[GSBN] Straw Bale House Fire

[Derek Roff]

I appreciate your comments, Martin.  I will respond with a couple of 
more points about carbon dioxide and fire extinguishers.

At 70 degrees F (21 C) ambient temperature, liquid carbon dioxide has a 
vapor pressure of 830 psi (57.2 bar).  Unless I am forgetting my 
physics, this means that every CO2 tank or fire extinguisher, large or 
small, will have an internal pressure of about 830 psi, at that 
temperature.  That's plenty of pressure to push some CO2 into the SB 
wall cavity, although the internal temperature of the tank will cool 
off rapidly as gas is released.

The boiling point of CO2 is listed at -69.9 deg. F (-56.6 C), so we 
have plenty of "coolth" to work with.  The actual temperature of the 
gas delivered through the hypothetical hole in our hypothetical 
plastered SB wall will depend on the rate and duration of gas delivery.

One reason that carbon dioxide fire extinguishers are not used more 
widely, for more types of fires, is that in open air, the gas disperses 
quickly (as mentioned in Martin's quote).  Injecting it into a 
plastered wall gets around this problem, and should make it possible to 
get high CO2 concentrations in small areas of the wall.  While oxygen 
will eventually seep back in, I am optimistic that elevated carbon 
dioxide levels, and decreased oxygen levels, will persist for quite a 
while, if the plaster isn't removed.  I think that combustion will be 
inhibited chemically, through displacement of oxygen, as well as 
thermally, through cooling.  Once the smoldering stops, there will be 
no new source of heat to re-ignite the straw.  While anything is 
possible, I think it is unlikely that the straw would re-ignite once it 
was extinguished, because smoldering straw is just barely above the 
required ignition temperature.  Just a little cooling would be enough 
to avoid re-ignition.  I would still favor removing the charred straw, 
but testing may show that this is better done after a certain delay 
than quickly.

While researching this question this afternoon, I discovered that it 
would probably be possible to inject liquid carbon dioxide into the 
wall.  This would mean that almost all the expansion and cooling would 
take place outside the CO2 tank, and inside the wall itself.  This 
would deliver cooler gas more quickly to the hotspot.  The liquid 
carbon dioxide would turn to gas almost instantly, absorbing a great 
deal of heat in the process.  I learned that some paint ball 
enthusiasts do this to power their guns in cooler weather.  It might be 
as simple as turning the fire extinguisher upside down before opening 
the valve.  On the other hand, it might require a different valve. More 
things to test.

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



------------ Forwarded Message ------------
Date: Wednesday, April 1, 2009 1:02 PM -0700
From: martin hammer <mfhammer at pacbell.net>
To: "(private, with public archives) Global Straw Building Network" 
<GSBN at greenbuilder.com>
Cc: Don Fisher <fishdl48843 at yahoo.com>
Subject: Re: [GSBN] Straw Bale House Fire

[snip]

CARBON DIOXIDE extinguishers are most effective on Class B and C
(liquids and electrical) fires. Since the gas disperses quickly, these
extinguishers are only effective from 3 to 8 feet. The carbon dioxide
is stored as a compressed liquid in the extinguisher; as it expands, it
cools the surrounding air. The cooling will often cause ice to form
around the ?horn? where the gas is expelled from the extinguisher.
Since the fire could re-ignite, continue to apply the agent even after
the fire appears to be out.

The question remains whether a CO2 extinguisher would sufficiently cool
a hot spot.  (The last sentence of the CO2 description is worth
noting.)  The simple answer is probably "it depends".  Finding the
effectiveness and any limits of this technique would be very valuable.


Martin

---------- End Forwarded Message ----------

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