[GSBN] FW: Straw Bale House Fire

[martin hammer]

Derek,

Re: the typical ineffectiveness of CO2 cooling a fire in open air, but
considering if for a straw bale wall, Don also said - " It may work because
of the closed compartment fire."  So he sees the concept.  I think we should
test and compare CO2 and water (and possibly other methods).

I appreciate your quantitative analysis, which further suggests CO2 might
work well.  And I really like the thoughts in your last paragraph, that the
CO2 would probably take the same path that a smoldering fire did, and water
wouldn't do the same.

And I think you would make a fine Captain Carbon Dioxide.

MH



On 4/3/09 4:22 PM, "Derek Roff" <derek at unm.edu> wrote:

>> 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
> 
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