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Vacuum to replace helium

by Carlos4558
Tags: helium, replace, vacuum
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Carlos4558
#1
May27-08, 05:55 PM
P: 5
Hia, Im not a student, im a gas engineer in the UK. Had a thought and tried out some theory but didn't get very far.

Basically; if a helium ballon rises in air as it is lighter than the air it displaces, what if I could get a tube (say internal dimensions of 15mm in diameter and 1000mm long making a volume of 0.177m^3 (ish)) and create a vacuum within it. obvously serous levels of vacuum would be difficult to achieve so im not looking to go too far down with it.

(oh and in case your wondering why I picked a tube, I figure it's easier to work with than a sphere, less likly to collapse. Can't remember why, I think thats how it works. (and if it works it'll be more fun to play with!!!)

Can anyone help me pick a suitable material to make the tube from? Any ideas how thick the wall would need to be etc...

Oh and a thought ive had just this seond. If I filled the tube with helium then vacuumed that out, then the bits I can't be bothered to wait for would be ligher than air anyway.

Any help would be greatly appreciated.
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tiny-tim
#2
May27-08, 06:23 PM
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Hi Carlos! Welcome to PF!
Quote Quote by Carlos4558 View Post
Can anyone help me pick a suitable material to make the tube from? Any ideas how thick the wall would need to be etc...
hmm … I think you've identified the essence of the problem.

The advantage of helium is that it generates its own pressure, so the balloon can be made of extremely light material without collapsing.

A vacuum, of course, generates no pressure at all, so the balloon would have to be extremely strong to prevent collapse.

I don't know what the current price of vacuum is (is it sold by volume, or by weight? ), but I expect that, even if it is cheaper than helium, the expense of the balloon itself would far outweigh any saving.

(and of course there's safety … a puncture of a helium balloon will lead to slow decompression, but a puncture of a vacuum balloon would lead to very fast decompression, unless the balloon had internal bulkheads, which would make it even costlier and less efficient.)
Mech_Engineer
#3
May27-08, 06:26 PM
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You won't be able to make a tank that is strong enough to witstand a 1 atm pressure difference accross it and still be lighter than a baloon full of helium. Vacuum tanks are heavy because they have to be stiff enough to avoid buckling.

phlegmy
#4
May27-08, 06:44 PM
P: 119
Vacuum to replace helium

i'm not sure if i have the sums right but the idea is:

if you take a football and cut it in half to make a dome, now imagine the dome immensly strong, now place it like an upturned bowl on the floor and suck all the air out to have a complete vaccume, so that its like a suction cup stuck to the floor, even better stick it to the ceiling, you can now hang a landrover discovery [about 3 tons]from it and it wont fall, that how much force the atmosphere exerts.

as for tubes being stronger than spheres: NOT SO!
in a tube you have hoop stress and axial stress (hoop around the curvy part, axial along the straight part)
in a sphere you have only hoop stress!

for thin walled clyinder the hoop stress is twice the axial stress

for a thin walled sphere under the same pressure, the hoop stress will be same as the axial stress for the cylinder (half the hoop stress of the cylinder)

so the max stress present in the cylinder will be TWICE that in the sphere!

iirc^

i like your idea of prefilling it with helium though, that'd be quite ingenius!
jablonsky27
#5
May27-08, 10:49 PM
P: 74
i dont get the idea.. tell me if i m getting it wrong.. a balloon filled with helium floats because helium is lighter than air.. so carlos wants to make a balloon and *fill it* with vacuum.. and vacuum balloon is lighter than helium balloon.. so it better.. am i getting the idea right?
mgb_phys
#6
May28-08, 01:51 AM
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Quote Quote by jablonsky27 View Post
.. so carlos wants to make a balloon and *fill it* with vacuum.. and vacuum balloon is lighter than helium balloon.. so it better.. am i getting the idea right?
Correct - except a balloon is generally made with very thin light walls. A vacuum tank is generally made with very thick heavy walls - see the problem ?

Of course the force on the walls is constant and the mass of the wall only inceases with the surface area (lenght squareD) while the lifting power increases with volume (length cubed) so as an excersice to the student, how big would the balloon have to be for this to become practical if you assume say a 1cm thick wall of Aluminium (2.7g/cm^3)
Carlos4558
#7
May28-08, 01:55 PM
P: 5
well this sure has put the physics forums into thought, I think pre-filling the space to be vacuumed with helium (just as an example) could possible help with decompressing the volume, as you might be lucky enough to avoid that nasty stage of water vapour on the walls of the chamber (read that somewhere as being a final and difficult stage of decompression) PATIENT PENDING ON MY PRE-FILL IDEA! lol. cost and fabrication isn't so much of a problem as im a fairly skilled metalsmith. Though I must admit that I had no idea of how much radial and axial stress came into it. I figure if you take away the axial stress you would double the radial. I guess im misinformed again. though i would seal the tube to make a doughnut (in reference to my first statement.)

