PDA

View Full Version : How long does smoke take to clear out?



Taejang
05-13-2015, 10:54 PM
I need to know how long it takes a room to clear of smoke after a fire. I know there are a thousand variables that change this, so below are hopefully the most important details.

The room is 25 by 20 feet with 6.5 foot-high ceiling. The floor is wooden, wooden panels on the walls, and a thin-wooden ceiling. The room contains assorted materials and furniture from the Middle Ages: some cloth, animal leather, paper, clay pots, and basic iron implements. A table, one chair, small bed, small chest. There are small amounts of black ink, coal, and tar/pitch, but nothing else particularly toxic. No lead. Nothing inside is plastic, nylon, or other "synthetic" materials.

The only way into the room is a solid wooden door, five feet wide by six feet tall. From the door is a thirty-foot long hallway, five feet wide and 6 feet tall, with a 10 foot deep, five-foot wide crawlspace below the floor. The floor, walls, and ceiling match that of the room (all wooden), and the divider between crawl space and floor is wooden. The room and hallway are underground and surrounded by dirt, with only enough ventilation to keep a single adult occupant from experiencing problems while sleeping, eating, and doing light duties (like reading, writing, etc, not exercising or heavy labor).

At the end of the hall is a five foot by five foot chute with a wooden ladder. The chute goes up for 20 feet before reaching a trap door that opens into a poorly constructed hut (poorly constructed should be translated as very well ventilated).

Ignoring weather patterns, humidity, and exterior temperature, and ignoring differences in the exact type of wood used in construction, let's assume the room's door catches fire. The door, trap door, and the hut's door above are all open. All the wood is quite dry and untreated. Let's assume neither the hallway nor the room collapse as the fire spreads, and the hut above it does not catch fire, but the hallway and room do. Would the fire eat up too much oxygen and put itself out? If it did burn, how long would it take for the smoke to clear sufficiently afterwards so that 3-5 people could be inside the room for ten minutes without passing out?

Thanks in advance!

jclarkdawe
05-13-2015, 11:29 PM
This is underground, correct? With ventilation in only one direction -- the hallway then up the chute?

Was this tunneled or a pit that was buried?

Even if this was tunneled, the chances of the overhead not collapsing is slight, unless made out of heavy timbers.

The door is unlikely to burn well, as it's going to be rather thick timbers. You need a lot of fire before it starts going.

Basically you'd get heat and air flow rising with minimal oxygen coming in. The fire would either suffocate and go out, or more likely, smolder. The higher towards the ceiling that you go, the hotter and smokier it will be. But it would be an extremely heavy smoke condition, with trapped air reaching 1500 degrees or higher at the ceiling.

It would continue in this state for days, and potentially months and years. Look at mine fires.

No one would be able to enter. Most likely if someone was dumb enough to enter, the movement of his body would stir up the air enough to cause it to flash over, and he'd be one very dead and crispy critter.

It's a lot easier to tell the expert what you need and let the expert come up with something. You're going to be limited to a small fire, but the conditions will be extreme enough.

Best of luck,

Jim Clark-Dawe

Taejang
05-14-2015, 12:06 AM
This is underground, correct? With ventilation in only one direction -- the hallway then up the chute?

Was this tunneled or a pit that was buried?

Even if this was tunneled, the chances of the overhead not collapsing is slight, unless made out of heavy timbers.

The door is unlikely to burn well, as it's going to be rather thick timbers. You need a lot of fire before it starts going.

Basically you'd get heat and air flow rising with minimal oxygen coming in. The fire would either suffocate and go out, or more likely, smolder. The higher towards the ceiling that you go, the hotter and smokier it will be. But it would be an extremely heavy smoke condition, with trapped air reaching 1500 degrees or higher at the ceiling.

It would continue in this state for days, and potentially months and years. Look at mine fires.

No one would be able to enter. Most likely if someone was dumb enough to enter, the movement of his body would stir up the air enough to cause it to flash over, and he'd be one very dead and crispy critter.
Great information. I don't know why I didn't think of mine fires, but that's a great comparison to make.

