Industry

Aerofoil Axial Fans
with pitch adjustable blades

The construction of an axial flow fan is influenced by a number of factors prominent amongst which is the design of the blades and their aerofoil section. This construction increases the performance for the user, with machines up to 88% of performance. However, a brief explanation of the “Aerofoil” may be useful.
An Aerofoil blade section may take a variety of forms but basically it is as shown in figure A. It tapers from a comparatively blunt leading edge to a thin trailing edge. The length of the section is referred to as the chord.
The shape of the mean line A-A, between the upper and lower surfaces is called the camber, and the angle at which the cross-section is set in relation to the direction of rotation is called the angle of incidence or pitch. These three factors influence performance.
Pressure development is also affected by hub diameter. Generally speaking, fans with relatively large hubs develop higher pressures at the same tip speed than those with smaller hubs, though at some sacrifice of air flow. The purchase price tends to be higher with larger hubs for a given duty.
For equal hub and tip diameters and equal speed, pressure development is dependent on total blade area, though a maximum limit is set by mutual interference if the blades overlap too much.
The number of blades is not important; a small number of broad blades is equivalent to a large number of narrow blades having the same total chord and area.
The pitch angle affects volume. A range of volumes for a given speed and diameter can be provided by different pitch angles.
Figure-B illustrates a construction method that enables a variety of volumes adjusting the pitch angle.
This device enables the blades to be set at any angle to give exactly the volume required.