How can mine fans be made reversible?

Reversible mine fans can be achieved by changing the direction of rotation of disc fans. This mechanical alteration allows the fan to alternately blow air in one direction and then in the opposite direction, thereby effectively controlling the airflow within the mine. When the rotation direction is reversed, the airflow changes, which can be crucial for ventilation management, ensuring fresh air supply or diluting hazardous gases.

Other options do not address the fundamental mechanism of how fan operation is altered. A manual switch could control the fan's operation but wouldn't facilitate the reversible functionality of the fan itself. Altering the fan speed might affect the airflow volume but doesn’t reverse the air direction. Repositioning fans outside the mine does not change their operational characteristics or their ability to reverse airflow, making it ineffective for creating reversible fans.