Page 425 - Dust Explosions in the Process Industries

392 Dust Explosions in the Process hdustries

high-power electric cables, and lightbulbs that have become buried in powder or dust,
as examples. Practical situations where the temperature of the hot surface is not influ-
enced by the thermal insulation properties of dust accumulationsmay, in fact, be com-
paratively rare.
In her constant heat flux ignition experiments,Beever (1984) used samples of wood
flour contained in a cylindrical stainless steel wire mesh basket of 0.8 m length and 0.1
m diameter. The ignition source was an electrically heated metal wire coinciding with
the axis of the basket. To generate differentratios of the radius of the central cylindrical
hot surface and the thickness of the cylindrical dust sample, the heating wire was
enveloped by ceramic tubes of different diameters. Some essential properties of the
wood flour are given in Table 5.1. Here, E is the activationenergy of the exothermicchem-
ical reaction, R is the gas constant, Q is the heat of reaction, andfis the preexponential
frequency factor.

Table 5.1 Properties of wood flour used in self-ignition experiments reported by Beever (1984)

Bulk density, p 220 k 10 kg/m3
E/R 1.275, IO4 K
Thermal conductivity, 1 0.346 kJ/mhK
___ 7.678 . 1Ozo Wm2
p.Q.f E
a~

Figure 5.5 shows some of Beever's experimentalresults for a hollow cylindricalwood
flour deposit surroundinga cylindricalhot-surface ignition source.A curve predicted from
an approximatetheory is also shown. The agreement of the theoretical predictions,using
a step-functionapproximationwith the experimentalresults, is reasonable, except when
the radius of the hot surface is very small in relation to the thickness of the dust layer.

-PREDICTEO FROM THEORY
L
$ e MEASURED EXPERIMENTALLY
I i
0
10-2 10-1
RADIUS OF CYLINDRICAL HEAT SOURCE OR
THICKNESS OF DUST LAYER

Figure 5.5 Minimum heat flux for ignition of a centrally heated, infinitely long cylindrical wood flour
deposit (From Beever, 1984).