Page 347 - Dust Explosions in the Process Industries

3 I6 Dust Explosions in the Process Industries

Schonewald(1971) derived a simplifiedempiricalversion of equation (4.75)that also
applies to dusts containing a mass fraction (1 - a)of inert substance, a being the mass
fraction of combustible dust:

CJa
c*- (4.77)
- 1-2.966(1- a)c,C, la
where the minimum explosible dust concentration without inert dust is C1= -1.032 +

1.207 106/Qo,Qobeing the heat of combustionper unit mass (in J/g), as determined in a
bomb calorimeter.As can be seen from Freytag (1965), equations (4.75) and (4.76) were
used in the Federal Republic of Germany for estimatingminimum explosible dust con-
centrations, but later, this method was replaced by experimental determination.
Table 4.11 gives examples of minimum explosible dust concentrationscalculated from
equations (4.75) and (4.76), as well as some experimental results for comparison. The
calculated and experimental results for the organic dusts polyethylene, phenol resin,
and starch are in good agreement. This would be expected from the assumptions made
in Zehr’s theory. However, the result for graphite clearly demonstratesthat Zehr’s assump-
tion of complete combustion of any fuel as long as oxygen is available is inadequate

Table 4.11 Minimum explosible dust concentrations (g/m3) calculated by the theory of Zehr
(1957)
Calculated minimum explosible dust

Bituminous coal 35 48 70-130, constant volume (Cashdollar,
Hertzberg, and Zlochower, 1988)
Source: Most data from Freytag (1965); comparison with experimental data.