The Ventilation of Mines; Designed for Use in Schools and Colleges and for Practical Mining Men in Their Study of the Subject (Paperback)

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. 1894 Excerpt: ...of power, in proportion to the yield, than the straight-paddle fan. The main point of difference between these two types, with reference to their working, is the speed: working under the same conditions and delivering the same amount of air, the speed of the Murphy fan is much the greater of the two. The Murphy fan is in some sense a screw-motor, and as such differs essentially from the straight-paddle fan in its working principle. Certain it is that some of the air is carried around by the revolving blades and expelled by virtue of the centrifugal force thus developed: the amount of air thus contributing to a centrifugal pressure will vary according to the greater or less inclination of the blades; the blades could be so wreathed into a spiral as to convert the fan wholly into a screw-machine, when its action would cease to be anything other than mechanical; and its necessary speed would have to be enormous in order to compete as an air-motor. The reasoning that would lead some to adopt a curved blade is to establish as little friction as possible between the radial passage of the air through the fan and the rotary motion of the blades; but we must remember that whatever tends to decrease the rotary motion of the air in the fan will decrease in the same proportion the efficiency or transmitting power of the fan, because the less the air is revolved the less will be the centrifugal force developed. The equations developed in Chapter VI giving the work, yield, and horse-power of straight paddle fans, do not apply to fans having curved or inclined blades. (See Addenda.) Expansion of Casing.--The peripheral expansion of the fan casing is another important point in the economic construction of the fan. The reason for such expansion is simple and obvious. Each s...