A lot of industrial motors are now variable speed to save electricity. They're generally either induction (AC) motors run on an inverter-based variable frequency drive, or brushless DC motors. In either case, the AC supply current is first rectified to DC, then chopped up as needed to run the motor at the required speed. The rectifier wouldn't care about the frequency of the AC supply (within reason, anyway).
That's a long way of saying that if the recirc fans are variable speed, it's probably just as easy to run them off the frequency-wild 235 VAC.