In this video we will look at the short initial pressures when using water mist inside a fire compartment, this time with a Coldcut Cobra ultra high pressure system (UHPS).

Water mist for exterior fire suppression

The water mist is introduced through the simulated window frame of a closed window.

The open window is used to look at flames, smoke color and pressure inside the fire compartment. At a real incident, this window would probably be closed.

Bi-directional flowpath

At the beginning of the video we can see a bidirectional flowpath (or airtrack) in the open window. The fire compartment is roughly at neutral pressure (or atmospheric pressure).

Varying gas densities inside the fire compartment causes the upper part of the window to be an outlet, and the lower part of the window to be an inlet. It is a gravity current.

You could also say that the fire is in a steady state, as it is probably not growing or reducing in power (Heat Release Rate).

Mixing of everything

The water mist spray cause massive smoke movements inside the fire compartment which mixes everything. The hot smoke will rotate back into the water mist and start to cool down.

In this fire compartment this mixing is probably only a good thing as it increases the absorption rate of energy, and cools the room faster.

Steam expands and cooling contracts

When water mist (or any liquid water) turns into steam, it expands in size.

When hot gases in the smoke cool down, they contract in size.

If the total expansion of steam is greater than the total contraction of smoke, the total gas volume in the room increases. And the other way around is true too.

Smoke cooling always cause gas contraction

If the water only cools the smoke, it would always lower the total gas volume in the room. The total effect is the negative pressure we see in the fire compartment.

At the end of the video, the open window has turned into a full inlet. This is an indicator that the overall pressure inside the fire compartment is lower compared to the atmospheric pressure outside.

The room is at a relative negative pressure. The total gas volume has reduced in size.

Exterior water mist at rescues

If our objective is rescue, the total room pressure is possibly important.

A negative total pressure in the fire compartment indicates that we are pulling hot smoke into the fire compartment from the adjacent rooms, not pushing hot steam out from it.

To optimize survivability inside the house, maybe we should stop injecting water mist when the flames disappear and pressure start to return to atmospheric pressure.

Firefighting is complicated

There are still so many things I do not know about firefighting. How water mist should be used at potential rescues is one of those issues for me.

The above approach would reduce the application of water mist at potential rescues to below 15 seconds. After that you would not have a negative pressure. For me that would be a good conservative baseline for residential spaces.

Or it might be true that you should flow for as long as you can. I do not really know.

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