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SF6 escape tube

Straight down ...
  [vote for,

Evacuating tall buildings in an emergency is a problem.

The usual method is via stairs. This can be difficult for those with mobility issues.

The proposal is for a fireproof tube about 1000mm internal diameter, with a very smooth liner, which runs nearly the entire height of the building.

Entry to the tube is through a curved duct, like a helter-skelter, gradually steepening into the vertical tube via a flap valve.

At the base, the tube curves gently from vertical to horizontal, ending in a padded chamber.

In an emergency, the tube is flooded up with SF6 from pressure tanks. Escapees don an oxygen mask with a 2 minute supply of air. They then enter through a port, and accelerate, but the dense gas acts as an efficient brake, a coolant, and is fire-supressing.

On arrival at ground level the escapee is helped out of the (pressurized) arrival chamber by trained staff wearing line-fed air masks.

In order to accustom the public to using such systems, they would also be offered as amusement park rides.


8th of 7, Sep 10 2016

Most Viscous gas http://boards.strai...x.php/t-143785.html
Some info here. [Ling, Sep 11 2016]

Should be combined with this idea Alternative_20Tube_...for_20dare_20devils
Using the technology set out in this idea, the tube should deposit people on a train. [hippo, Sep 11 2016]

Spinniest concept in a while. https://www.youtube...watch?v=eohpyx8NzMg
[2 fries shy of a happy meal, Sep 11 2016]

Foam http://www.stevenab...nts/foam-making.php
[Ling, Sep 11 2016]



       Would SF6 make much difference to terminal velocity? The density per se would not have much effect - it would reduce your effective mass by maybe 1kg.   

       However, its viscosity (according the the internet) is about 10 times higher than that of air. Does that mean that terminal velocity would be tenfold lower than in air? If so, that would make it about 12mph - seems unlikely, but then again I've never fallen any significant distance in SF6.
MaxwellBuchanan, Sep 10 2016

       // I've never fallen any significant distance in SF6. //   

       Well, now's your chance. We need replacement test subjects.   

       It's the viscosity, not the density, that makes the difference, plus the piston effect.
8th of 7, Sep 10 2016

       Oops, accidentally deleted [MB]'s anno ... something about 20% oxygen.   

       Because of the huge difference in densities, the oxygen will quickly get displaced to the top of the column, even with the mixing effect of users hurtling through.
8th of 7, Sep 10 2016

       The SF6 may be a fire suppressant, but you certainly don't want to be exposed to any that has been suppressing any fires. It starts to thermally decompose at around 300 degrees C and forms some really nasty products. Some of those products will spontaneously ignite on contact with moisture-containing objects, such as people-things...
lurch, Sep 11 2016

       //the oxygen will quickly get displaced to the top of the column//   

       I don't think that's the case. Gas mixtures don't spontaneously separate due to differing densities, just as a true solution of uranium salts will not "settle out".
MaxwellBuchanan, Sep 11 2016

       // However, its viscosity (according the the internet) is about 10 times higher than that of air. Does that mean that terminal velocity would be tenfold lower than in air? //   

       If the drag coefficient was increased tenfold then the terminal velocity would reduce by the square root of ten. However, the drag coefficient can be surprisingly insensitive to viscosity so my guess is that the actual effect would be much less than this.
EnochLives, Sep 11 2016

       // I don't think that's the case //   

       The density ratio of SF6 to normal air is about the same as that of air to helium, and helium tends to float away pretty quickly.   

       Incidentally, if you put SF6 in a bucket, you can make a boat that will float on it.
mitxela, Sep 11 2016

       //The density ratio of SF6 to normal air is about the same as that of air to helium, and helium tends to float away pretty quickly. //   

       No, it doesn't. If you have a helium-air mixture, it stays mixed indefinitely. Obviously if you have a balloon, or even just a bolus, of helium it will float in air. But a gas mixture doesn't separate spontaneously.
MaxwellBuchanan, Sep 11 2016

       I think I would rather take the tube with a large diameter at the top, funelling down to a diameter which gives >120mph updraft from a large fan.
Ling, Sep 11 2016

       // a gas mixture doesn't separate spontaneously //   

       I was under the impression it forms a gradient. If different density gases don't float about under gravity, how does convection work?
mitxela, Sep 11 2016

       //If different density gases don't float about under gravity, how does convection work?//   

       It will not form a gradient to any detectable extent, or over any manageable distance. In fact, if you fill a box with helium at the top, and air at the bottom, before long the gases will interdiffuse, undoing your gradient.   

