310
8 Secondary Ventilation
where
• L i : Fumes throwback length, i.e. the distance the fumes move backward,
immediately after detonation (m);
10
• K: 25,000;
• F a : Face advance (distance between two consecutive faces) (m);
• D: Rock Density (kg m
−3 );
• A: Face transverse area (m
2 ); and
• M: Mass of explosive (kg).
2. Estimation of the volume of gases generated by the blasting. Generally, Table 8.3
is used, and calculations usually refer to NO 2 due to the fact that it is the most
toxic (its TWA is 3 ppm) and difficult to dilute of the potential pollutants.
3. Estimation of the concentration of gases in the throwback region (Eq. 8.22):
C i
ppm
=
V gas
V i
=
V NO 2
A L i
× 10
6
(8.22)
where
• A: Transverse section of the gallery (m
2 ), and
• V i : Initial volume (m
2 ).
4. Calculation of the concentration of pollutants when the exhaust fumes retreat into
the main airway. At this stage, the gases uniformly fill the length of the gallery
(often termed the drive) but have not yet entered the main airway:
C f
ppm
=
V gas
V
=
V NO 2
A L
× 10
6
where
• V: Volume of the gallery (m
3 ), and
• L: Length of the gallery (drive) (m).
5. Calculation of the time it takes for diluted gases to reach the main airway (t 1 )
(Eq. 8.23):
t 1 =
(L − L i )A
Q
(8.23)
where
• Q: Ventilation airflow rate at the face (m
3 s
−1 ).
10 In the Russian literature L i = 2.44M + 10, is frequently quoted (e.g. Borisov et al. 1976, p. 381).
This expression provides very different values from those found using Eq. 8.21, at least for ANFO.