Contra-rotating axial flow fan
The contra-rotating type which has two separate impellers of opposite hand arranged in series, invariably with separate motors rotating in opposite directions. By this means, swirl from the first impeller is removed by the second impeller. The rotational energy is recovered and converted into useful static pressure. Thus instead of twice the single stage fan pressure being developed, this approaches three times that of a single stage tube axial fan (See Figures 1.84 and 1.85) Pitch angles are generally
Figure 1.84 Contra-rotating axial flow fan
Set at 24 ° first stage pitch angle, 21 ° second stage
Figure 1.85 Contra-rotating axial flow fan— typical characteristic curves
Adjusted so that each impeller takes equal power around the best efficiency point. This automatically secures an output flow free from swirl.
Posted in Fans Ventilation A Practical Guide