Page 110 - Advanced Mine Ventilation

Mechanical and Natural Ventilation 91

In a shaft of depth, D, the relation between P1 and P2 can be expressed as:

P 2 D
ln ¼ (6.7)
P 1 RT d
T 1 þT 2
where T d is the average temperature ¼ in the downcast shaft.
2
P 3 D T 3 þT 4
Likewise, lnð Þ¼ where T u ¼ , average temperature on upcast shaft.
P 4 RT u 2
Because P 1 and P 4 are known and often equal, P 2 and P 3 can be determined.
NVP ¼ðP 2 P 1 Þ
6.7.1.2 McElroy Method [3]
It is an empirical formula but surprisingly accurate.

NVP ¼ 0:03=10 F=100 ft in. of water (6.8)
6.7.1.3 Rees Method [4]
It uses only the dry-bulb temperature in the downcast and upcast shafts.

T u T D
NVP ðin. of waterÞ¼ W.D. (6.9)
5:2 T
where T D , T u are the average temperatures in upcast and downcast shafts, respectively
T (with bar on top) is the average surface temperature.

6.7.1.4 Hinsley Method [5]
This is the most recent and refined measure of NVP:

D D
" #

P 1 T 2 RDT D T 3 RDT U
NVP ðin. of waterÞ¼ (6.10)
5:2 T 1 T 4
where DT D is the (T 2 e T 1 ) and DT u ¼ (T 3 e T 4 ). (Refer to Fig. 6.1)
Problems

1. Calculate the head developed by the (a) radial bladed, (b) backward bladed, and (c) forward
bladed fans, given the following data:
Q ¼ 100,000 CFM
n ¼ 800 rpm
D ¼ 8ft
Width at outlet ¼ 3ft

q ¼ 35 for backward and forward bladed fans

q ¼ 90 for radial bladed fans.