Page 137 - Advanced Mine Ventilation

118 Advanced Mine Ventilation

8. The first sample was taken immediately by drawing 10 cc into the pipette slowly. It took
20 s to withdraw the sample.
9. The sample was drained into a preweighed aluminum container and dried in an oven at
105 2 C for 24 h. It was then weighed with an accuracy of 0.25 mg on a Mettler analyt-

ical balance. The weight of the first sample gives the initial dust concentration, and it agreed
well with the calculated concentration.
10. Subsequent samples were withdrawn at precalculated intervals to have experimental data
uniformly spread over the range 0e37 mm.

8.5.2 Calculation of Stoke’s Diameter for a Given Time, t

Settling velocity was earlier calculated in Eq. (8.5) as:

2
2r ðr r Þg
f
v S ¼
9h
where r f is the fluid density.
v S is also equal to h/t, where h is distance shown in Fig. 8.2 and “t” is the time
elapsed since the clock was started.
Hence,

2
2r ðr r Þ g
f
h=t ¼
9h
or

1

9h h 2
r ¼ (8.27)
2ðr r Þgt
f
The r values for each time interval, t, was calculated for plotting on an RR graph.
The weight of each successive sample was expressed as a percentage of the first
sample that gave the cumulative mass (weight) frequencies. The results were plotted
on an RR graph as shown in Fig. 8.4.
Table 8.6 lists the n and k values of 37 mm dust from various coal ranks [12].
It is clear from Table 8.6 that the “characteristic n” is very much rank dependent.
Rank is decided by the volatile content of coal on a dry ash-free (DAF) basis.
Fig. 8.5 shows a plot of 16 data points. This reconfirms that in general, high rank
coals, but particularly anthracite, yields more submicron dust leading to higher “respi-
rable dust dose” and consequently a higher rate of CWP in anthracite mines.
Fig. 8.5 shows a variation of distribution parameter, n, with volatile matter (DAF).