Flow control at different RPMs of the Pulsator

I am working on a device that can be used on different voltage ranges and frequencies. The device is a blower that blows hot air. It has a pulsator, heater and a motor. The motor is a single phase AC. Due to the different frequency specification used in different countries, the motor RPM fluctuates accordingly resulting in varied airflow. I would ideally like to have a constant air flow from the outlet. Also i want a complete mechanical solution for this as the electronics is already been frozen and running in the field. Please let me know if you have any suggestions.