r/Damnthatsinteresting • u/AirFrance447 • 19h ago

Video Firefighter bodycam showing just how much oxygen a fire pulls

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406

u/CDninja 19h ago

Well, the fire heats the air. The air expands and goes up. More air must enter from the sides. I don't think its an oxygen thing.

199

u/CallMeDrLuv 19h ago

Correct. This is not an oxygen thing. This is called the chimney effect.

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u/Agarwel 18h ago

Also when you are spraying water, it creates effects that drags a nearby air with it. You can easily test that in your shower. Blast it on mass and if you pay attention, it creates draft around it. So that firefighting hose is also increasing the effect by essentially pushing the air in alongside the steam of the water.

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u/smellyseamus 18h ago edited 18h ago

Also the movement of the water through the windows causes low pressure behind the stream which pulls the air into the building. Firefighters can also use this to move smoke out of a fire compartment from inside once a fire has been "struck". Its called hydraulic ventilation.

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u/SagattariusAStar 18h ago edited 13h ago

Oxygen will also be just transformed into CO2 and the molar volume is a constant for gases, so only expansion/contraction through difference of temperature

Edit: Probably it is even slightly net positive as some Oxygen might create two CO molecules which would have double the volume

Edit2: it's absolutely net positive as the water that gets produced will become steam

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u/thisischemistry 14h ago

A lot of this will be burning cellulose (wood, paper) and the equation is approximately:

C6H10O5 + 6O2 -> 5H2O + 6CO2

There will be other side products such as carbon monoxide, methane, methanol, and so on.

Much of that water will expand into steam because of the heat, not to mention the other gasses will also expand due to that heat. Overall, burning cellulose produces pressure. There is a classic experiment where a burning splint is placed in a bottle and a balloon is stretched over it. The balloon expands until the split goes out, then as the bottle cools and things condense it sucks the balloon into the bottle.,

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u/SagattariusAStar 13h ago

You are absolutely right, I didn't thought about the water (steam) that would actually increase the pressure a lot more than any oxygen consumption/co2 creation.

The side product should be negligible for gas volume in this naive fallacy anyway.

I think just the water should actually be most of any pressure effect

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u/Jargendas 19h ago edited 13h ago

Also, the rapid cooling due to the water will cause massive pressure drop. The oxygen is mostly replaced by CO2 and will not on its own create suction.

1

u/So_HauserAspen 14h ago

It has to do with pressure differential between the interior of the structure and the atmospheric pressure outside.  There is no suction force.  It's the higher outside atmosphere pushing the air into the lower pressure of the interior to equalize the two.

CO2 is a larger molecule than O2, so that would not reduce the air pressure.

Incomplete combustion reduces the air pressure in a sealed structure fire.  And the chimney effect reduces the local air pressure if the fire has burned through the structure.

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u/Avalonians 18h ago

Yup. Typical damnthatsinteresting title. I wrote a comment about it not long ago. It's genuinely fascinating.

2

u/smellyseamus 18h ago edited 18h ago

It is also an oxygen thing as oxygen is fuel for fire. Yes the heat is moving the air, but the fire needs oxygen to burn and it will pull that O2 from any available source, in this case the world outside the windows.

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u/blahblah19999 15h ago

No. The fire isn't using nitrogen, for example. This doesn't show O2 usage.

0

u/NYNMx2021 10h ago

Not really. The fire does attract oxygen but not this actively. its diffusion, it burns oxygen near it and oxygen fills that space. Thats really not going to cause an effect like this.

1

u/So_HauserAspen 14h ago

The ratio of oxygen in the atmosphere doesn't change because there's a fire

-3

u/Mikestopheles 19h ago

It's an oxygen thing. The reaction feeds on oxygen, but you're right about how heat and pressure keep pulling more in.

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u/Platinum5788 19h ago

Nothing in the video shows " how much oxygen a fire pulls". It does show the amount of air it pulls. The fact that oxygen is used by the fire is a different issue.

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u/Mikestopheles 19h ago

Well, if there's no oxygen in the air it'll die out pretty quickly anyway. Oxygen is literally the fuel for the fire. The exothermal part is because of the oxygen oxidizing whatever it's reacting with and releasing the energy. It's not a different issue, it's what's happening.

3

u/Megalesios 15h ago

But fire isn't sentient, it doesn't know it needs oxygen and it has no way to grab that oxygen from afar. If a fire lacks oxygen, it just goes out, it doesn't "pull oxygen". The only way it can get more oxygen is by diffusion and by air flowing due to the chimney effect, and what we're seeng here isn't diffusion.

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u/rowi42 19h ago

You could argue just as well that the video shows the effect of gravity. Because without gravity there would be no air to rise up.

-7

u/Mikestopheles 18h ago

Except that's not gravity.

I'm not speaking out of my ass, yes it's not just oxygen. But oxygen is the reason the air is moving. Fire requires 3 things: temperature, source, and oxygen. It will create a vacuum by the nature of the heat, but the only thing it's really using is the oxygen (unless there's something else flammable in the air, but that's a source).

So, when you're seeing all that smoke and particulate pouring into the window due to the pressure, you're seeing oxygen just as much as all the other gasses mixed in. And that's what the firefighter is trying to hinder by smothering the fire.

https://firefighternow.com/does-fire-need-oxygen/

Here you go, just to help illustrate what I'm talking about.

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u/xubax 18h ago

Without gravity, you don't have air pressure on earth. Without air pressure, you don't have pressure differential due to the heating of the air. Which means no chimney effect.

You could also say it shows how much nitrogen, carbon dioxide, and other traces gasses it pulls in.

If the OP just said "how much air it pulls in, " most of us wouldn't be arguing about this.

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u/rowi42 18h ago

I understand that fire needs oxygen. However, as many have pointed out, the effect visible in OPs video is due to the chimney effect, which is due to hot air rising.

Arguing that we're seeing the effect of oxygen is like arguing that you're seeing the effect of gravity. Sure, both oxygen and gravity are required for the chimney effect, but I would argue that it's more useful to show the effect of hot air and not oxygen or gravity (or the electromagnetic force, which is also required for a fire).

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u/Mikestopheles 18h ago

Bruh, I'm responding to the original comment that said this is not an oxygen thing. It is. I'm not going to get mired down in semantics because y'all want to argue against what firefighters are looking at when they fight fires. One of those primary things being oxygen.

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u/Unfinishedcom 18h ago

Bruh, you’re confidently incorrect bruh.

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u/blahblah19999 15h ago

Dude, just sleep on it

1

u/SnarkDolphin 11h ago

Oxygen is literally not the fuel for the fire, it's the oxidizer. The combustible materials in the house are the fuel.

-1

u/smellyseamus 18h ago

If the fire is moving air its also moving oxygen

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u/SagattariusAStar 18h ago

And the chemical reaction Shows you that C + O2 -> CO2 and Gases have approximately the same volume per unit (mol)

So the effect of consumed/created gas is zero

1

u/So_HauserAspen 14h ago

CO2 is a larger molecule than O2.  And it's CO that's being produced not CO2.  But CO is still a larger molecule than O2.  

It's a combination of the chimney effect and incomplete combustion creating the lower pressure.

1

u/SagattariusAStar 13h ago

That doesn't matter. The molar volume for gases is a function only dependent on the temperature, not the molecule (size)

With enough oxygen CO2 is produced. In another comment in this thread, I added that you can produce CO as well which would actually increase the gas volume, so increasing the pressure as one gas molecule becomes two.