Why are centrifugal air compressors built with backward curving blades? 
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1$\begingroup$ Because of the velocity triangles. $\endgroup$– Solar MikeCommented Oct 18, 2020 at 15:06
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$\begingroup$ Helps push the air outwards to the maximum diameter? $\endgroup$– Jonathan R SwiftCommented Oct 18, 2020 at 15:13
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$\begingroup$ To maximize the tangential velocity component of the impeller discharge, thus delivering more work to the flow. $\endgroup$– AlgoCommented Oct 18, 2020 at 22:29
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2 Answers
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As the air moves from the center out it accelerates in a pattern similar to a hurricane. the velocity vector angle changes to reduce the rotational whirlpool and angular momentum component of the airflow but add to its pressure and centripetal momentum.
The blade angle of attack to airstream follows air velocity vector more or less under 14 degrees to give an optimal lift to the stream and not create local turbulence.
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They aren't universally.
Centrifugal fans use all sorts of blade configurations - forward, backward, radial, curved (either forward of backward) and straight (also prismatic or aerofoil). Different blade configurations give different fan characteristics, some relating to how effective it is as a fan (pressure, flow rate) and some relating to things like how tolerant it is of dust in the gas stream, how strong it is, how it reacts to fluctuating pressures.
For some initial pointers: https://www.sciencedirect.com/topics/engineering/centrifugal-fan
