Page 543 - Dust Explosions in the Process Industries

5 10 Dust Explosions in the Process Industries

Figure 7.35 Silicon dust explosion following elec-
tric spark ignition in an apparatus of the type illus-
trated in Figure 7.34.

for breakdown to be impossible at that voltage. Pneumatically or spring-driven dis-
placement of one spark electrode toward a shorter spark gap, allowing sparkover, is
synchronized with the occurrence of the transient dust cloud, for example, via sole-
noids. Boyle and Llewellyn (1950) were probably among the first to use the electrode
displacement method. Its drawback is that the actual spark gap distance at the moment
of the discharge is not known.
One way of avoiding the synchronization problem is to work with a semistationary
dust cloud and charge the high-voltage capacitor slowly until breakdown occurs natu-
rally at the fixed spark gap distance chosen. Because of arbitrary variations, the actual
voltage at breakdown differs from trial to trial and must be recorded for each experiment
to obtain the actual given spark energy '/2CV2.
Figure 7.36(b) illustrates two versions of the direct high-voltage discharge circuit, with
and without a significant series inductivity, on the order of 1 mH. This difference can be
significant with respect to the igniting power of sparks of similar energies. The induc-
tion coil makes the spark more effective as an ignition source by increasing the discharge
duration of the spark. Such an induction coil is automatically integrated in both the orig-
inal U.S. Bureau of Mines circuit and the CMI circuit, as shown in Figures 7.36(a) and
7.36(c) (see Chapter 5 for further details concerning the influence of the spark discharge
duration).