Page 29 - Dust Explosions in the Process Industries

2 Dust Explosions in the Process Industries

(a1 SLOW COMBUSTION (b) FAST COMBUSTION Id EXPLOSION

Figure 1.I Illustration of how the combustion rate of a given mass of combustible solid increases
with increasing subdivision.

enough space for its unrestrictedburning, the combustionrate is very fast and the energy
required for ignition very small. Such a burning dust cloud is a dust explosion. In gen-
eral, the dust cloud is easier to ignite and burn more violently the smaller the dust par-
ticles are, down to some limiting particle size that depends on the type of dust material.
If such an explosive combustion of a dust cloud takes place inside process equipment
or work rooms, the pressure in the fully or partly enclosed explosion space may rise rap-
idly; the process equipment or building may burst; and life, limb, and property can be
lost.

1.I .1 .3
Specific Surface Area-A Convenient Measure of Dust Fineness
The degree of subdivision of the solid can be expressed in terms of either a characteris-
tic particle size or the total surface area per unit volume or unit mass of the solid. The
latter characteristic is called the specijic surface area of the subdivided solid.
Figure 1.2illustrates the relationshipbetween the particle size and the specific surface
area. After subdivision of the original cube to the left into eight cubes of half the linear
dimensionof the original cube, the total surface area has increased by a factor of 2, which
indicates that the specificsurface area is simply proportional to the reciprocal of the linear
dimension of the cube. This can be confirmed by simply expressing the specific surface
area S as the ratio between surface area and volume of one single cube of edge length x.
One then finds

This is also the specific surface area of a powder or dust consisting of monosized cubes
of edge length x.
The same result applies to spheres of diameter x,because

m2 6
S= - -
(n/6)x3- x