# Can gas pass a wall?

• 09-04-2012, 09:14 AM
ndrini
Can gas pass a wall?
Hi everybody,
I'm investigating (is it the right forum session, isn't it?) a fire that broke out from inside a wall (the insulating foam caught fire due the heat coming from the pipe of a exhaust flue of a fireplace).

I'm wondering if it's possible to estimate the flux rate of gas (essentially CO) out coming from the wall.

I know which are the wall layers:
mortar 1 cm
brick 20 cm
polyester 2 cm

Unfortunately I know only “water vapor permeability” of these materials.

Do you think I can use the water vapor permeability to assess whether and how a gas (carbon monoxide) has gone through a wall?

Recalling the definition of 'resistance to water vapor μ
“The value μ of a material construction is a parameter, without dimension of the matter itself, which indicates how many times the building material is more insulating vapor, compared with a layer of still air of the same thickness.
The larger the parameter μ, the greater will be the impermeability to vapor of the material construction!”

I can use it to get an indication (or exact values, note the differential pressure) of a gas other than water vapor?

Here the wall values, with μ equal to:
mortar 38
brick 8
polyester 45

Using Fick's law - for one dimension - that ∂ c / ∂ t = - D ∂ ² c / ∂ x ² ; dimensionally
∂ c / ∂ t [kg / m³ / s] = - D [m · s] · ∂ ² c / ∂ x ² [kg / m³ / m²]

in steady state I can transform this one as:
0 = - D · ∂ ² c / ∂ x ² and integrating - K/D = ∂ c / ∂ x
but I can go further...
Any idea?

Thanks.

Andrea
• 09-04-2012, 04:43 PM
fire49
Why do you want to know this???

you might post the question here also

http://www.forumworld.com/arson-inve...ons/list.php?3

some of the people on the site are not to friendly!!!!!!!!!!!!
• 09-04-2012, 10:41 PM
EastKyFF
I'm friendly, but I have no idea what your answer is. I do have a question. Can a wall pass gas? Because it'd make a heck of an excuse for me sometimes. "No, honey. That was the wall."

I'm only posting this to bump this thread in hopes that somebody who knows your answer will see it.
• 09-05-2012, 03:15 AM
LaFireEducator
Damn.

I know that I've been told that my gas passes through every wall in the fire station ..... But I have gotten one heck of a headache just looking at your question.
• 09-05-2012, 04:04 AM
ndrini
[quote "Why do you want to know this???"]
Because I'm investigating a fire that burn insulating foam and I wanna know where the gases gone.

Is the volume of gas increasing because of stoichiometric transformation ( with n=1: 6 O2 + (C8H8)n → 8CO + 4 H2O from 6 mole to 12 mole)

Is it drive outside the wall from the pressure?
And how much smoke does pass a wall?

These are the questions :D :eek:

Andrea

PS I ampliated my post here
http://www.chemicalforums.com/index.php?topic=61823.0
• 09-05-2012, 02:30 PM
fire49
well more than likely depends on the wall construction

If voids, yess will travel through voids, if openings in the wall, such as electrical outlets, will seep out of them. Not sure you could calculate any wall.

Only thing I can think of is to rebuild the wall and burn it!! to see what happens

yes a fire can create pressure

also you might post here:::

http://www.forumworld.com/arson-inve...ons/list.php?3
• 09-06-2012, 05:06 AM
ndrini
Thanks.
I was talking of an close wall!
:D
• 09-06-2012, 12:14 PM
lniles77
The short answer is no, you can't use the water vapor permeability as an approximation of the carbon monoxide permeability. Differences between H2O and CO molecules in terms of size, polarity, hydrogen bonding, etc. mean that the gases will interact differently and therefore diffuse at different rates. For example, there is a table in http://www.arkema-inc.com/literature/pdf/775.pdf that shows very different permeabilities for different gases across different types of barriers -- some barriers are relatively better at blocking water vapor than carbon dioxide, while other barriers are relatively better at blocking carbon dioxide, and so on.

You need permeabilities for CO specifically if that's what you're interested in. I have no idea where to find that information. You may need to measure it yourself, as someone else suggested.