Other types of axial flow fan
Reversal of the direction of rotation of an axial fan reverses the direction in which the airflows. The performance of guide vane fans in reverse is extremely poor, but non-guide vane and contra-rotating fans will deliver 60% to 70% of the forward volume flow when reversed on a given system. The reduction is due to the fact that the aerofoil sections are operating tail-first and have their camber (curvature) in the wrong direction.
A truly reversible impeller can be built from standard parts by rotating every other blade through 180°. Half will then be running nose-first and half tail-first, the volume flow being about 85% of normal in each direction. A more recent innovation has been to design blades with two top surfaces (Figure 1.87) when the performance can be over 92% of normal in each direction.
Figure 1.87 Truly reversible flow blade section
Posted in Fans Ventilation A Practical Guide