Page 50 - Advanced Mine Ventilation

P. 50

Air Flow in Mine Airways 33

Hence

r ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
1:275
Q ¼ 10 ¼ 35; 704 CFM
1
10 10
r ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
1:275
Q ¼ ¼ 29; 154 CFM
2
15 10 10
r ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
1:275
Q ¼ ¼ 25; 249 CFM
3
20 10 10
Q 1 þ Q 2 þ Q 3 slightly exceed 90,000 owing to computational rounding.

2.9 Calculation of Air Horsepower

When the total ventilation rate, Q, and pressure required to circulate this air through
mine airways are known, it is required to determine the size of the prime mover, typi-
cally an electric motor.
If Q is expressed in CFM and P is expressed in inches of water,

5:2PQ
Air horsepower ¼ (2.22)
33; 000
Brake horsepower (bhp) of the motor is equal to air horsepower/h, where h is the
efﬁciency of the electric motor. It can range from 0.8 for an induction motor to almost
1.00 for a synchronous induction motor.
An example:
Ventilation planning of a coal mine shows that 500,000 CFM of air is needed at
14 in. of W.G. Calculate the size of the electric motor to drive the fan. Assume motor
efﬁciency equals 0.8.

500; 000 14 5:2
Brake horsepower ¼ ¼ 1; 378:8
ð0:8Þ33; 000

A 1500 horsepower drive would be a good choice.

Problems

2.1 Calculate the pressure loss in a mine tunnel 3000 ft long and a cross section of 20 8ft if
150,000 CFM of air is needed to keep methane level in compliance. Assume a K of 100.
2.2 Calculate the friction factor, l, for mine airway with K values of 50, 100, and 200. How
does the mine airway compare to a steel pipe?

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