Adiabatic Decompression Chamber

Similar problems have been solved for nuclear research for the "bubbles chamber".

The problem is more complex as it appears at 1st look:

- the ration contact area /kg mass of gaz has to be as small as possible AND the thermal difusibility of the walls has to be also as small as possible. This is the comment for the design and for the "maintained" decompression.

- the volume change has to be fast since this is the only way to reduce dynamic heat exchanges. This is for the case that a "very adiabatic" process is analysed.

It can be done but requires a lot of optimisations one cannot do on a forum.

I could help but not at this superficial level.

Thank you for the thoughtful reply. I agree with you comment this is more difficult than most reflexive people would give it credit for. I am simulating what occurs in a hurricane or tornado. The difference is hurricanes and tornados spread over vast areas to create their adiabatic conditions and pressure drops. I am trying to accomplish adiabatic decompression in a human controlled mechanical device. I am not as familiar with nuclear research, but that is the whole point of this kind of forum to through out a problem to the smartest people in the room and let them bring to bear experience from a multitude of industries.

Your comment about the speed of the decompression to avoid heat transfer I would agree with. This does not seem to be difficult to accomplish. For example, we should be able to create a 500 mbs decompression within 1 second or less, and that should be sufficient to avoid the dynamic heat exchange. For simplicity purposes I can see creating a piston cylinder out of PVC. PVC is fairly well insulated and comes in many diameters going as large as 3 feet/1meter. It should not be impossible to fashion a piston out of PVC with appropriate O rings to create the necessary seal. In fact a human being can provide the energy needed to draw the piston back and create a 500mbs decompression. A simple latch could be used to hold the piston in place in order to take some measurements.

I hope that someone else has needed a similar device and built it, then I will not need to fabricate such a machine. NASA or some other governmental agency must have needed to create an adiabatic decompression without evacuating the air in order to create the vacuum. My real desired outcome from this forum to identify an existing device that I can rent/lease to conduct some testing on. My plan "B" is to fabricate the device myself.

This might also be accomplished by a high powered vacuum pump sucking air through a tube that expands in volume and then shrinks back to its original diameter before exiting through the pump. This would create a continuous flow and probably create a more efficient way to create the adiabatic decompression, but I would not be able to stop the process during maximum decompression. Maybe I can live with this. Another way to look at this is using a bellows vs a piston cylinder. There are other design issues that arise taking this approach. I have been told that using PVC impregnated canvas (like material used in inflatable boats - zodiacs) is highly insulated and might be a very good material to consider for a bellows approach. Here in America we have these disposable blow up small lounging pools. Parents blow up rings of plastic that create the walls of the pool with a flat plastic bottom. This concept would be ideal to create the low mass and high insulating properties needed, and if the material was the inflatable boat fabric it might have the rigidity to sustain the deformity coming from decompression.

Has any of my comment sparked any additional thoughts by anyone?

Thanks for your comments. I hate to be presumptuous, but this is going to be a challenge for even the smartest people in the virtual room.

A few remarks:

- from the design point of view, "O"-rings are relatively "stiff"; it would be better to use lip seals as in other pneumatic devices since the lip is more adaptable to geometrical errors.

- if we compute the force and the energy required to decompress a volume of air if the piston is 1m in diameter values are impressive. It is of course better to use big cylinders since the ratio I mentioned is (A/V) decreases if dimensions increase (and with it the errors in comparison to theoretical values).

A= Π*( 2* D^2/4+D*L) V= Π*D^2*L; L=λ*D;

A/V=K*f(λ)/V which shows the interest for bigger volumes.

For V= constant it results as optimal value λ = 1.

From the function point of view the pressure will be reduced to half if the volume increases by 64% (adiabatic). The stroke would be S= 0.64 m (D=1m).

It must not be 1m in diameter, smaller dimensions can be satisfactory but it depends on the sensors you will use since they are in touch with the air and participate at the exchange and influence of the "adiabatic" process. It is possible to determine even only by computation the minimal still acceptable dimensions considering how much the reality will derive from the theoretical adiabatic process.

Your aim is 0.5 bar pressure difference to ambiance = 5E4 N/m^2. Piston area 0.7854 m^2 (D=1m)→ Force=39.27 kN !!! For a human, even if well build, it is not as easy at it seems to be.

It is a full design required with complex strength computations and a lot of knowledge in PVC behaviour and structural manufacturing.

The installed power for a stroke/s is several kW!

As I mentioned several times an idea is OK if the numbers are OK, unfortunately engineering is a "precise" business.

