Page 109 - Advanced Mine Ventilation

90 Advanced Mine Ventilation

where B is the barometric pressure in inches of mercury and ‘e’ is the water vapor
pressure.
Let the depth of the shafts be D, in feet.
Pressure at the downcast shaft bottom caused by the air column ¼ D(W 1 þ W 2 )/2.
Pressure at the upcast shaft bottom, likewise, is ¼ D(W 3 þ W 4 )/2.
Then NVP (natural ventilation pressure) ¼

W 1 þ W 2 W 3 þ W 4
D D
2 2
D 2
or ððW 1 þ W 2 Þ ðW 3 þ W 4 ÞÞ lb ft
2
D
¼ ðW 1 þ W 2 W 3 W 4 Þ=5:2 in. of water.
2
An example:
Calculate the NVP for the following conditions:
3
D ¼ 3000 ft; W 1 ¼ W 2 ¼ 0.075 lb/ft t

t 3 ¼ t 4 ¼ 90 F; e ¼ 0.5 in. of Hg.
B ¼ 30 in. of Hg.
Here, W 1 ¼ W 2 and W 3 ¼ W 4 , hence
NVP ¼ D(W 1 eW 3 )/5.2

1:3253ð30 0:378ð0:5ÞÞ
W ¼
460 þ 90
39:51 3
¼ ¼ 0:0718 lb ft
550
hence NVP ¼ 3000 (0.075e0.0718)/5.2 ¼ 1.85 in. of water.
The net NVP will always be less than the calculated NVP because of pressure losses
in the downcast and upcast shafts. If the mine workings are inclined and they trend
downward, additional NVP is created that adds to the shaft NVP. On the other
hand, if mine workings trend upward, the NVP becomes negative and works in oppo-
sition to the shaft NVP.

6.7.1 Historical Review of Natural Ventilation Pressure
Calculations
6.7.1.1 Weeks Method

The earliest derivation of NVP was by Weeks [2]. The pressures P 2 and P 3 (Fig. 6.6)
were calculated using the basic gas laws, Eq. (1.3).