The purpose of covering the top of the fan-shaft in Mine
If the top of the fan shaft were left open, air would flow through the lower resistance path between the top of the fan-shaft and the fan drift. Instead of its intended path through the mine between the top of the non-fan shaft and the fan drift. In such a situation, mine would get only a small quantity of air handled by the fan. In order to avoid that, the top of the fan-shaft is covered thus making airflow through the mine. In case of forcing fan, downcast shaft is the fan-shaft; and in case of suction/exhaust fan, upcast shaft is the fan-shaft. As coal mines are generally ventilated by suction fans, coal mines generally have the top of the upcast shaft covered. Such covering is generally termed as surface air lock. Surface air lock is normally made of fire resistant material.
