Need help with pressurized air

[CapriRacer]

Tire engineer here.

I can't seem to get my head around the relative humidity of air inside a pressurized tire.

What I'd like to know is if water condenses out of the air inside a tire when it gets cold. I'm trying to figure out the implications of using nitrogen instead of air as an inflation medium.

For example, if water doesn't condense out of the pressurized air inside a tire when it gets cold then it doesn't matter if one uses air or nitrogen, the gas medium reacts the same.

 

[Lnewqban]

Welcome!
The water vapor contained in trapped atmospheric air changes volume much more dramatically than air with changes in tire temperature.
For each combination of internal pressure and temperature, humid air absorbs and condensates different amounts of water.
Water may affect metal parts of the rim.
Pure nitrogen does not have those problems.

 

[berkeman]

Welcome to PF.

What I'd like to know is if water condenses out of the air inside a tire when it gets cold. I'm trying to figure out the implications of using nitrogen instead of air as an inflation medium.

I don't know much about the subject, but it seems like dehumidifying the air before compressing it would be simpler than having to stock a bunch of $N_2$ cylinders. No?

 

[Baluncore]

I'm trying to figure out the implications of using nitrogen instead of air as an inflation medium.

The primary reason why nitrogen is used, is to prevent fire supported by compressed air, in the wheel well of an aircraft. Nitrogen also prevents aircraft wheel imbalance, due to internal ice formation at high altitude, low temperatures.

The problem with humid air in a tire is that, if you inflate the tire to the correct pressure on a hot and humid day, when the tire cools, the tire pressure will fall as the water condenses to a liquid in the tire.

If you pour an excess of liquid water, into the wheel before inflation, then with changing temperature, the tire pressure will vary as a function of the changing vapour pressure of water, also, the rim and valve will probably corrode.
How bad could it get? There is a code table below showing how gauge pressure changes in a tire with temperature, for dry air or nitrogen, and for saturated wet air. Those are my computations from a year ago when I was considering truck tires. The effect will be proportionally greater at lower pressures, subtract a constant. They have not been checked by others, so take them with a grain of salt, (which may accelerate corrosion).

As tire pressure varies with temperature due to moisture, the contact patch and the tread wear across the tire will change. At lower pressures, the contact patch will increase in area, so the sidewall will also deflect more, reducing tire life.

The tension and the speed of sound in the belt and tread will be greater at greater pressures. Low pressure at high speeds can destroy a tire when road speed approaches the sound velocity in the belt, as a noise front builds up at the first point of road contact, a sound barrier.

 Temp °C  kPa(g)    bar(g)  psi(g)   Psi(g)
    0.0   635.524    6.355    92.2     92.3
    5.0   649.012    6.490    94.1     94.3
   10.0   662.500    6.625    96.1     96.3
   15.0   675.988    6.760    98.0     98.3
   20.0   689.476    6.895   100.0    100.3
   25.0   702.964    7.030   102.0    102.4
   30.0   716.452    7.165   103.9    104.5
   35.0   729.940    7.299   105.9    106.7
   40.0   743.428    7.434   107.8    108.9
   45.0   756.916    7.569   109.8    111.2
   50.0   770.404    7.704   111.7    113.5
   55.0   783.892    7.839   113.7    116.0
   60.0   797.380    7.974   115.7    118.5
   65.0   810.868    8.109   117.6    121.2
   70.0   824.356    8.244   119.6    124.1
   75.0   837.844    8.378   121.5    127.1
   80.0   851.332    8.513   123.5    130.3
   85.0   864.820    8.648   125.4    133.8
   90.0   878.308    8.783   127.4    137.6
   95.0   891.796    8.918   129.3    141.6
  100.0   905.283    9.053   131.3    146.0

 

[Baluncore]

Here are a couple of other typical situations, a car and a tractor.

For a typical light vehicle, normally inflated to say, 25 psi.

