5.3 Natural Ventilation
179
Turning the Main Fan Off and On
The natural draught can be estimated from measurements taken in the main fan drift
with the main fan off and operating. Thus, when the fan is on and the surface airlock,
also termed shaft cover is closed, the resistance is:
R =
P v + P n
Q 2
v
where
• P v : Pressure supplied by the fan, and
• Q v : Flow rate with the fan running.
Afterwards, the main fan is turned off, the fan drift is closed with a damper, and the
shaft cover is open. Then, we have:
R =
P n
Q 2
n
with:
• Q n : Flow rate with the main fan off (flow rate supplied by natural ventilation).
Therefore:
P v + P n
Q 2
v
=
P n
Q 2
n
Finally (Eq. 5.12):
P n = P v
Q
2
n
Q 2
v − Q 2
n
(5.12)
Exercise 5.5 The upcast shaft in a mine registers an airflow rate of 800 m
3 min
−1
when the depression developed by the main fan is 6000 Pa. When the fan stops the
flow in the upcast shaft, it reduces to 250 m
3 min
−1 . Determine the value of the NVP
in this mine.
Solution
If the fan is on, we have:
R =
P v + P n
Q 2
v
=
6000 Pa + P n
800
m 3
min
2
When the fan is stopped: