello friends. I had an interview at an aerospace company and asked me a question about this scenario. Two pressurized bottle filled with some gas to some high pressure with valves on each end are opened. One has insulation the other doesn’t not. Which one empties first or do they empty at the same time? I don’t think I answered it right. I said since the insulation keeps the bottle from cooling off so fast it’ll maintain density longer and the mass flow will stay the same.


Cooling is from the gas expansion. Insulation keeps the ambient air from warming it up.


Insulation keep the tank to be in adiabatic process. Qin=0


Yes which is why the inside will be cooler in the insulated tank.


Realize when they ask you things like these they are not solely looking for the right answer, they are looking for the way you use logic to get through the problem and break it down. So make sure you walk the interviewers through your thought process step by step. Anyways, assuming ideal gases, as the tanks are pressurized, the gas will warm up and, in turn, will warm up the pressure vessels. Assuming that the surrounding temperature is colder than the now pressurized gas the noninsulated vessel will transmit heat and begin to cool down the gas inside dropping the pressure in its vessel. The insulated vessel will have less of this effect. Given that the driving force for the gas out of the valves is the pressure and the non-insulated has less, the non-insulated will empty more slowly. This would reverse if the temperature was higher in the ambient air than in the tanks. This is my take, someone please correct me if Im wrong.


Counterpoint- why would that matter?


Why does having an understanding of high pressure gasses leaving a container apply to aerospace engineering?


Maybe this person meant: I would tell the interviewer that, while the answer is clearly the bare tank, I'd have to be convinced by running calculations that the difference for most cases isn't functionally quite small, especially since that insulation isn't perfect in real life.


Exactly, among other things. If you could even measure the difference, and if that difference had some sort of meaningful application to some concrete real world situation, you wouldn’t (at least I hope not) rely on your first order opinion of the problem. If it was safety critical you’d probably build a bunch of test equipment and see what the actual values were under a lot of situations. Indeed the “physics-y” answer may not even be practically right when you actually build the thing. You might have hidden heat transfer that you didn’t even envision as having an impact. Something that you couldn’t even foresee until you built a model. We don’t actually know the gas involved - I think someone posted some stuff here about the Joule-Thomson effect (which admittedly I know very little about) but the effect can be different for different gasses. This is a bogus bullshit question- it is created to convince the interviewer that they’re smarter than the applicant and has very little bearing on how that person will actually pan out as an employee. Has nobody here ever hired anyone?


This is a brain teaser not a good interview question.


If they are both pressurized, unless they were pressurized to the point of being liquids, insulation doesn't matter. When the valves are opened, the pressure escapes until it is equal inside and out. If the pressurant is cold and liquid like LOx or LN2 or a very hot liquid like steam then the insulation might matter, but it seems like this is all gasses. The theoretical answer is the insulated tank empties slower but there would be no functional difference in the common/likely case. Though I guess then you might want to walk through the specific equations to get to your specific answer that proves your general answer.




What’s your solution?


Are you a new grad or early career engineer? As some of the other posters have said, this would be a weird question for someone with experience - it just isn't nearly as important as practical aspects of the job. But it's often hard to ask new grads or early career engineers practical questions, whereas questions like this can be a good combination of "what is the candidate's mindset like?" and "can they apply the knowledge they learned in school?". I believe this is the correct answer: If you are venting a high pressure bottle to atmosphere, the system will be choked at some point. When flow is choked, it flows at Mach 1 (velocity proportional to **sqrt( k * R * Temperature )** and with a density proportional to **Pressure / Temperature**. Assuming an ideal gas, **Pressure = Mass / Volume * R * Temperature**. Mass flow will equal **Area * Density * Velocity** Getting rid of the volume and ratio of specific heats and gas constant and area, the mass flow out of the bottle is proportional to **mass / sqrt( Temperature )** On a slightly different track, both bottles will cool down as the gas inside of them flows out. One bottle will be kept slightly colder than the other due to the insulation, which will reduce its temperature. If the mass flow out of the bottle is proportional to **mass_remaining / sqrt( Temperature )**, then the colder bottle will have higher mass flow out. This means that the insulated bottle will empty first.