Hello AE,

Just a thought, perhaps you could use 'weather' balloons inside a measured box of known volume. They would expand for the test you want to do and can also have a 'tap' to control the air 'in and out'?

What do you think?

bb

Hello AE,

It sounds as if you have a 'minds eye picture' as to what you want to do, but, perhaps not how to achieve it? Take a look at this site.

It sounds like a pretty complicated thing to try to do and, at the same time, to measure any parameters? Have you done anything like this before? With Thermodynamics, it is 'keeping control' and measuring 'instant' changes which could cause some difficulties?

Good luck and please keep in touch, this sounds really interesting!

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

http://www.google.com/search?client=safari&rls=en-us&q=adiabatic&ie=UTF-8&oe=UTF-8 .

bb

Try looking at the design of the Alnor Dewpointer. The chamber is smaller than you are looking for, but it may give you some design clues.

Welderman, I went to the Dickey-john website, but they do not provide any descriptive information on the Alnor Dewpointer. Looking at it I am skeptical that it will create an adiabatic decompression. From what little is discussed on the site it sounds like the device was designed to help determine humidity/moisture level/dewpoint at the given ambient conditions. There looks to be a significant amount of mass including metalic mass surround what must be a very small chamber. I can't tell what the device does due to the lack of data on the website, but just looking at the device I think that I can judge the book by the cover.

Thanks for trying! AE

Although I have never studied the design details of the dewpointer, it is an adiabatic decompression chamber. I believe a piston in a glass chamber adiabatically lowers the internal pressure until the water vapor in the gas sample condenses into a visually identifiable cloud. By knowing the pressure ratio of the expansion, one can calculate the dew point of the gas. If the process is truly adiabatic, the mass of the container should play a minor part in the result.

If you consider the fact that water vapour will change phase to water droplets the process as a whole is NOT adiabatic since heat comes from the phase change. There is no heat exchange with external sources but there is a not neglectable heat source in the gas: water vapour! Thus the gas does NOT present an adiabatic decompression!

It is only a remark.

Welderman,

Thank you for the additional comments. Even though you have not studied the design details of the dewpointer you seem to know quite a bit about how it is made. You mention that the device lowers the pressure in the "GAS" sample contained in the glass cylinder. If this gas is ambient air then you will definitely NOT see the condensed water droplets. I can't tell what gas the device would ingest other than ambient air. Even though the water vapor will have changed phase back into liquid form due to the pressure/temp change the condensed water would still be impossible to see with the naked eye. This is why you only see fog or a cloud when you look through 20 or more feet of condensed water particles. There is NO way to see the condensed water particles in a small diameter glass filled cylinder. There simply is not enough water in the volume of air within the cylinder. Now if you tell me the gas in the chamber is something other than air and that there is a much larger amount of water contained in the gas then maybe you could see the fog.

Again, I am skeptical that this is a truly adiabatic decompression. Further when you tell me that you can see the fog in the glass chamber - this tells me that the gas can not be ambient air. I am sure that the device performs well, as evidenced that it has a commercial customer base. But you have now convinced me to call them tomorrow when they are back in the office. I will get to the bottom of this. We don't need to waste anymore mental effort on debating how we think this device works - I will go to the source.

Nick Name made a comment about the heat released during the phase change of water vapor back to liquid. As you probably know solar energy is used to evaporate water into vapor and the water vapor stores this thermal energy until it changes phase back into liquid water. BUT the thermal energy is not realized during the decompression. You will only feel the thermal energy if you remove the water during the decompression and then bring the air mass (less a majority of the water) back to the ambient pressure. The reason this is true is as follows - if you reverse the decompression back to the original ambient pressure all the thermal energy in the air will be used to re-evaporate the condensed water back into vapor. Thus you will wind up with exactly the same temp, and relative humidity when you go back to the original ambient pressure. This is exactly what happens in thunderstorms and hurricanes and all the most violent weather conditions on Earth. I am not sure why you say the process could not be adiabatic just because the water vapor gives up its thermal energy after the phase change. In fact, that release of thermal energy is a nature part of the adiabatic process. If the air comes to saturation during decompression and the vapor changes phase that is natures sign that this is an adiabatic decompression. If there were temperature conductivity between the ambient conditions and the glass cylinder then the air would not cool down enough during the decompression and the vapor would not change phase.

I am on the record as being HIGHLY skeptical because you can NOT SEE a cloud in a small diameter glass cylinder of AIR. Can't be DONE. I will call them tomorrow to better understand how this device works.

