Page 106 - Advanced Mine Ventilation

Mechanical and Natural Ventilation 87

Table 6.1 Fan Law Summary (Air Density Remains Constant)

Speed, n Diameter, D
Performance Criteria D, Constant Dn, Speed Constant
Quantity, Q Q is proportional to n Q is proportional to D 2
Pressure, H H is proportional to n 2 H does not vary
Power, P P is proportional to n 3 P is proportional to D 2

6.5 Fan Testing

It is necessary to test all main fans in a mine after installation and at intervals thereafter
to make sure they are working efficiently. Changing mine resistance also makes it
often necessary to readjust the fan blade angles. The quantities that need to be
measured and the procedures are described in the following section.

6.5.1 Air Quantity
The area behind the fan is divided into 2 2 ft by wires, and an anemometer is kept in
each tiny square for 10 s to get an average air velocity. If the air velocity is very high, a
pitot tube can be used. Air quantity is the product of average velocity and the area.

6.5.2 Total and Static Pressures

Each fan house has a static pressure reading gauge. Several readings are taken and
averaged over the test period. The velocity head is calculated from the average velocity
obtained with anemometers. The total pressure is the sum of static pressure and veloc-
ity pressure.

6.5.3 Air Density
If the pitot tube is used for velocity measurement, air density must be calculated from
barometric pressure and wet- and dry-bulb temperatures as discussed in Chapter 1.

6.5.4 Fan Speed

Typically, the fan rpm is measured with a revolution counter and stop watch, but a
tachometer can also be used. Several readings should be taken to get a good average.
If the fan is run by a synchronous electric motor, the fan speed is the same as the motor
speed.