       Convection works because a relatively large body of gas is warmed and becomes less dense than that around it, and so rises en masse.   

       As noted, a bolus of helium will indeed rise in air; likewise, a bolus of warm air will rise in cooler air - convection. But in time the helium or the warm air will mix with the cool air by diffusion; and once that mixing has happened, the gases will stay mixed.   

       Of course, if you apply a big enough G, you will be able to create a gradient. That's how uranium enrichment centrifuges work; but they use very large G's. Equally, Earth's atmosphere under 1G would eventually form some sort of gradient (with CO2 enriched at the bottom), but the gradient would be _very_ shallow and would take a long time to form, even in the absence of convection and wind. Entropy wins over gravity, for gases.
MaxwellBuchanan, Sep 11 2016

       Yadda yadda yadda ... c'mon [Max], you gonna jump down this pipe, or what ?
8th of 7, Sep 11 2016

       [8th], I am generally willing to try anything that has a >70% chance of being survivable without major injury. I have even tried a couple of things that had a <25% chance, though perhaps I had not thought them through at the time. So, you build it, I'll try it.
MaxwellBuchanan, Sep 11 2016

       Hmm, it seems you are right. I just attempted to calculate how tall your box would have to be in order to get a 1% difference in partial pressure between helium and air, and got 21.1km. For 0.01% it's 212m, so a skyscraper's SF6 escape tube at equilibrium _might_ have a detectable difference, but only just. (Feel free to check my calculations as it's quite likely I made a mistake.)
mitxela, Sep 11 2016

       But.... If the pipe is open at the bottom so people can escape, doesn't that mean the gas is moving downwards through the pipe, at at least the same velocity as the escapees? This spells bad news for your terminal velocity theory, I think you'd actually be shooting people downwards...   

       A tube that is somewhat elastic and tries to shrink around you would likely work rather well? Especially if there's also an updraft.
Custardguts, Sep 11 2016

       The tube is a good idea. You've seen these right? [link]
Just turn descent into spin.

       I did notice that they never once mentioned motion sickness during the video though...   

       // , doesn't that mean the gas is moving downwards through the pipe, //   

       No, because the base of the pipe is a sealed (airlock, gaslock) chamber. The piston effect of the descending escapee raises the pressure below them, and the gas then has to force its way past them, acting as a cushion.
8th of 7, Sep 11 2016

       Why not just have the fire in a lower building?
MaxwellBuchanan, Sep 11 2016

       You know everybody is going to grab their hand luggage; this has been proven in many aircraft evacuations. Therefore the shaft has to be a bit bigger. To offset the decrease in effectiveness, each rescuee can be issued a parasol.
lurch, Sep 11 2016

       Question about the piston effect on terminal velocity.   

       It seems to me that if the object falling down the tube had a cross sectional area of half the cross section of the tube, then the gas would be going past the person at twice their rate of decent, essentially cutting the terminal velocity in half.   

       That's probably way oversimplified, but it's good enough to point out that the person will need to fill much more than half the cross section of the tube for the piston effect to do much. considering that cross sectional area of the population caries by much more than 2:1, the piston effect won't help.   

       Now using a parasol as suggested by [Lurch] could make the piston effect useful, but they'd have to be very strong parasols and the users would have to hold on tight.   

       Instead we should use those big rubber exercise balls. throw one in the tube and jump after it. It would also provide extra cushioning at the bottom. Maybe then we could skip the gas. I guess we'd still need the pressurized room at the bottom.   

       But where do you store all those exercise balls? Simple, a trend a few years back was to use them to replace office chairs. Issue one per person. If someone wants a traditional chair it's up to them to figure out where to store the ball.
scad mientist, Sep 11 2016

       Maybe make a foam with 20% O2/ Rest SF6. The viscosity could be tuned.
Ling, Sep 11 2016

       I know.. Vests with strong magnets built in (and equally strong crotch straps). The bottom part of the tube, where it curves back to horizontal could be made of something conductive, but not magnetic. As the wearer passes this point, the magnet/induced-currents will cause a deceleration proportional to speed. The faster someone falls, the more they are slowed. The pipe may get a little toasty, so some water may need to be added.   

       Of course, SF6 must be involved, so I propose that the person at the receiving end of the tube take giant inhalations and greet the now-safe individuals with slow motion movements and a big slow, "You're going to be fine..."
TIB, Sep 12 2016

       // The faster someone falls, the more they are slowed.//   

       Doesn't a pavement also have this effect?
pertinax, Sep 17 2016

       Apparently the Coroner's jury is still out on that one ...
8th of 7, Sep 17 2016


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