changed volume of fan air controller 46 is actually determining what future air flow demand will need to pass through the diffuser vanes and will cause an apparent curvature change of the diffuser vanes to accept the new flow rate more efficiently diffusers are present in a fan casing to aid in converting velocity head to static pressure or head for diffusers vanes to operate correctly and efficiently the entrance angle of the diffuser vane is designed so that the air leaving the rotor enters the diffuser vane at zero or small angle of attack for a given rotor exit angle and fan speed there is only one flow at which air enters the diffuser vane at optimum angle of attack at off design flow the angle of attack may be large enough to cause the vane to stall thus generating shock looses in the passages between the vanes these shock losses are such that the efficiency of the diffusers is usually less than that of the fan without diffusers mechanically adjusted vanes are known in the art for maintaining efficiency over a wide range of flow conditions but are not altogether satisfactory from the standpoint of efficiency because complicated mechanical linkages necessary to change the vane angles are subject to high maintenance the above discussion of prior art applies to fans operating.