                             Dry      Sat
 Temp °C  kPa(g)    bar(g)  psi(g)   Psi(g)
    0.0   153.696    1.537    22.3     22.4
    5.0   158.364    1.584    23.0     23.1
   10.0   163.033    1.630    23.6     23.8
   15.0   167.701    1.677    24.3     24.6
   20.0   172.369    1.724    25.0     25.3
   25.0   177.037    1.770    25.7     26.1
   30.0   181.705    1.817    26.4     27.0
   35.0   186.373    1.864    27.0     27.8
   40.0   191.042    1.910    27.7     28.8
   45.0   195.710    1.957    28.4     29.8
   50.0   200.378    2.004    29.1     30.9
   55.0   205.046    2.050    29.7     32.0
   60.0   209.714    2.097    30.4     33.3
   65.0   214.382    2.144    31.1     34.7
   70.0   219.050    2.191    31.8     36.3
   75.0   223.719    2.237    32.4     38.0
   80.0   228.387    2.284    33.1     40.0
   85.0   233.055    2.331    33.8     42.2
   90.0   237.723    2.377    34.5     44.6
   95.0   242.391    2.424    35.2     47.4
  100.0   247.059    2.471    35.8     50.5

For a water-ballasted tractor tire, normally inflated to say, 12 psi.

                             Dry      Sat
 Temp °C  kPa(g)    bar(g)  psi(g)   Psi(g)
    0.0    70.180    0.702    10.2     10.3
    5.0    73.319    0.733    10.6     10.8
   10.0    76.458    0.765    11.1     11.3
   15.0    79.598    0.796    11.5     11.8
   20.0    82.737    0.827    12.0     12.3
   25.0    85.876    0.859    12.5     12.9
   30.0    89.016    0.890    12.9     13.5
   35.0    92.155    0.922    13.4     14.2
   40.0    95.295    0.953    13.8     14.9
   45.0    98.434    0.984    14.3     15.7
   50.0   101.573    1.016    14.7     16.5
   55.0   104.713    1.047    15.2     17.5
   60.0   107.852    1.079    15.6     18.5
   65.0   110.992    1.110    16.1     19.7
   70.0   114.131    1.141    16.6     21.1
   75.0   117.270    1.173    17.0     22.6
   80.0   120.410    1.204    17.5     24.3
   85.0   123.549    1.235    17.9     26.3
   90.0   126.688    1.267    18.4     28.5
   95.0   129.828    1.298    18.8     31.1
  100.0   132.967    1.330    19.3     34.0

Of note: A Californian study on permeable pavements found that asphalt pavements, with a typical albedo of 0.1, can produce surface temperatures of 70-80°C on a hot and sunny summer day in cities such as Phoenix, Arizona.
(Li et al. 2013).
We can significantly exceed that in Australia.

 

[Lnewqban]

Also, please see:
https://www.engineeringtoolbox.com/water-content-compressed-air-d_1275.html

https://www.atlascopco.com/en-us/compressors/wiki/compressed-air-articles/what-is-condensate-in-air

https://www.cs-instruments.com/us/a...-point-measurement-in-compressed-air-systems/

 

[Ranger Mike]

During a stock car race, we need to maintain a proper tire diameter once the tire is brought up to proper operating temperatures. usually 180 to 210 degrees F. depending upon the weight of vehicle and particular tire compound. in order to maintain the proper diameter, we use nitrogen instead of air because this gas does not have moisture in it, has larger molecules and will not bleed through the tire wall. The lack of moisture means the tire will expand at a more consistent rate vs. moisture rich air which could steam up on you. Yes the moisture ( water) turns to steam and you can grow tire diameter up to an inch or more. Two things happen when diameter grows. The tire contact patch is reduced on all four tires and stagger grows again on all four tires. Stagger is used to assist the rearend turning when a locked differential is used. Both of these impact handling dramatically.
Just about all air compressors will use moisture latent air and unless you use air filtered through an expensive air dryer, $1100, you will have moisture in the air. I sell expensive machines requiring instrument quality compressed air. A bottle of nitrogen from local welding supply shop is cheap and lasts the entire season.
Go to a junk yard and buy an old refrigerator and take out the motorized pump. Suck the air out of the mounted and balanced tire and replace it with nitrogen. Will decrease your lap time!

https://www.mscdirect.com/product/d...hAiXhamaRtBIjPBN9vzcU2YEBfL_HfCaqz30OeM00Cjbo

 

[Baluncore]

If you can draw air into your compressor from a refrigerated cold store, there will be significantly less water dissolved in the air.

The partial pressure of water in saturated air, halves approximately, for each 10°C drop in temperature.
At +20°C, ppH₂O = 2337 Pa.
At +10°C, ppH₂O =1227 Pa.
At +0°C, ppH₂O = 611 Pa.
At -5°C, ppH₂O = 421 Pa.
At -10°C, ppH₂O = 286 Pa.
At -20°C, ppH₂O = 125 Pa.