Which of the following are common causes of recirculation in an auxiliary ventilation fan?

• A. Baffle wrongly set; fan velocity too high; fan placed too close (<10 m) to the cross-cut; door or stopping open.
• B. Low ambient temperature.
• C. Humidity too high.
• D. Electrical outage.

Answer: (A)

Recirculation occurs when air that has already left the face and is on the return path gets drawn back into the intake, creating a looping flow instead of delivering fresh air to the workings. The factors listed in the same setup describe practical ways to create that short-circuit path. If the baffle is wrongly set, the flow is directed poorly and can loop back toward the intake. Running the fan at too high a velocity adds momentum that helps air slip back into the intake area or around obstacles, increasing the chance of recirculation. Placing the fan too close to a cross-cut means exhaust air can rapidly reach the intake path, promoting mixing and re-entry of exhausted air. If doors or stoppings are open, air can bypass the intended routing and mix back into the intake, completing the recirculation loop. The other options don’t inherently create that short-circuit path—low ambient temperature, high humidity, or an electrical outage may affect conditions or stop the fan, but they don’t by themselves cause recirculation.