Practical Mine Ventilation

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1922 Excerpt: ...with different types of fans. In some it is in excess of the tip speed and in others it is less than the tip speed. Let us assume that the constant is 20 per cent in excess of the circumferential speed, the formula then for the velocity of the air leaving the circumference of the wheel would be: F=(n D +.20 n D) X N In which V is the velocity of the air, D is the diameter of the wheel in feet, N is the number of rotations of the wheel and n is the ratio of circumference of the wheel to its diameter. With the velocity of the air as shown in the table, we can transpose this formula to determine the number of revolutions of the wheel. The formula then becomes: (YlD +.20 n£) The speed of the wheel determined, how can we determine the width? To obtain this dimension, we must know the volumetric capacity of the wheel and how this capacity changes under varying conditions of mine resistance. This can only be determined by observation, as it depends on the curvature of the blades, their number, and the angle of advance or retreat of the blades at the periphery of the wheel. These factors combined give the fan its characteristics of volumetric capacity. So that this feature of the calculation is necessarily empirical. It is different with each type of fan and even in the same type will vary with the mine resistance. The periphery speed determined and the volumetric capacity assumed, how shall we determine the orifice? THE INTAKE ORIFICE OF A FAN The intake orifice should be determined from the speed of the air that is necessary to overcome the resistance or water gage of the mine. In the discussion of the equivalent orifice of the mine and the orifice of passage of the fan, we discovered a certain relation between the two. Let us now discuss the intake orifice ...