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compressed air: Boeing tyres

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mechanicaldup

mechanicaldup

Mechanical
Jun 30, 2005
155
what is the typical compressed air requirement for inflating Boeing 737 tyres ( CFM @ ...PSI)?

 
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Mechanicaldup, I believe it isn't compressed air but nitrogen that fills the tires to avoid combustion when heat is developed from friction as when landing and braking.
 
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BigInch, I cannot grasp the truth of IR's statement under the heading of Better air pressure retention:
With nitrogen the diffusion is 30 to 40 percent lower than oxygen.
Click to expand...
which appears to contradict the laws of effusion and diffusion of gaseous molecules. Can you ?
 
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No, I certainly can't. I must confess that I can only see two reasons to use N2, theoretically, you can save an ounce or two of weight, and reducing potential combustion dangers, although I believe that titanium will combust under 100% N2 atmosphere at certain conditions. Weight does not seem to be a valid reason, but then again, I don't own 5000 trucks running 24-7. As for this IR article, I just happened to discover it when I was looking for a compressor, so never missing an opportunity to heat up a discussion, I posted it here to see what response it would generate. So, now that the hook got set (in my mouth), I've decided the best way to get this whole thing explained is to invite the horse to talk. I've asked Ingersol Rand to explain it. It will be interesting to see what witchcraft they are relying on as a basis for making that statement. I will report back on any answer they may be willing to give us.

Going the Big Inch! [worm]
 
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Still running this out,

Aircraft reasoning is combustion, stability and oxidation; nothing about diffusion.

[COLOR=black yellow][/color]

3. USE DRY NITROGEN GAS (WHEN REQUIRED)
Nitrogen will not sustain combustion and will reduce degradation of the liner material, casing plies and wheel due to oxidation."

[COLOR=black yellow][/color]

"A maximum-energy RTO is a dramatic test that involves accelerating an airplane with fully worn brakes to a takeoff speed of nearly 200 mph, and bringing it to a complete stop using only the brakes and the wing spoilers. Engine thrust reversers are not used to help slow down the airplane in this test. The brakes glow orange-red and reach temperatures of more than 3,500 degrees Fahrenheit from the energy absorbed while halting the airplane. Shortly after the stop, wheel fuse plugs melt to safely release tire pressure."

[COLOR=black yellow] [/color]

"The space shuttle tires are filled with nitrogen (as are most aircraft tires) due to its stability at different altitudes and temperatures. Due to the extremely heavy loads these bias ply tires are inflated to 340 psi (main gear) and 300 psi (nose gear)."


[COLOR=black yellow][/color]

"Inflating With Nitrogen

Many regulatory agencies require the use of nitrogen when inflating tires for aircraft above a specified Maximum Takeoff Weight (MTOW). Michelin recommends the use of nitrogen when inflating all aircraft tires. Nitrogen provides a stable, inert inflation gas while eliminating the introduction of moisture into the tire cavity.

Aircraft operating procedures for initial inflation and adjustments must comply with applicable instructions as given in FAR 25 or JAR 25.
Oxygen concentration should never exceed 5%."


Going the Big Inch! [worm]
 
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I believe the mentioned limit % O[sub]2[/sub] has something to do with the partial pressure of oxygen to start a chemical reaction at the temperatures in use.
 
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Here's the word from IR, phone number censored, e-mail me if you want it. I'm letting this one die on the vine. I must do some real work now.

"Unfortunately, I am not one of the "experts" on our nitrogen system. The
explanation that I have heard is that the nitrogen molecules are larger
than oxygen molecules so it will take longer for them to permeate the tire
wall.

If you have other questions about the nitrogen system, you may contact:

908-###-###

Thanks"
Click to expand...

Going the Big Inch! [worm]
 
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Both molecules have about the same diameter, the nitrogen bond length is even smaller (110 pm) than oxygen's (121 pm). Thus, it cannot serve as an explanation for the massive selective escape of oxygen from a tire.

Assuming the reported comparative tire pressure loss is correct, I tend to believe that oxygen is preferentially adsorbed (physically or chemically) on the carbon-rich material of the tire, and as a result its molecule dissociates.
The generated atoms are indeed smaller and lighter than the molecules and can permeate out easily. Let's recall that the energy of oxygen's endothermic dissociation is about half of nitrogen's.
 
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I think automobile-wise its an urban myth. From what I read, I believe it is justified in the case for aircraft tires that are seldom at full pressure and run with soft walls. Otherwise I feel it is of highly questionable value and even less technical merrit, except for the case of extremely hot brakes, or if it has any merrit at all, its definately not worth the trouble or expense. I can say I've had quite a few vehicles by now and I've never ever used N2 to fill the tires and I've never experienced a blowout due to tire oxidation, so really I ain't worried.

For terrestial use, it must be a Madison Avenue [2thumbsup] ploy.


Going the Big Inch! [worm]
 
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See:
for diffusity for different gases in different materials. Note the last entry is Butyl Rubber.

Note also that the diffusivity difference between oxygen and nitrogen appears to be even bigger than stated in the supposedly biased statement from the Irtools website.

The difference in diffusivity might be a by-product of the reactivity difference of oxygen vs nitrogen, rather than size difference.


TTFN
 
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Lange's Handbook of Chemistry, 14th Edn. pages 10.66 to 10.69 gives permeation constants for gases through polymers, including various rubbers & blends. In agreement with data for butyl rubber in IRstuff's link, oxygen permeates about 3x as fast as nitrogen through various rubbers. Water vapor is much faster, 100 -1009x, than nitrogen. After parking a plane an extended period in a humid, tropical clime, considerable water vapor would be present inside the tires. Would not want to land a plane with ice crystals on the inside walls of tires after a high altitude flight.
 
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A tire dealer once told me that tires filled with N2 last longer because the rubber does not lose it's plasticity in an N2 atmosphere as fast as it does in an oxidizing O2 one.
 
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Now it becomes a clearer situation, at least for me.

[•] Coming back to the subject of nitrogen-inflated tires. It is not a diffusion process as claimed by the nitrogen supporters, it is permeation, as highlighted by kenvlach, a totally different concept. Permeation refers almost exclusively to polymers and is caused by differences in chemical potential. Example: loss of carbon dioxide from plastic bottles for carbonated soft drinks.

[•] Permeation is characterized by a coefficient P, which is the product of two seemingly counteracting transport effects: the diffusion coefficient D through the pores (favouring nitrogen), and the solubility (molecular transport) coefficient S of the permeant gas in the matrix (adding high mobility to oxygen).

[•] The process of permeation through a polymeric barrier is said to consist of four steps: absorption of the gas into the polymer wall; solubility in the matrix; movement by diffusion along a concentration gradient; and desorption from the outer wall.

[•] It appears that the polarity of the polymer functional groups, chain symmetry, attraction between polymer chains, glass transition temperatures, and crystallinity all affect the permeation rates. Example: oxygen permeates 10 million times faster through poly(dimethylsiloxane) than through poly(acrylonitrile).

[•] Permeability is also influenced by temperature, differential pressures, humidity, fillers, additives, thickness, etc. Some of them, by increasing the pathway and its tortuosity, decrease the permeation of a gas molecule.

[•] Gases, in general, have lower permeation coefficients in thermoplastics than in elastomers because segmental movements of the former are frozen-in below the transition temperatures.
 
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