WTC Challenge, page 33

Originally posted by white4life420
You are over-complexing the issue. The fact is, fire eats air. If there is a fire that is raging so heavy that it melts steel, that fire is consuming air at a rapid rate. The air that fire will be grabbing at most would be coming from out side of the building more than the inside (hence why when there is a fire, you should not open windows or doors).

Ok, let me try to explain it a different way.

The atmosphere is a thin layer of gas that surrounds the planet and consists of a relatively stable mixture of several hundred different gases. It has a mass of about 5.15e15 tons, held down to the planet's surface by gravitational attraction. Excluding water vapor, the proportions of gases are nearly uniform up to around 80km high. The major components of this region are Oxygen (21%), Nitrogen (78%), and Argon (0.93%), with minute amounts of trace gases also present. This part of the atmosphere also contains water vapor, the amount of which depends on local environmental conditions.

The weather report for Central Park, New York, on Sept 11, 2001 was very nice. It was a clear day with a relative humidity of 60 (nice and low for a coastal city when it's a nice 75 degree day.) So, we can safely assume that the water content of the air was less than 1%.

Now, everyone knows that fires don't burn Nitrogen, right? And everyone knows that fires use Oxygen right?

Does that not mean that when a fire burns it uses up (as you want to put it) 21% of the air? Actually, what it does is it converts ~ 21% of the air from Oxygen to carbon dioxide and carbon monoxide and other carbon based gasses depending on what is burning (to the tune of ~21% of the air). No air is lost, no air is "used up" -- 21% of the air content (O2) is converted into another gas (CO CO2 etc). Fires use oxygen, fires don’t destroy oxygen.

The reason a fire can become explosive at "flashpoint" is because the smoke in the air is superheated in a contained area and it is right at the verge of being able to combust but lacks an *ogygen* source. When you (or a fireman) open a door this super heated air suddenly has an oxygen source and instantly flames (flashes) in a violent fire. Perhaps this analogy is what's confused your idea of how fire and air interact?

Would a significant amount of air be moving upwards in the WTC towers during the fires? Most likely yes - heat from the fires rises (that's why smoke goes up, because of the heat - the soot and carbon content in smoke is heavier than O2 & N but it's hotter and thus rises). The act of the smoke and air rising due to heat would mean it would have to be replaced by another volume of air from outside the building and from floors below the building (just like a chimney works). So there would be no (or minimal) negative air pressure in the building, and from what I know about most modern office buildings they actually have a slightly positive air pressure to keep cold (winter) or warm (summer) air out of the building (it's cheaper to heat/cool a building when you maintain a slightly positive air pressure). We also know that power existed in the buildings to the 78th floor (and above) because we know that the phone system worked in the restaurant and most offices almost to the point of collapse (and in some cases at collapse). We also know that the vast majority of office telephone systems require power to work (VISTA, Avaya, NorStar, Lucent, etc). So it stands to reason that most, or at least some, of the building's air system was operating during the fire unless somebody shut it off manually.

But, even with the air system shut down, air will still freely move through ducts, vents, shafts, stairwells, giant holes in the walls from airliners, etc., to flow to the floors where air was being lofted away by the heat of the fire. There was probably just as much air on each floor after the fire as there was before the fire.

Simpler for ya?

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As a little aside, if you'd like to try a science experiment at home to see if a fire does or does not consume air, you can do two simple tests. First, get a birthday candle (smaller the better), an empty jar (something decent sized and thick like a pickle jar with clear glass - don't use plastic) and a lighter. First place your lid from the jar upside down on a counter and melt some candle wax onto the jar lid so you can "stick" the candle to the lid (blow out the candle). Fill your kitchen sink with water - if you want to be extra clever you can also add a few drops of food coloring to the water to make it more visible. (A large bowl filled with water will work as well, and will actually be easier to see.)

Now, with the lid still upside down on the countertop, and the candle standing upright on it, light the candle and quickly (and carefully) place your jar over top and screw it tight to seal the jar. Sit and watch to see how long it takes for the candle to burn out (no need to time it). What's happened is the candle has used up all the oxygen and converted it to CO2. Let that jar sit and cool down for a few minutes.

Now, place the first jar (upside down!) in your water filled sink and carefully unscrew the lid. Did any water get sucked into the jar? If you carefully, and slowly, raise the jar until the lip is even with the surface of the water you can see how much water has been drawn into the jar. None should have if you sealed the jar properly! Obviously no air was used up, it was merely converted into another gas.

Dry your jar, lid and candle off and let the jar lay open on it's side for a few minutes.

You can repeat the same experiment again from the beginning and skip the cool down time and try unscrewing your jar it in the water soon after the candle goes out. You should notice a positive pressure (bubbles will come out) because the air is heated and has expanded creating a positive pressure (and not a vacuum). I think you'll have proved to yourself at this point that fire does not consume air, it merely converts one gas to another gas.

If you're under 16 be sure to get a parent to help you with this.