Page 268 - Dust Explosions in the Process Industries

240 Dust Explosions in the Process Industries

Although a substantialamount of work has been carried out during the years sincethe
statementof Hartmann et al. to overcomethis basic problem in dust explosionresearch,
their words are still valid.
It appears, however, that the problem does not merely arise from experimental diffi-
culties. The basic question is, perhaps, not how to produce the “perfect” experimental
dust cloud but rather whether a realistic definition of such a dust cloud can be given at
all. The ideal static, fully dispersed, and uniform dust cloud is impossible to realize in
practice, whether in the laboratory or real life. In any realistic dust cloud, the particles
and supportinggas are in motion, the dust concentration is only to some extent uniform,
and the dispersion of agglomerates may not be complete.
Sophisticatedmeans of overcomingthe problems have been attempted. These include
the use of free-falling explosion chambersby which the influence of gravity is eliminated
(Ballal, 1983;Gieras et al., 1986)and steady-statesuspensionof the dust cloud in a strong
electrostatic field (Gardiner, Caird, and Bardon, 1988). However, while such methods
may provide useful insight into basic details of ignition and combustion of dust parti-
cles and clouds, they do not representpractical industrial conditions. Disregarding such
highly sophisticated techniques, the methods used for the formation of experimental
dust clouds for dust explosion research may be classifiedin the three main groups, illus-
trated in Figure 3.29.

Fan I Dust in

. _... . .

1 Dust out
I Air blast
(a) lb)

Figure 3.29 Three basic principles used for generating dust clouds for dust explosion research:
(a) transient clouds generated by dispersing a given quantity of dust by a short blast of air, (b) stationary
circulation of a given quantity of dispersed dust in a closed system, and (c) stationary formation of
a dust cloud in an open system.

3.9.2
TRANSIENT DUST CLOUDS GENERATED BY A SHORT AIR BLAST

Due to the relatively simple equipmentand minimal dust quantitiesneeded, the transient
dust cloud method has been adopted in the major part of published investigations,both
in small and large scale.
According to Brown and James (1962), the transient air blast method was probably
first introducedby Holtzwart and von Meyer (1891).Their very simple explosion appa-
ratus consisted of a glass tube of 50 cm3capacity, fitted with a pair of platinum electrodes,