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Tests were performed to measure the ventilation parameters, under practical circumstances, required to prevent smoke from a fire within an enclosure (ventilated to the mine return) from reversing through the intake air opening and contaminating the intake air course. The tests were made in a structure simulating an underground fireproof enclosure formed between two masonry stoppings in a crosscut between intake and return air courses. Each stopping contained an opening through which ventilating air flowed from the intake air course to the return air course. Although the initial intent was to determine a single velocity which would control the airflow direction through a ventilated underground enclosure in the event of a fire, preliminary tests showed that the complexity of the airflow system generated by a fire precludes a simple velocity application. Based on the results obtained under test conditions, the smoke from a fire in a ventilated fireproof structure or area located underground adjacent to a return air course and on a separate split of air should travel in the same direction as the normal ventilating current when the following conditions are met: the centerline velocity of the ventilating air current through the intake opening is in excess of approximately 1100 fpm as measured with a 4-inch rotating vane anemometer; the maximum dimensions of the intake opening are 16 inches high and 24 inches wide; the discharge opening dimensions are 16 inches high and 24 inches wide; and the intake and discharge stopping walls are 10 feet or more apart and the enclosed volume is approximately 640 cubic feet or more. The conditions stated are valid for all cases tested; however, in specific cases, lower air velocities may be adequate. The most effective location of openings resulted with the intake at the bottom and the discharge at the top. The air quantity through the structure may be regulated by decreasing the dimensions of the intake opening. (ERA citation 08:048554)