Need help with a research project for firefighting

[Zbelgie]

Good evening everyone! I'm currently working on building conclusions to support changes to operational tactics at building fires. I'll list what I have figured out so far and then what I'm still searching for.

-Air expands and pressure increases within a room or structure during a fire; is there rate at which those two changes take place? For every degree increase, pressure increases?

-Fire nozzles when opened, discharge water at a velocity of roughly 85 fps, with that velocity it causes air entrainment into a structure. "fog" nozzles entrain between 11,000 and 16,000 CFM. "Smooth bore" nozzles are roughly 4,500 CFM.

-When we ventilate, either vertically (cutting a hole in the roof) or horizontally (breaking a window or windows), the reasoning is to allow the superheated gases, smoke and other byproducts to exit up and out; high pressure to low pressure. This tactic MUST be coordinated with a hose crew, if water isn't being applied to the fire, things will get really bad as we've essentially allowed the combustion to "breathe" more efficiently, especially if a front door or other doorway or window is opened. At that point we've created a bilateral flow path.

The hang ups for me at the moment are:

-We teach our crews to cut an 8'x8' hole in the roof to allow adequate ventilation. 64 square feet. How much air can move through that opening? I found a formula Area x Velocity = meters3/sec. But running numbers through that doesn't seem to give solutions that "add up" to applied practices and experiences.

-If a structure has a rough volume of 23,000 CuFt under normal ambient conditions. Does that number increase when a fire is burning inside, creating increases in pressure and expanding the air?

-When a nozzle is opened from an exterior position or in the doorway, the extra air we are putting in to the structure will increase overall atmospheric? pressure inside? Which will in turn force the high pressure contents into lower pressure spaces? This leads me back to the first hang up, theoretically, what size roof opening would we need to create the proper ratio for intake and exhaust.

I'm sure there are more things that will come to mind and as they do I'll comment on the thread. Thanks for any help any one can provide!

 

[berkeman]

Welcome to PF.

Are you a FF? I'm a part-time EMT for the last 15 years -- all EMS and no Fire, but lots of great shared experiences and calls with local FFs.

Can you post some links to the Fire references you've been researching so far? That will help us to better answer your questions. Thanks.

Edit/Add -- is this research work of yours covered under a grant from NFPA or FEMA or a similar agency?

 

[Zbelgie]

Yep, sorry I should have added some more background.
I've been in the fire service for 18 years, currently working for a professional department in South Carolina and I am one of the lead instructors for a large part time department in the county I reside in.

Not working under any grants or agencies, this is all self-initiated research. To put it simply, for 14-15 years of my career I cared about the how and what, now I'm very invested in the why. It's my hope that with science backed ideas and concepts, we can remain aggressive but fine-tune our "why".

I will gather all the links, I have a bad habit of doing research, filling up pages in my notebooks and not writing down the source. Much of the information has been gathered from UL/FSRI (Fire Safety Research Institute) classes. They do a fantastic job of doing large scale testing sessions and posting the scientific findings in their classes. I will post the link to their online training page but it requires an account (free) and a little time to go through the curriculum. Most likely I will type up a synopsis from my notes and post it tomorrow to provide a better foundation to my research.

 

[berkeman]

I've been in the fire service for 18 years, currently working for a professional department in South Carolina and I am one of the lead instructors for a large part time department in the county I reside in.

Thank you for your service, FF.

-Air expands and pressure increases within a room or structure during a fire; is there rate at which those two changes take place? For every degree increase, pressure increases?

There is the Ideal Gas Law relating those variables, but with dynamic combustion involved, I'm not sure that will give a good number.

-Fire nozzles when opened, discharge water at a velocity of roughly 85 fps, with that velocity it causes air entrainment into a structure. "fog" nozzles entrain between 11,000 and 16,000 CFM. "Smooth bore" nozzles are roughly 4,500 CFM.

It's hard to believe that the water stream entering the room will increase the air pressure in the room, unless you are shooting through a small break in a window that you made with the water stream. I would think that the pressure in the room is pretty much atmospheric pressure, with the changes from the combustion and heat, not from an incoming water hose stream (but I could be wrong).

-We teach our crews to cut an 8'x8' hole in the roof to allow adequate ventilation. 64 square feet. How much air can move through that opening? I found a formula Area x Velocity = meters3/sec. But running numbers through that doesn't seem to give solutions that "add up" to applied practices and experiences.

Paging our local HVAC expert @russ_watters

-If a structure has a rough volume of 23,000 CuFt under normal ambient conditions. Does that number increase when a fire is burning inside, creating increases in pressure and expanding the air?

