Enclosure fires

Ignition time
Ignition time can also be calculated using Equation 2, which is a reformulation
of Equation 1. Note that the heat resistance from the surface has been omitted
and that the ignition temperature most often lies in the range of 300 °C–400 °C.
When the ignition temperature T sa is known the ignition time t a can be
calculated:
(T sa – T i ) 2
t a = 4(q") 2 k3c × p
Let us take as an example a fi re room where a fl ashover has occurred. If the
temperature in the room is around 600 °C all the surfaces will be affected by
radiation in the order of 30 kW/m 2 . If we calculate the length of time it takes to
ignite combustible chipboard, for instance, the calculations to be carried out are
as follows, assuming that the ignition temperature T sa = 400 °C. The k3c value is
taken from Table 1.
(400 – 20) 2
t a = 4(30 000) 2 120 000 × p u 15 seconds
Equation 2
This is a rough estimate and must not be regarded as a precise value. In actual
fact, the material will heat up at the same time as the surface cools down as a
certain amount of heat radiation leaves the surface. If you decide beforehand
that the surface should not be heated up beyond a certain temperature, you can
calculate the length of time the surface can be subjected to a certain amount of
heat, i.e. a certain amount of incident radiation, until it reaches the preset
temperature.
Material k
(W/mK)
c
(J/kgK)
3
(kg/m 3 )
k3c
(W 2 s/m 4 K 2 )
Chipboard 0.14 1,400 600 120,000
Wood fi bre
board
0.05 2,090 300 32,000
Polyurethane 0.034 1,400 30 1,400
Steel 45 460 7,820 160,000,000
Pine tree 0.14 2,850 520 210,000
Table 1. Thermal
inertia for different
materials.
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