Seeking Mechanical Engineer for tank study

[VikingDives]

His LinkedIn page says he has a Bachelor's in Business from a community college. . .

Maybe a mistake, or maybe they offered a BA at one time, but that seems unlikely.

 

[Angelo Farina]

Ex firefigheter here, but also engineer with a PhD on Applied Physics and university professor.
When a firefighter needs an extended range and little weight, he employs a CC rebreather, as this Drager unit:

Draeger.Web WWW - Draeger Master

www.draeger.com

Using air tanks is good only for quick actions, when in service I had never entered an huge smoky building using just a small SCBA system, with the risk of finishing air inside.
We ere trained with CC rebreathers, both for scuba diving and for firefighting, so why bother with anything else?

 

[Wookie]

I looked at his Linkedin and he has zero engineering background. Only formal education is a business degree and the USMC SGT program. And no real relevant experience where he would pick it up on the job.

Well, hell.

I'm an engineer, Nuclear and Master's in Marine Engineering, and I don't know a small percentage of what Mark knows about cylinders.

All I can do is fill them and drain them. I got a lot of experience with that....

 

[Manatee Diver]

Well, hell.

I'm an engineer, Nuclear and Master's in Marine Engineering, and I don't know a small percentage of what Mark knows about cylinders.

All I can do is fill them and drain them. I got a lot of experience with that....

I am not questioning that, but more likely he is distilling the works over others rather than doing his own unique research. That doesn't mean he doesn't have value but in this case you it would probably best to have someone with an engineering background within the field.

As that is the difference between "This probably not going to work because it is similar to X attempt" vs "This definately isn't going to work, and here are the numbers."

 

[Cthippo]

FWIW modern SCBAs are 5500 PSI kevlar wrapped aluminum and come in three sizes, 30 minute, 45 minute and 60 minute. To keep up with the filling requirements a lot of departments have gone to higher volume compressors. I looked it up once and our older 4500 psi 45 minute bottles are about 68 CuFt.

Rebreathers for emergency work are almost unheard of in North America, and where they do exist it is usually for cave or tunnel rescue. FDNY has some, I think Oakland CA has some, and the mine reacue teams in Nevada use them. Not aware of any others out there. Only time I have ever seen one was at a the BC Mining Museum at Britania Beach where a mine rescue team was practicing in one of their tourist tunnels. They may be more common in Europe.

 

[Tanks A Lot]

...scba tank that is 1/3 the size of standard tank but holds same volume of air...

The math behind most of this is not terribly difficult. The design must be a composite, as it would otherwise be a nightmare weight wise.

SCBA cylinder range widely in size and pressure rating. From as small as 1.1L to 9.0L and 150bar to 379bar. As your post lacks more specifics, I will assume the "S80" of the SCBA world, which is a 6.8L composite cylinder charged to 310bar in the US or 300bar in Europe. This is the most ubiquitous cylinder in use world wide for fire departments etc.

Unfortunately air does not behave like ideal gas at higher pressure and models badly as an ideal gas above 140bar at room temperature. Thus you have to treat the gas as a real gas and include the corresponding formulae.

The air volume a 6.8L cylinder charged to 310bar holds is roughly 1859L. Luxfer specifies the amount this cylinder holds at 1840L, which is due to them using a different compressibility model then I used above, but those 19L shouldn't bother us for an armchair calculation.
Your design occupies a third of the size. I will assume a third of the volume and disregard material sizes, which are already very thin on modern composite cylinders. I know this is not entirely correct/fair, but again probably good enough for an armchair calculation.


This means we are looking at for a composite cylinder with a 2.27L volume, holding 1859L of air, charged to an unknown pressure. If you plug in the numbers, you end up with a pressure of 4750bar. At 4750bar the Z-Factor for air is roughly 5.7130.

Luxfer has H₂ composite cylinders on the market today which are rated to 520bar. To my knowledge these are one of the higher rated composite cylinders under the TPED.

I'm not saying what you have been proposed to invest is impossible on an engineering scale, but it is an order of magnitude above what is on the market today. As others pointed out, just this new cylinder won't do, you need compressors and regulators to handle these very different pressures. I do not see that this is a serious or viable project. The benefits seem very small to the downsides.

Compressibility considerably diminishes with higher pressure, negating the gained benefits by these higher pressures. Do not let yourself be fooled by ideal gas law computations, as these do not apply here at all.

The above has been calculated with MiniREFPROP and Coolprop. If you really want better numbers, obtain REFPROP or ESS.

 

[broncobowsher]

4750 bar? At what pressure does air turn liquid (at roughly room temperature)?
Quick search found liquid Nitrogen has an expansion ratio of roughly 700x going from liquid to gas. Air shouldn't be far off that. You are not going to compress air any more once it goes liquid.

 

[Tanks A Lot]

From the Engineering Toolbox:

Critical point: The end point of the pressure-temperature curve that designates conditions under which a liquid and its vapor can coexist. At higher temperatures, the gas cannot be liquefied by pressure alone. At the critical point, defined by the critical temperature Tc and the critical pressure Pc, phase boundaries vanish.

Critical temperature of air: 132.63 K = -140.52 °C
Critical pressure of air: 37.858 bar