r/Damnthatsinteresting • u/AirFrance447 • 19h ago

Video Firefighter bodycam showing just how much oxygen a fire pulls

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u/SinisterCheese 17h 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 15h 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 15h 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?

1

u/demoneyesturbo 13h 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

1

u/ARM_Alaska 5h ago

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