I just came across this statement and I had never heard this before. High temperatures and pressure changes have less effect on inert gases. Why is that?
Where did you come across the statement and what sort of effects where they talking about?
The question will benefit from context. If you are talking about physical properties like density, specific gravity, paramagnetism etc. then they are pretty much effected the same like other gases. There are some niche gases like Helium that become superfluid at low temperature.
As to chemical reactivity - i think it is obvious that inert gases are not effected since they are by definition inert. A gas like Nitrogen which behaves as inert at normal conditions reacts with oxygen to form nitrogen oxides at high temperatures. Other gases form free radicals in high temperature that are very reactive.
A person who is in the tire making industry was explaining why airplane tires are filled with nitrogen. One reason, she explained, is that “inert gases are less affected by high temperature and changes in pressure.”
I believe Nitrogen is less likely to makes it’s way past seals and/or through the tire itself. And I’m reading that it’s pressure won’t swing as much as ‘air’ in response to temperature changes.
Having said that, ISTM, Nitrogen may be used in airplane tires to help insure the tires are the expected pressure when they hit the ground.
Okay that makes sense. Airplane tires are typically inflated to 200+ psi. There have been cases when tires overheat during landing and give off vapors. if the tires were filled with air (21% oxygen) then the high pressure of oxygen and the temperature of the tires + vapors result in fires.
Its a requirement for certain aircraft types and here is a ruling from FAA going into more details - Federal Aviation Administration
So basically it comes to chemistry. AFAIK, they haven’t repealed the gas law pv=nrt where p = pressure, v = volume, n = number of molecules, r is a universal constant and t the absolute temperature.
Nitpick, n = number of moles not molecules
Or PV = NkT, where N is actually the number of molecules, and k is Boltzmann’s constant.
All gases will deviate from the ideal gas law because a) there are intermolecular forces, and b) molecules have non-zero size. Inert gases have much smaller intermolecular forces, and so smaller deviations from the ideal gas law Deviations from Ideal Gas Law Behavior: . Smaller molecules obviously have smaller deviations due to smaller size. Both these effects are small (I think) at normal temperatures and pressures though, so I think “less affected by temperature changes” means “less likely to cause fires at high temperatures” (as was suggested above by am77494)
effected -> affected
where -> were
it’s -> its
(Sorry - unable to restrain nitpickery.)
Maybe all the above will get accepted as normal usage with time. Like momentarily did.
How embarrassing. Your right.
I am not sure, I understand your post. In the OP’s context, the deviations from ideal gas laws have nothing to do with tire fires. Also smaller molecule gases like Helium, although inert, will be the worst gas to inflate tires since it will escape.
In the OP’s context, the use of inert gas is to avoid the presence of oxygen. And btw Nitrogen is not a inert gas but close enough for the OP’s context.
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That explanation is a little garbled. The primary reason for filling tires with nitrogen is because it’s dry. Air always contains some moisture, and some air contains quite a bit. When that water gets hot enough to turn from water to steam, it expands by a factor of about 1400. In short, moisture means tire pressure varies a lot with temperature.
This doesn’t matter much at all for passenger cars, but race car tires routinely exceed the boiling point of water inside the tire. Aircraft tires certainly could get that hot as well.
It’s true that oxygen diffuses through tires a little faster than nitrogen, but if you think about it, your tires are nitrogen filters. The first time you inflate a tire with air, you’ve got about 80% N and 20% O[sub]2[/sub]. After a while, some of the oxygen diffuses out, leaving maybe an 85/15 mixture. Then you top off with more air, and the ratio is perhaps 83/17. As that cycle continues, the nitrogen content asymptotically approaches 100%. 4-5 top-ups means you have very little oxygen in your tires.
As AM points out, oxygen content is a more pressing concern for aircraft tires; that’s obviously legit. But the dryness of nitrogen helps there too. It’s cold at altitude: -50 degrees C is common. That guarantees that any moisture in the tire will condense during flight. When that plane lands, it may very well have liquid water in its tires, and that liquid water can boil and (potentially) blow the tire off its rim. Dry nitrogen won’t do that, and it also won’t support combustion. Everybody wins!
It’s common for tire shops to upsell customers on nitrogen fills, but that’s a scam. (Costco doesn’t charge extra, which is nice). Nitrogen is harmless in passenger car tires, but it offers almost no benefit either. I’d accept a free nitrogen fill, but I wouldn’t pay extra.
That moisture in air is already in vapor form at temperatures below 100C. Water vapor is slightly less of a [censored] than liquid water when it comes to behaving like other substances, but still doesn’t behave in ways consistent with “gas with a molecular weight of 18”. But I’d expect the bigger problem to be that both water and oxygen can react with the partially-polymerized rubber which forms the tire walls; nitrogen won’t.
Ok. But aside from the fact that the original statement took Nitrogen to be an inert gas (of course it’s not), and is not a fire hazard, why is it less affected by changes in pressure?
I think EdelweissPirate addressed this; the key culprit is water.
For air inflation, you take air from ambient and compress it to say 400 psi at which point it is hot and then it cools down. Some water then condenses out (most compressors have a water drain) but some water remains in the compressed air. You use this air next to inflate the tires to 200psi when it cools a bit (due to the pressure drop) and some water condenses our but some water remains in the air. Now as the tire goes through temperature changes during flight - the water keeps condensing out and vaporizing due to temperate changes and hence it effects the tire pressure.
Nitrogen on the other hand is mostly made by cryogenic processes and is transported as liquid nitrogen and subsequently sold in cylinders at high pressure. Water is removed before the cryogenic liquefaction using mol-sieves which brings the dew point of Nitrogen to less than -70F. So the tires inflated with Nitrogen do not have the drawback of water condensation.
I ran some numbers just to make sure that the theory above made sense and it turns out that I was talking out of my a** (provided the air compressor system was working correctly).
So, here are the numbers
1> So say you take 102.5 volumes of ambient air at 70 F on a very humid day (relative humidity = 100%) then this air will contain 100 volumes of dry air and 2.5 volumes of water vapor.
2> So now you use a compressor to raise the pressure to 400 psi. The air gets hot (depending on the number of stages of compression and efficiency) but then it cools down back to ambient temperature of 70F. Of the 2.5 volumes of water vapor in the original, 2.39 volumes condenses out as water and 0.11 volumes remains in the compressed air. (The volume changes when compressed but for simplicity, I will keep referring to the original volumes)
3> So you take the original 100.11 volumes of 400psi air @70F and put it in the tire so that the tire pressure is 200 psi. The air cools down to 63.5F due to the expansion but no water condenses out. Eventually the air temperature rises to 70F (ambient) and still no water condenses out.
4> Next I cooled this air to -55F (approx the temperature at 35,000 ft) and as expected the water condenses out at this temperature BUT the water is only about 0.1% of the volume, so the pressure change in the tire due to water is negligible.
So CC, your intuition was correct. Nitrogen is only marginally less effected by changes in pressure/temperature in this application. But hey, the Chemical Industry has found another way to monetize the nitrogen they produce while making oxygen from air.