[GSBN] FW: Straw Bale House Fire

[Derek Roff]

> 1. What do you think of the idea of using CO2 instead of water
> to inject into hot spots?
>
> Using CO2 on Class A materials is not very effective as this does not
> remove the heat side of the fire tetrahedron.  It can work but
> requires a large amount of CO2.

It isn't my goal to set myself up as Captain Carbon Dioxide, but I 
think some of Don's statements are not applicable to the scenario that 
we have been discussing, of smoldering straw in a plastered SB wall. 
Having the fire effectively contained and choked by the the low oxygen 
atmosphere in the wall is very different than fires inside other types 
of walls.  Using carbon dioxide, especially liquid carbon dioxide, 
inside a closed wall is very different from using it in free air.  I've 
done some calculations, and the numbers look promising to me.

I'm estimating that smoldering straw within a strawbale wall with the 
plaster largely intact might have a average temperature in the 
smoldering area of something like 600 degrees F.  I couldn't find a 
clear reference, but I feel very safe in estimating that the 
temperature is between 400 and 1200 degrees F.

The figures for the heat of vaporization for carbon dioxide are 
available (574 J/g).  It looks like each gram of liquid carbon dioxide 
that we could inject into the smoldering hotspot would reduce the 
temperature of one gram of smoldering straw from roughly 600 degrees to 
a negative 60 degrees F.  I see that as a pretty effective way to 
"remove the heat side of the fire tetrahedron," with a very small 
amount of CO2.  Or one gram of CO2 could cool ten grams of smoldering 
straw down to less than 100 degrees F.

Liquid carbon dioxide looks to be somewhat more effective than water, 
gram for gram, in cooling the fire.  Water on a fire produces steam, 
and near-boiling water, both of which can (and do) scald people 
fighting fires.  Liquid CO2 is cold enough to "burn" the skin, but it 
turns to gas very quickly.  Overall, I think liquid CO2 would be 
somewhat riskier for the firefighter than water, and would require more 
care in use.  Assessing that risk differential would be another thing 
to test, if CO2 turns out to show any promise for extinguishing 
smoldering straw.

Something that I just thought of is that the carbon dioxide gas inside 
the wall would diffuse and convect most rapidly into the areas of 
lowest density in the straw.  These would be largely the same paths 
that the smoldering fire would take (would have taken), however narrow, 
twisted and tenuous they might be.  The CO2 would suppress combustion 
everywhere it traveled.  Water would only follow gravity.  Steam would 
follow the low-density path, until it condenses.  I would love to find 
out how those factors would influence fire suppression in an SB wall. 
It seems like the farther combustion had moved into the bales from the 
center of the hotspot, the more advantageous the use of CO2 would be.

Derelict

Derek Roff
Language Learning Center
Ortega Hall 129, MSC03-2100
University of New Mexico
Albuquerque, NM 87131-0001
505/277-7368, fax 505/277-3885
Internet: derek at unm.edu