Enclosure fires

Temperature impact on fl ammability limits
In the sample calculation on page 58, we were dealing with a
gaseous mass which was kept at around ambient temperature
at the start. The fl ammability limits change when the temperature
of the gaseous mass is raised.
Temperature °C Lower fl ammability limit
27 5.7%
127 5.2%
227 4.8%
327 4.3%
427 3.8%
The lower fl ammability limit falls when the temperature is
higher. Less energy needs to be released during combustion
for the gaseous mass to ignite and burn with a fl ame. This
means indirectly that a smaller amount of fuel is required for
combustion to be able to continue.
The temperature is a crucial factor when estimating the
risk of a smoke gas layer igniting. The higher the temperature,
the more pyrolysis gases have formed, thereby making it easier
for the layer to ignite. The fact that the fl ammability range has
widened is more often than not of minor importance.
Flammability limits for gas mixtures
Up until this point we have been discussing one gas or one
substance. But a smoke gas layer contains many different gases.
The principles for fl ammability limits apply in the same
way to gas mixtures as well.
Pure hydrocarbons, e.g. methane and propane, are special
cases. The amount of fuel, specifi ed in g/m3 , required for the
gases to ignite is almost not dependent on the molecular
weight. Figure 38 below shows the lower and upper fl ammability
limits for hydrocarbons with different molecular
weights. It seems clear that the lower fl ammability limit is almost
constant for most hydrocarbons.
The upper limit, by contrast, varies considerably with the
hydrocarbon’s molecular weight (see Figure 38).
Table 5. Temperature’s
impact on the lower
fl ammability limit for
methane.
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