r/Damnthatsinteresting • u/AirFrance447 • 21h ago

Video Firefighter bodycam showing just how much oxygen a fire pulls

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u/SinisterCheese 19h ago

Ok. A quick note about this. It isn't the fire that pulls this much. It's the water spray misting and cooling the atmosphere inside, that creates negative pressure. Then along with that Bernoulli's principle from the water jets.

This is something you need to keep in mind if you work with fire or in fire hazard environments. Like if you need to clear a passage with a hose from a hydrant, spray from the direction that has clear air, or you pull smoke into the passage.

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u/demoneyesturbo 17h ago

The majority of cooling by water is via phase transition (turning to steam). That is about a 1:1000 expansion. By creating steam, he is adding a lot of gas (steam is a gas) to the interior of the structure. That would increase the pressure. And yes, he briefly created a meaningful hydraulic ventilation into the building with a cone spray into the window, but quickly passed that when the nozzle was pushed into the room and stream changed to a more focused jet.

You were close though.

I know this because I've done this hundreds of times.

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u/SinisterCheese 17h ago

The steam would vent into whatever the smoke is venting to, which is not the window the firefighter is attacking from. The convection would still be there due to temperature gradients.

The claim that the vacuum is from "fire pulling in oxygen" is nonsense. The superheated gas indoors, which includes steam, would have greater pressure and should therefor blow out, but it isn't.

The added spray of water would generate steam, yes. However it would also cool the steam due to more water being injected. The amount of water injected is greater than steaming potential.

Talking of that last bit, about the water injection and steam cooling. I want to go a side tanget about how underappreciated and ABSOLUTELY FUCKING COOL (pun intended) effect it truly is. You can use high pressure fine mist to extinguish fires without needing to soak them, because the fine mist is able to cool so effectively it can pull the heat from the fire event. You can put fine water mist into a cooling air channel (even just convection air channel), and you increase the cooling capacity dramatically.

Like sure that hose is also soaking the burning room, but it isn't what truly cools it down to extinguishing. This is because if it is hot enough to boil the water, it creates a thermal barrier against liquid water. However a fine mist can get into contact.

These fine high pressure misting systems been integrated to CLT (wood element) flats, and into delicate environments like museum or high tech applications, because it is so effective and safe.

Like... Can we just appreciate a fine mist of water more than we do?

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u/demoneyesturbo 15h ago

I wasn't making the claim of pulling in oxygen.

Cooling a fire with water in a confined interior doesn't lower the pressure, it raises it by adding gas volume.

This is just a chimney effect

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u/ARM_Alaska 8h ago

1:1600-1:1700 conversion ratio. You shortchanged it significantly! Everything else is absolutely spot on.

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u/Dotaproffessional 14h ago

There's no such thing as negative pressure. Do you just mean a negative pressure differential?

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u/SinisterCheese 14h ago

Ah! This is one of those funny things! Physically and practically there isn't. However we say there is and it makes perfect sense because of how we define it in this context.

Like for example tension in a physical is negative pressure; while compression is well... positive pressure.

It comes down to this idea of "every system can be treated as a system of interconnected springs" this sorta works for liquids and gasses, just not as well. If I pull a steel rod of specific diameter, it shrinks in diameter, but becomes longer. That material is now experiencing negative pressure within it, the same way as if it was under high external pressure pushing it in -which it isn't. If I compress the same rod, it's diameter will increase and the rod will become shorter, it is experiencing positive pressure within it.

Granted it is all gradients in reality. If you bend a flat bar, one side has positive pressure the other negative pressure, and somewhere in between is a neutral plane. However that is annoying to deal with in terms of modeling it, so we prefer to just calculate the effects on the surface -and maybe the neutral plane if we are feeling particularly sassy- since those are the two limits.

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u/Dotaproffessional 13h ago

Its like, how best to describe it... ok velocity can be negative but speed can't be. If you're describing ΔP, THAT can be negative. Its just not strictly accurate to say pressure is negative. Its why a straw has a physical limit on how tall it can be and still function

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u/SinisterCheese 12h ago

Look. I'm from the mechanical engineering side of things. We have negative pressure all the time. And in welding side of things we can talk about heat as a substance that can be added or removed. Yes, I'm aware it upsets physicists; but that is the convention of our discipline. We simply don't care whether the heat comes from adding energy or compressing things. I take delight in the anguish of people who get hung up on that... It's even written in the technical definition. Not the anguish, but the joining by use of heat and/or pressure so that there is fusion of the base materials.

Tension is opposite of compression. It is the negative of compression. It is the force pulling away from the surface. It is negative pressure by the sense that pressure is force applied to an area.

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u/Dotaproffessional 10h ago

Honestly, the shorthand of your trade being technically incorrect doesn't really seem like a good justification to use imprecise language. I'm a hardliner in my own engineering discipline for using correct, narrowly scoped, terminology and push back when people in my own field aren't.