There are several reliable ways to tell what direction a fan blows, and they apply to both industrial fans and mining ventilation fans. You should always confirm airflow direction before installing or starting a fan in a duct, tunnel or mine airway, because reversed flow can severely reduce ventilation performance.
The simplest method is to look for arrows on the fan casing. Most modern axial and centrifugal fans have small cast, stamped or sticker arrows showing both the direction of rotation and the direction of airflow. One arrow is usually on the motor or hub end, indicating rotation, and another is on the housing, indicating the path of air from inlet to discharge. On site, cleaning dust or paint from the casing often reveals these arrows.
If no arrows are visible, you can use the geometry of the fan as a guide. For an axial fan, air almost always flows from the side where you can clearly see the hub and motor towards the guard or discharge cone, following the convex side of the blades. The concave (cupped) side of the blade faces upstream, and the convex side faces downstream. In a wall fan, this usually means air flows from the motor side to the grille side, but you should still verify with other clues.
For a centrifugal fan, the airflow direction is easier to see from the scroll housing. Air enters at the open eye of the impeller (the round inlet) and exits at the scroll outlet, where the discharge duct connects. The outlet angle (left-hand or right-hand, up, down or horizontal) tells you exactly where the fan is blowing. If you know the direction of rotation, you can also see which way the blades scoop and throw air into the scroll.
Another practical method is a brief test run. With all safety precautions in place, run the fan for a short time and use light materials such as tissue, a strip of plastic or smoke to see where air is being drawn in and where it is discharged. Never put your hands or loose clothing near the moving impeller. In mines and large ducts, you can use an anemometer or even light dust to see flow direction along the airway.
Finally, remember that rotation direction alone does not guarantee airflow direction if the fan is wired incorrectly or blades have been re-installed. You must always think in terms of inlet and discharge. Confirming fan blow direction with arrows, blade shape, housing geometry and a safe test run ensures that industrial and mining ventilation systems perform as designed.
