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Name: Tom V.
Status: other
Age: 40s
Location: N/A
Country: N/A
Date: Sunday, February 23, 2003


Question:
At what level (percent oxygen) does a nitrogen/oxygen atmosphere cease to support combustion?


Replies:
Every combustible substance (the fuel) has an upper and a lower flammability air concentration limit. That is, if there is too much air available the fuel can't get hot enough, fast enough to burn, and if there is too much fuel and not enough air (oxygen), not enough oxygen to support combustion and/or heat the substance to a high enough temperature to sustain the combustion.

This upper and lower limit depends upon many factors, for example, how finely divided the fuel is, if it solid, liquid or vapor, the orientation of the fuel with respect to the point of ignition, and others. You can see that a hanging sheet of paper ignited at its base will burn more easily (hence have wider flammability limits) than the sheet ignited at the top, which will be different from the burning characteristics if the ignition source and the sheet are perpendicular. The problem of defining flammability limits is very complicated, especially for solids and liquids. There are test methods (ASTM) but these methods only limit the number of confounding experimental variables; it does not control all of them. A chemical handbook, like the CRC Handbook of Chemistry and Physics, gives values for some materials but the limits are incomplete and still dependent upon uncontrolled variables.

Vince Calder



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