Page 273 - Dust Explosions in the Process Industries

Generation of Explosible Dust Clouds 24.5

causes of dust explosions and fires in flour mills. The flour was placed on a 100mm diam-
eter sieve of a suitable mesh size, and by vibrating the sieve, a controlled column of falling
dust was created in the region below the sieve. By means of an annulus, placed at the
bottom of the sieve, the diameter of the dust column could be controlled. The measure-
ment of the dust concentration in the column was carried out simply by inserting two
parallel plates, separated by a fixed distance, into the falling dust column, the plates being
perpendicular to the axis of the column. Since the diameter of the column could be
measured and the distance between the plates was known, the volume of the dust cylin-
der trapped between the plates was known; consequently,the dust concentrationwas given
by the amount of flour that settled on to the lower plate, divided by the volume of the
dust cylinder initially trapped between the plates.
Seventy years later, Weber’sidea was adopted by Jones and White (P948), who by siev-
ing the dust into a vertical cylinder, avoided the gradual distortion of the falling dust
column, which in Weber’s case occurred as the dust traveled away from the sieve. The
sieving method for dispersing dust was also adopted by Craven and Foster (1967) as part
of a more refined experimental setup and, on a comparatively large scale, by Palmer and
Tonkin (1968).
Schlapfer (1951) used a conveyor screw to supply the powder at a constant rate at the
bottom of the vertical dust explosion tube, in which air was flowing upward at a con-
stant rate. The bottom part of the tube, where the dust was introduced, was narrow to
ensure high turbulence during dispersion of the powder in the airstream. The upper part
of the tube was considerably wider, and hence more laminarlike flow of the dust cloud
could be obtained.
A similar arrangement was used by Cassel (1964). In this apparatus, to obtain a con-
stant fl~owrate and dust concentration, the dust was dispersed by ajet of gas from a hypo-
dermic needle directed vertically downward toward a horizontal rotating dust layer of
constant thickness. The arrangement also incorporated an electromagnetic vibrator, the
purpose of which was to prevent the powder from depositing on the inner walls of the
apparatus.
Line, Clark, and Rahman (1957) used an apparatus where a turbulent dust cloud was
initially formed by means of jets of oxidizer gas passing through a bed of the dust. This
cloud was then directed downward through a vertical cylindrical tube, under laminar con-
ditions. The combustion chamber, having a considerably larger diameter, was attached
to the tube; consequently, on leaving the tube, the dust cloud formed a continuous wall-
€ree column traveling downward through the combustion chamber.
To investigate the burning velocity of laminar flames of lycopodium, Kaesche-Krischer
and Zelv (1958) fed lycopodium into the lower end of a vertical 2 cm diameter tube, where
it was dispersed into a stationary dust cloud by an upward-moving stream of air. This
arrangement made it possible to obtain stable flames in the concentration range of dust
between 200 and 500 g/m3.Mason and Wilson (1967), who also studied the burning veloc-
ity of stationary flames of lycopodium, described a dispersing arrangement where the
lycopodium was elutriated from a fluidized bed. These workers could obtain stable dust
flames in the concentration range 125-190 g/m3.
Ballal(l983) also supplied the dust to be dispersed to a worm conveyor, from which
it was fed at the desired rate at the top of the apparatus into the controlled downward
airflow. In very fine powders, particle clusters or agglomerates tended to form at the output
of the worm conveyor. The problem could be somewhat reduced by modifying the design