Page 102 - Advanced Mine Ventilation

Mechanical and Natural Ventilation 83

Hence,

Q 50; 000
¼ V r ¼ ¼ 16:58 ft=s.
A 60 50:26

Using Eq. (6.3) for radial bladed fan,

2
0:075 ð209:4Þ
H ¼ ¼ 19:6 in.
5:2 32:2
Using Eq. (6.4) for backward bladed fan,

0:075ð209:4Þð209:4 50:26 1Þ
H ¼ ¼ 14:9 in.
5:2 32:2
Using Eq. (6.5) for forward bladed fans,

0:075ð209:4Þð209:4 þ 50:26 1Þ
H ¼ ¼ 24:3 in.
5:2 32:2
The theoretical head, H, is never realized in practice because some energy is use-
lessly spent in overcoming the following conditions.

6.1.3.1 Frictional and Shock Losses
The fan provides a narrow path for air flow, and hence the velocities are higher. Conse-
quently, some of the pressure generated is consumed in overcoming these frictional
and shock losses.

6.1.3.2 Conversion Losses

A considerable part of the theoretical head developed by the fan is in the form of
velocity pressure in the air leaving the blade tips. This must be converted to static pres-
sure, otherwise it will be lost. This is done by letting the air go through a “volute cas-
ing” of uniform cross-sectional area. The volute ends up in an “evasée,” which has a
gradually increasing cross-sectional area. As the air velocity decreases, the velocity
pressure is converted to static pressure. The conversion is, however, incomplete
because the size of the evasée has to be practically limited and air will leave it with
some velocity pressure unconverted to static head.

6.1.3.3 Recirculation

There is a tendency for air at the blade tip to flow back to the center and get recircu-
lated. It mostly happens when the air quantity is much lower than the optimum quantity
for the fan. The energy required to maintain these eddy currents of air reduces the theo-
retical head and is mostly dissipated as heat.