Page 103 - Advanced Mine Ventilation

84 Advanced Mine Ventilation

6.1.3.4 Bearing Friction

The bearing friction causes loss of energy provided to the fan by a prime mover. It has
no impact on the static head developed by the fan. When the fan is working at its
optimum (most efficient) point, the power loss is less than 1%.
6.1.3.5 Fan Efficiency

The work done by a fan is measured by the air horsepower, which is the product of air
2
3
flow rate (ft /min) and total pressure (lb/ft ) divided by 33,000. Air horsepower
divided by horsepower to the fan shaft and multiplied by 100 is the percent efficiency
of the fan.

6.2 Fan Characteristics

Because of many measurable and some immeasurable variables, it is not possible to
mathematically relate the pressure generated by a fan to the different quantities, but
it can be done graphically by running tests on the fan. The pressureevolume curve
is called the “fan characteristic” curve. For a given speed, pressure, efficiency, and
horsepower consumed can be plotted against air quantities. Fig. 6.2 shows a typical
characteristic curve for all three types of centrifugal fans.
In all cases, pressure declines as the air quantity goes up. The efficiency has a peak
in each case, which is the ideal operating point for the fan. The power consumption
continues to increase with increasing volume for radial bladed and forward bladed
fans, but it begins to decline for backward bladed fans after reaching a high point.
This is called a “nonoverloading” characteristic of the fan, and it is very desirable.

6.3 Axial Flow Fans

The idea of a fan where the air flows axially and does not suffer change of direction is
obviously attractive. The early axial flow fans were manufactured in the 1910s using
aircraft propeller blades. Modern aerodynamically designed axial flow fans were
developed soon afterward.

Figure 6.2 Pressure, power, and efficiency versus air quantity.