What stinks about the fact that I probibally can't even do this without some sort of diamond tube and ive already though of applications for my "silly tube" :p

So, someone mentioned making a 10mm thick aluminum sphere and vacuuming that sucker out (it would be easier if I could get away with 2.5mm but im sure you guys know much more about material strength and atmospheric forces than I do). what would the internal diameter have to be before It started to benefit from the effects of buoyancy? (Forgive my terminology, it has been 3 years since I sat in a physics classroom and had all my inventions blown away by the unruly power of science.)

-A moment of pause while his dreams of an aluminum vacuum zepplin temporally come to life-

Thanks guys!
Carlos4558
#8
May28-08, 02:12 PM
P: 5
A list of advantages of a vacuum zepplin over a helium or hydrogen one;

1. Content, there isn't any, and of course nothing is always readily available! so if a leak were repaired, just suck out what went in!

2. Strength, I believe a zepplin of old was made from canvas and whatnot, MUHAW EAT ALUMINUM!

3. Potential for disaster, Id rather be under a plummeting ball of aluminum as opposed to a plummeting ball of burning hydrogen (the vacuum would hopefully implode the zepplin, making a smaller...projectile.) -gasp! another use!-

4. Storage, oki, I admit it. You probably can't fold up a metal zepplin quite like you can canvas one, but who cares! just leave it floating.

5. Cost, almost zero maintenance of parts (vacuum pump and directional fans). the aluminum would cost a fair bit but oh well, nothing ventured = no metal zepplin!

The list goes on (but my pc is broken for the moment and typing on an asus eee pc keyboard is frustrating with the worlds largest hands!)
pallidin
#9
May31-08, 03:21 AM
P: 2,292
Positive bouyance(that which povides lift) demands that the external environment is of greater pressure than the "balloon" Even more, this demands a negative gradiation of external pressure as one goes higher in order for lift to occur.
The later is a given aspect of our atmosphere... air is denser close to earth and less dense towards space, thus balloons will rise if the total weight of the balloon is off-set by the bouyancy.
As such, balloons will rise only as high as there is an equalization of internal/external pressure and weight. This rise can be significant, but far short of "orbit"
Special techniques can be used to provide further lift, such as super-heating the light gases in the balloon...etc, but eventually the limiting physics is the structual weight of expansion containment.
That is, if the containment weight of, say, a vacuum ballon, is greater than the external atmospheric bouyancy, further lift will cease. And, since any containment is heavier than a vacuum, a vacuum balloon could never reach earth orbit by itself alone.
skywolf
#10
May31-08, 12:33 PM
P: 80
or you can use light to achieve the pressure!
but then youd have to carry around a powerful emitter. unless you beamed it from a light source on the ground, then youd have to keep the "balloon" within sight of the laser, and youd have to get the light in and keep it in the "balloon"
also i imagine the power requirements would be immense, i mean unless i have my math wrong:

100 kPa is external pressure

the sun exerts 4.6 µPa at the earths surface, so we would need about 10^14 times the energy that we feel from the sun....
akhmeteli
#11
Jun4-08, 12:37 AM
P: 592
Quote Quote by mgb_phys View Post
Correct - except a balloon is generally made with very thin light walls. A vacuum tank is generally made with very thick heavy walls - see the problem ?

Of course the force on the walls is constant and the mass of the wall only inceases with the surface area (lenght squareD) while the lifting power increases with volume (length cubed) so as an excersice to the student, how big would the balloon have to be for this to become practical if you assume say a 1cm thick wall of Aluminium (2.7g/cm^3)
I am afraid the scaling of this problem is somewhat different: the mass of the wall increases as length cubed, because as length increases, the radius of curvature also increases, so you need a thicker wall to withstand the same pressure.
A sphere is clearly superior to a cylinder (because one of the radii of curvature is zero for a cylinder), but even a sphere with a homogeneous shell made of any existing material will be either heavier than air or won't withstand the atmospheric pressure.
You may wish to look at our US patent application 20070001053 (11/517915) (at http://portal.uspto.gov/external/portal/pair enter the verification code and then 11/517915). We propose an evacuated sandwich spherical shell with two thin face sheets and a light core between them. Finite element analysis confirmed that the structure using commercially available materials (e.g., boron carbide face sheets and aluminum honeycomb core) can be light enough to float in air and strong enough to withstand the atmospheric pressure with decent safety factors for strength, buckling, and intracell buckling. Actual manufacturing, while definitely possible, is not easy. So I bet Carlos won't be able to make a vacuum balloon in his garage, sorry. I could give him all the dimensions, but I don't think his metalsmith's skils will help him with structural ceramics. The only metal that can be practically used to make the face sheets is beryllium, and beryllium is expensive and hard to work (it is not malleable), and its dust is highly toxic.
pallidin
#12
Jun4-08, 02:57 AM
P: 2,292
The above is a fascinating post. Thanks for sharing, as it seems that a vacuum balloon is at least possible, though not easy as you say.
Great info!!


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