Reversible axial fans are having low efficiency ? why?

[Rumananoor]

Please clarify why reversible axial fans are having low efficiency respectively comparing with normal operation fans.

I have bit confusion is it because the impellers have been modified to allow for reversible operation i.e flow 'A' orientation equals flow 'B' orientation. To achieve this efficiency of the impeller is reduced due to 50% of blades being reversed.

 

[KiwiMace]

A fan flowing in one direction can be optimised for that direction. Blades are curved and twisted, you can't do this for both directions, so compromises are made that effect efficiency.

 

[berkshire]

The most efficient reversible flow fans I have seen, use a motor running in one direction only, a variable pitch fan with symmetrical airfoil blades.
B.E.

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