Page 272 - Dust Explosions in the Process Industries

244 Dust Explosions in the Process Industries

3.9.3
STATIONARY DUST CLOUD IN A CLOSED
CIRCULATION SYSTEM

Several workers tried to experiment with systems in which a given quantity of dust,
suspended in a given volume of air, circulates in a closed loop. An early attempt was
made by Bauer (1918), who simply used a fan located near the bottom of the verti-
cal cylindrical explosion chamber, rotating at 90” with respect to the axis of the
chamber, to keep the dust in suspension. However, the assumption that this arrange-
ment could produce a uniform dust concentration was not, according to Selle (1957),
justified, because the dust could not be completely raised from the bottom of the
vessel.
Eckhoff (1970) attempted to adopt a similar system as that used by Bauer, by using a
coaxial fan at the bottom of the cylindrical 1liter bomb and a “beater” rotating at 90” to
the cylinder axis. However, it was impossible to generate a stationary dust cloud. The
dust initially introduced in the bomb was raised into suspension, but the dust concen-
tration decreased systematically and comparatively fast with time due to the inevitable
deposit of dust on parts of the vessel wall and such.
Brown and Woodhead (1953) arrived at the same conclusion for another version of
the closed circuit apparatus.These workers studied the dust cloud formed in a closed loop
of 7.5 cm diameter glass tubing, through which the dust dispersion was circulated at var-
ious rates of flow. They concluded that it was impossibleto obtain a uniform cloud, and
very high circulation velocities were required to prevent dust from depositing at the
bends of the loop. Furthermore, at these high circulation speeds, it was noted that sig-
nificant comminution of the particles could take place.
A special version of the method based on the circulationof a constant quantity of dis-
persed powder in a closed apparatus was developed by Gliwitzky (1936). To keep the
dust dispersed,two propellers, rotating at different speeds and in oppositedirections, were
situated coaxially inside a horizontd cylinder with open ends, which was placed inside
the 43 liter explosion chamber.The closed circuit thus consisted of the inside of the inter-
nal cylinder and the annulus between this cylinder and the wall of the explosion cham-
ber. However, this system was not found to be satisfactory, since even with easily
dispersed aluminum flakes, the dust dispersion was incomplete.
It therefore appears to be justified to conclude that none of the various versions of the
closed circuit system developedfor the formation of experimentaldust clouds has proven
to offer an acceptable solution to the problem.

3.9.4
STATIONARY DUST CLOUD IN AN OPEN-CIRCUIT SYSTEM

Because of the limitationsand shortcomings of the two other categories of methods, open-
circuit dust cloud generators have been used by a number of workers, despite the com-
paratively large dust quantities and more-complicated apparatus required.
A simpleversion of the open-circuit principlefor dust dispersion was in fact described
by Weber (1878) more than a century ago as part of a comprehensive investigation of