Page 99 - Dust Explosions in the Process Industries

72 Dust Explosions in the Process Industries

01,
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I
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8 0.3
8 0.3
0
0
G 0.2
n
5 0.1
c
a
0
00 I
0 20 40 60 80 100
ACTUAL DUST CONCENTRATION Ig/dI
Figure 1.75 Optical density of clouds in air of wheat grain dust containing 10% moisture, length
of light path is 150 mm, optical density Dlois defined as
( incident light intensity
loglo light intensity after 150 mm

(From Eckhoff and Fuhre, 1975).

11111 Illill ARRANGEMENT FOR
FLUSHING WINDOWS
BY COMPRESSED AIR

LIGHT EMITTING PHOTO
DIODE [LED) DIODE
Figure 1.76 Light attenuation probe for measurement of concentration of dust clouds, used by
Eckhoff et al. (1985).

for detecting transmitted light. This concept was probably first introduced by Liebman,
Conti, and Cashdollar (1977), with subsequent improvement by Conti, Cashdollar, and
Liebman (1982). The particular probe design in Figure 1.76 was used successfully by
Eckhoff, Fuhre, and Pedersen (1985) to measure concentration distributions of maize
starch in a large-scale (236 m3)silo. The compressed air for flushing the glass windows of
the probe was introduced via the metal tubing constituting the main probe structure.
However, in the case of dust explosions in the silo, the heat from the main explosion
and from afterburns,required extensive thermal insulation of the probes to prevent damage.
A light path length of 30 mm was chosen to cover the explosiblerange of maize starch
in air. The calibration data are shown in Figure 1.77. If this kind of probe is to be used
for continuous monitoring of dust concentrations below the minimum explosive limit,
such as in the range of 10 g/m3,paths considerably longer than 30 mm are required to
make the instrument sufficiently sensitive. Other dust materials and particle sizes and
shapes may also require other path lengths. In general, it is necessary to calibrate light
attenuation probes for each particulate dust and concentration range to be monitored.
The use of dust control in dust extractionsystemsis most likely to be successfulif a small
dust fraction is to be removed from a coarse main product, such as grain dust from grain or