The whole thing was tunneled, not buried. Ventilation could be in two directions if I want (one the hallway, another from a small vent bored from above and directly into the room. Kinda like a miniature chimney for air.) If necessary to prevent a collapse, I could say the room and/or hallway is reinforced with stone supports. The door wouldn't be set ablaze from a match or something incidental, we're talking more like a blowtorch here (it is actually closed when the fire starts, but the room's occupant doesn't want to stay in a burning room. It is either opened or smashed open within a minute of the fire starting).

I didn't realize the smoke could reach such a high temperature. I knew it would be far too hot to be in, but I was guessing more like 500 degrees. 1500... wow. And with the height of the ceiling, I imagine the smoke would be lethal temperatures from floor to ceiling, even if it is significantly lower than 1500 at the floor. That's good to know.


It's a lot easier to tell the expert what you need and let the expert come up with something. You're going to be limited to a small fire, but the conditions will be extreme enough.

I actually don't have something I "need," at least not from a plot perspective. I'm exploring options without a specific end-goal in mind. One option (setting the place on fire and then going back later) doesn't seem terribly realistic, unless they are willing to wait a very long time before going back in.

King Neptune
05-14-2015, 02:29 AM
You might consider giving the room and the corridor stone floors.

WeaselFire
05-14-2015, 04:58 AM
I know there are a thousand variables that change this...

Yep, and you adjust those variables to meet the needs of your story. You could write this however you want if you simply change the details to match.

Jeff

jclarkdawe
05-14-2015, 05:34 AM
Smoke and heat layer very nicely in a room with no ventilation. Heat rises, and cold sinks. Your ceiling could be 1500 degrees and the floor maybe 100 degrees, with different temperature layers in between. This is why firefighters go low and slow in a fire. You hug the floor because it's that much cooler.

When this room with no ventilation suddenly gets ventilation, all of a sudden you have oxygen, and in the heated air, tar balls. The oxygen ignites the tar balls, and suddenly the room is full of fire, and the room develops a more uniform temperature. This happens with almost explosive speed, and can blow someone out of the room.

Now the condition of no ventilation can last for minutes to hours, depending upon the construction of the room. What firefighters do is control the ventilation, opening up the roof to cause the heat to rise, and then opening the windows or door to blow the heat and fire out the roof.

First problem you face is this extreme heat can cause things to warp and bend, and stone to crack. But without good ventilation, the heat and smoke are trapped and has nowhere to go. On the plus side, you don't have much of a fire load. I'd think about making it mostly stone, with just a few wooden objects. This would burn on the oxygen in the space, with some smoldering left after the oxygen gets used up. Some ventilation, causing the heat to escape and limiting the smoke in the area.

Best of luck,

Jim Clark-Dawe

Taejang
05-14-2015, 06:09 PM
All good comments. One of the characters involved used to be a miner, so it isn't a stretch to say he may have some experience with fires in similar situations. The characters will avoid it altogether.

...if they can. I may have the fire start accidentally (in that case, not at the door), and the characters will then have to deal with it. But I can do that now, since I have a decent-ish idea of the mechanics involved.


Yep, and you adjust those variables to meet the needs of your story. You could write this however you want if you simply change the details to match.
I find it is often fun to start at the beginning with a situation and think about what the characters would do instead of starting at the end and working backwards. Naturally I adjust accordingly to make key plot elements happen (or to make the story actually interesting), but this was one of those exercises: characters A, B, and C have this goal and find themselves with these resources at this place. What do they do, and how does character D respond?



When this room with no ventilation suddenly gets ventilation, all of a sudden you have oxygen, and in the heated air, tar balls. The oxygen ignites the tar balls, and suddenly the room is full of fire, and the room develops a more uniform temperature. This happens with almost explosive speed, and can blow someone out of the room.
This is a horrible thing in real life, but ever since I learned of it not long ago, I've wanted to include it in a scene somewhere. It can be hard to include non-magical explosions in fantasy writing, and everyone knows explosions are *required* for a good story. (Joking, obviously, but still want to make it happen in a scene sometime.)