I don't think so. The structure is rigid, so there is no mechanism to increase the internal volume with heat expansion.

-When a nozzle is opened from an exterior position or in the doorway, the extra air we are putting in to the structure will increase overall atmospheric? pressure inside? Which will in turn force the high pressure contents into lower pressure spaces? This leads me back to the first hang up, theoretically, what size roof opening would we need to create the proper ratio for intake and exhaust.

I don't think so. See above.

 

[Lnewqban]

Welcome, @Zbelgie !

It is very difficult to significantly increase internal pressure in a closed structure.

If happening slowly, air will leak out through doors and windows, crevices in walls, ventilation ducts, electrical conduits, drain and ventilation pipes, elevator shafts (if any), etc.

If happening quickly enough, like in a gas explosion, the weakest parts of the building envelope will break, normally glass windows, swing doors and perhaps slender areas of exterior walls and roof.

 

[Rive]

-Air expands and pressure increases within a room or structure during a fire; is there rate at which those two changes take place? For every degree increase, pressure increases?

Yes, but usually rooms (accessible for people) has some kind of basic ventilation what would keep that pressure around atmospheric. Unless there is some kind of explosion, of course. The pressure difference should be around the pressure differences related to ventilation speed (ventilation will limit the pressure buildup).
That also answers the question about volume change (usually negligible).

-We teach our crews to cut an 8'x8' hole in the roof to allow adequate ventilation. 64 square feet. How much air can move through that opening?

As calculated for ventilation only, that's some decent over-sizing. I guess it's partly to clean up the visibility close to the floor, fast.
To calculate the air flow through such a hole would be difficult. It's not just stationary pressure difference, but stack effect (what would drag not only hot gases but cold smoke too) and others. I would leave it as experimental/experience based, with the urgency and required effort of the task factored in.

-When a nozzle is opened from an exterior position or in the doorway, the extra air we are putting in to the structure will increase overall atmospheric? pressure inside?

Yes, but that pressure difference is also at the level of (aggressive) forced ventilations.
Feels windy, but as actual pressure difference, it's not that much.

Those pressure differences being big or small - this depends on the perspective. Usually people has some experiences with tire pressure and such, while pressure differences in air flow and ventilation business are usually several decades lower than those, thus always 'around atmospheric'.

 

[willem2]

-Air expands and pressure increases within a room or structure during a fire; is there rate at which those two changes take place? For every degree increase, pressure increases?

The pressure can't get all that high. Houses are leaky. If the house was airtight, you could get 3x or 4x atmospheric pressure if you'd heat up the air to the temperature of a house fire. (This could be hard to do without sufficient air).
10 % more than atmospheric pressure, would mean 10000N = The weight of 1000kg on a square meter. I don't think a window can take that, and the pressure really can't get higher than that.
While the pressure doesn't go up much, the hot air will be much lighter than cold air. This will produce a large air flow if you let out the hot air at the top, and let in cold air at the bottom.

-We teach our crews to cut an 8'x8' hole in the roof to allow adequate ventilation. 64 square feet. How much air can move through that opening? I found a formula Area x Velocity = meters3/sec. But running numbers through that doesn't seem to give solutions that "add up" to applied practices and experiences.

The formula is correct. The problem is that you not only need to let the hot air out, but you also need to let cold air in. Houses are leaky, but not nearly leaky enough to get a fast air stream through a 64 square feet hole. The air has to come in through other ventilation openings, or the air will also have to come in through the hole in the roof. Having air move through it in both directions won't help with ventilating.

 

[LT Judd]

I have some experience, (theoretical and drills only) from merchant marine and have actually done something similar on a fire fighting course. Typically, as non professional fire fighters we would always seek to use existing opening like doors , windows, access hatches rather than cutting anything. 8X8 seem to be a little larger than necessary. A typical marine access hatch is 600 X 600 mm and on a fire fighting course a few years back that worked quite well to evacuate smoke. But then again we only have 50 mm hoses at 8 bar.
I understand you are trying to "fine tune" the dimensions to balance the inflow and outflow. I just wonder if it is that critical. There are just so many variables, like the location of all of the openings and leak points in relation to the seat of the fire, the layout of the room, obstructions and amount and structure of fuel etc. It seems a bit pointless to try and be too exact. If there is insufficient inflow the pressure will start to pull down, which should also help to dampen any fire down.
I would encourage you to try and get some grant money for this if you feel its a worthwhile project. It could be something that would be suited to some empirical analysis by mock up or model testing. It could also be approached using CFD . The company I work for can do that but I am sure many others could too.