This is the remains of the fan at the Glyn Pits. In the background is the brick Air Duct that carried the foul air out of the upcast shaft to the brick-lined hole you see in the side of the Air Duct. From here it was drawn out by the fan and exhausted to atmosphere.  The foul air passed out of the mine through a tunnel exiting the shaft and linking to the Air duct six feet below the surface.  The Air Duct rose out of the ground and along the surface for around about thirty feet to just beyond the fan outlet. At this point it was secured and sealed by a steel door, thus making the round hole in the side of the Air Duct the only place for the foul air and gas to exit.

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Below is a photograph at the end of the Fan Drift minus its steel door. The round brick outlet is about  9 feet inside on the right.