The last dryer we set up simply had a control loop that used the signal from the anemometers to the VFD for the fan. The power source was 60 Hz so the full open speed was 60 Hz. We did not care what the rotational speed of the fan was (obviously the fan was sized properly, well almost the motor was actually oversized...).
Just tune the air speed reading to the frequency output of the VFD. Worked like a champ.
Edit to add - Over time the actual air flow from the rotational speed of the fan can change due to debris build up or loading of the dryer, etc., but if you use the actual air speed at the product location in the dryer then changes in the efficiency of the fan doesn't matter.