To tell if fans are exhaust or intake, you must identify which side is pulling air in and which side is pushing air out. The fan itself often looks similar in both roles, so you rely on airflow arrows, blade orientation, pressure side vs suction side and how the fan is installed in the ventilation system.
The easiest method is to look for arrows on the fan casing. Many industrial and mining fans have two arrows stamped or labelled on the housing: one for direction of rotation and one for direction of airflow. If the arrow shows air moving from the room or duct towards the outside or a stack, the fan is configured as an exhaust. If the arrow shows air moving from outdoors or a clean plenum into the room or process, it is configured as an intake or supply fan.
If no arrows are visible, you can use the suction and discharge sides of the fan. The side where you feel air being pulled in is the intake side of the fan; the side where air is blowing out is the exhaust side. In a wall-mounted unit, if air is being sucked from the room and blown outside, that fan is working as an exhaust fan for the room. In a duct system, the fan’s suction side is connected to the source of air and the discharge side is connected to where the air needs to go next.
The location in the system also provides a clue. Fans installed at fresh air inlets, drawing outdoor air into an air-handling unit, compressor room, control room or mine intake shaft are acting as intake or supply fans. Fans located at stacks, roof vents, scrubber outlets, dust collectors or mine return airways are almost always exhaust fans, pulling used or contaminated air away from the working area.
In some axial fans you can judge by blade orientation and tilt, but this is less reliable unless you are familiar with the impeller design. For safety reasons, it is better to rely on airflow arrows, measured airflow with a handheld anemometer, smoke tests or simply feeling the air movement with your hand at a safe distance (and never putting fingers near moving blades).
In summary, you tell if fans are exhaust or intake by checking airflow markings, feeling which side is suction vs discharge, and understanding where the fan sits in the ventilation system. The same physical fan model can often serve as either exhaust or intake, depending on how it is installed and connected.
