Page 428 - Dust Explosions in the Process Industries

Ignition of Dust Clouds and Dust Deposits 395

Table 5.3 Specific heats and heat conductivities of some combustible materials in solid and pow-
dered form

Cork - 0.074 95 2.5 - I 0.033
Brown coal 1 1.16** I 0.39 I 74 I 1.05 I 0.61 I 0.067
*63 vol% pores in solid.
**33 vol% pores in solid.
***Depending on orientation of fibers.
Source: Data from Selle, 1957.

The heat conductivities in Table 5.3 for the powders, except for aluminum, are very
low, and in fact lower than for air. Selle did not describethe method of measurement and
further analysis of his data is not possible.
However, in more recent years, John and Hensel(l989) developeda hot wire cell allow-
ing more accurate measurement of the heat conductivity of powder and dust deposits.
The cell was a vertical cylinder of diameter about 50 mm and height about 200 mm. The
heat sourcewas a straight, electrically heated resistance wire coinciding with the cell axis
and generating a constant power. The temperature was measured as a function of time
at a point in the powder midway between the hot wire and the cell wall. John and Hensel
used the Fourier-type equation

(5.12)

for calculating the heat conductivity of the powder from two measured temperatures
TI and T2at times tl and t,. Here, ilis the heat conductivity and q is the heat generated
by the hot wire per unit time and wire length. This is a valid approach as long as the
two measured temperatures are within a range where the temperature is a linear func-
tion of the logarithm of time. A set of data from measurements with this cell are given
in Table 5.4.
Faveri et al. (1989) presented a theory for the heat conduction in coal piles, using the
following expression for the heat conductivity 1in a powder, developed for porous
oxides by Ford and Ford (1984):
+
a = asFi- (1-aa,/as)E~r[i(a - llE1 (5.13)
where

a= 3%
us+ ag