Page 422 - Dust Explosions in the Process Industries

ignition of Dust Clouds and Dust Deposits 389

5.2.2
§OME EXPE RIM ENTAL INVESTI GATIONS

5.2.2.1
lsoperibolic Experiments

En the isoperibolic configuration, the outside of the dust deposit is kept at a constant tem-
perature while the temperature development at one or more points inside the deposit is
monitored. The dust sample may either be mechanically sealed from the surroundings
or air mxy be allowed to penetrate it, driven by the buoyancy of heated gases inside the
dust sample or external overpressure or suction.
Leuschke (1980,1981) conducted extensive experimental studies of the critical param-
eters for ignition of deposits of various combustible dusts under isoperibolic conditions,
with natural air draught through the sample, driven by buoyancy. Figure 5.3 shows a plot
of the minimum ambient air temperature for self-ignition of deposits of cork dust sam-
ples olf vardous shapes and sizes as a function of the volume-to-surface ratio of the sample.

520
'+
- 500 \
Y
w
III A
I--
3 \
a 480 - 8
%
CL
E
+
5 460
I- -
z
U
1
W
;440
z + CYLINDERS A
Y
03 +\
3 -- CUBES ..A
i AA TETRAHEDRONS +
' 420 - Q SPHERES \
0
@ CONES '.
- e
-tc
LOO I I
0.1 0.5 1 5
VOLUME/SURFACE AREA RATIO Icml
Figure 5.3 Minimum ambient air temperature for self-ignition of cork dust deposits of various sizes
and shapes as a function of the volume/surface area ratio (From Leuschke, 1981).

This correlation can be interpreted in terms of the critical Frank-Kamenetzkii pararn-
eter for self-ignition (equation (5.11) later), which was discussed extensively by
Bowes (1984). Note that the abscissa scale in Figure 5.3 is linear with the logarithm
of the volumehurface area, whereas the ordinate axis is linear with the reciprocal of