Thanks again for the lively discussion. Does anyone have new suggestions or comments/advice.

AE

Welderman, Thank you for the pdf reference. This was very helpful to better understand how the device works. I quickly went through the document and then spoke to a representative at the company. I am not sure that this will satisfy my needs, but it is about as close to a bulls eye as I have seen. I did not not have high expectations about finding something off the shelf. The operator claims that it will work with ambient air, but is primarily used for testing the dewpoint of other gases. The decompression chamber is only 6 sqr inches, and I would like a device to be at least 1 cubic meter. I need to have my partner read through the literature before we determine if this will really accomplish what we need. Thank you again for the suggestion.

Does anyone else have a suggestion for a 1 cubic meter adiabatic decompression device?

You haven't told us what you are trying to accomplish with this test. The Alnor will measure the dewpoint of water in any gas including air. Dew point is independent of the carrier gas. If it is dewpoint you are trying to measure, there are much less expensive and straight forward methods to make the measurement. We use a thermoelectrically cooled dew point sensor made by General Eastern (now General Electric) and others. They are good to about .02 degrees C but are available in higher accuracy versions.

I am in the process of building a proof of concept device. I have developed some technology that harvests latent energy of condensation. I am in the process of proving the concept prior to a full blown prototype. The larger the chamber the more air I process per cycle, the more air processed the more money to be made. My next challenge will be to find the most efficient way to rapidly remove the suspended condensed water in the air mass. We plan to use the water so we do not want to contaminate the water nor do we want to spend to much $$ to get the water out, but we need to remove it from the chamber rapidly.

Any suggestions? You can email me directly to discuss further.

I am really pleased that this is getting the attention of some of the more experienced and seasoned engineers that use this site. As Nick Name pointed out the it becomes much easier to achieve an adiabatic decompression the larger the chamber volume becomes. This is because the mass of the machine becomes smaller to the mass of the air plus the natural insulating characteristics ensure that there is little heat transfer to the center of the air mass from the air in contact with the outer walls of the chamber. So for example, a 100 foot wide cylinder could be made out of metal and still allow a nearly adiabatic decompression. But the smaller the volume becomes the more impossible it becomes to perform an adiabatic decompression. I also agree with your suggestion concerning the lip seal. I used the O ring as an example of what could be used not as the preferred embodiment/solution. But as you can imagine it is much easier to come across O rings in various sizes than lip seals. Remember I am hoping to use off the shelf and not fabricate, so my skunk works team needs to use readily available materials. You are also correct that it would be quite a feat of strength to pull the vacuum needed for a 1 meter diameter piston. I did not qualify my earlier comment that a person could pull the vacuum necessary. I pulled a 500 mbs vacuum on a PVC tube roughly 5 inches in diameter, a far cry from 1 meter in diameter! The comment concerning the measurement devices is well put. It goes without saying the any device inside the chamber will have an effect on the adiabatic process, so if the chamber is small this will drive a more scrutinizing material choice, but if the chamber is larger then I can get away with more off the shelf materials and devices. Currently I was hoping to use an RF operated measurement device so there would be no physical connection to the ambient environment.

Baby Bear's weather ballon idea is VERY interesting! I am not sure how you envision the balloon expanding? But here is how I could see using your idea, and I will just describe the concept don't get hung up on my details those can we worked out if the concept makes sense. I could see a large box say 3 sqr mtrs with a ballon inside. The ballon would be filled with air via a non conductive material tube (PVC or ??), there would be a shut off valve between the balloon and the tube. The balloon would be filled with 1 sqr mtr volume of ambient air, then I would draw a vacuum inside the box and this would hopefully expand the balloon thus creating an adiabatic decompression inside the balloon. What do you think would that work?

Baby Bear we have thorough calculated the thermodynamics of the process using well understood and non controversial atmospheric physics. So we are going into this assuming that our measurement will be confirming what our scientific calculations predict will be occurring within the balloon. At this point in our proof of concept we don't need to be uber precise - in that we capture temp & pressure changes in microseconds. I would be practical concerning my measurement devices, so balancing cost vs speed of response. That may very well change as we get further along the commercialization path. I will look at the site that you refer to shortly.

Welderman, I will also take a look at the site you suggest as well.

Thanks to everyone who took the time to share your expertise and I look forward to your responses to my comments.

Cheers,

AE

To all, I should point out that my partner is a PhD Atmospheric Physicist from a major research university. So I feel like I have a very strong subject matter expert on the adiabatic process on my team, but I am lacking engineering talent. That is why I am reaching out to this forum.

AE