1st, 2d, 3d, : 25 rev. 30 rev. :: 44450: 53340 cubic feet of air. 33000 34 horse-power in the air. 4th, (25)2: (30)2:: 3.62: 5.21+ pressure on piston. 5th, (25): (30): 5.32: 9.19 effective HP. of engine. Assuming the quantity of air discharged for each revolution of the fan to be 1,778 cubic feet, then, at 25 revolutions per minute, the number of cubic feet passing will be 25 x 1778 = 44450 cubic feet. This does not always hold good when the number of revolutions are greatly increased, from the fact that the baffling of the air does not admit the water-gauge to register correctly. "The theoretical quantity of a fan at High Colliery was short, when compared with the measurements, 3,048 cubic feet per minute." The fan gave off 1,584 cubic feet per revolution: therefore the quantity that should have been delivered was 82,368; but from measurement 81,923 cubic feet were delivered at 1.3 inches water-gauge. From the formula was all that the water-gauge called for, the measured quantity was 102.713 cubic feet, making a difference in the second case of 3.048 cubic feet per minute, as stated above. 42. The Guibal Fan belongs to that class of ventila tors called centrifugal, because of the air being thrown off at the tip of the blade tangent to the circumference of the fan. There are probably more of these fans in use than of any other kind: seemingly they have the precedence among the best mining engineers, maybe from their simplicity of construction, or from their nonliability to get out of order. The committee of the North of England Institute, appointed to determine the useful effect of different fans, reported: Thus placing them nearly on a par with a displacement machine in the van. Mr. Howe, in his experiments quoted above, places the useful effect of the Guibal at about sixty-four per cent. The fan is enclosed in a house, the air being discharged through a chimney which gradually expands towards the top. That there is a certain amount of benefit derived from this chimney has been proved by experiment, simply because by its use there is a saving in final velocity by the opportunity it affords the air for expanding. The opening for the discharge of the air is regulated by an adjustable shutter. To find the most advantageous position for this shutter, a series of experiments must be instituted. First, the shutter is fixed at a certain point, then the amount of air ascertained by measurement, as before explained; the shutter is now lowered and the measurement again taken; again the shutter is lowered and the measurement taken. If, however, in the last position, a less amount of useful effect is shown than in the previous position, the shutter has been lowered beyond its most useful position, and must be returned to the second position. The position of the shutter depends upon the speed at which the fan runs. Should the shutter vibrate when the speed is increased, it shows that the fan is not working properly, and that the shutter needs regulating. A modification of the Guibal Fan is used in the anthracite regions, having, instead of the shutter, a spiral casing, commencing at the orifice of discharge (throat), and extending sometimes the whole circumference, according to the notion of the engineer. Whenever this spiral casing is too short, thumping will take place. The discharge is regulated at the throat by a short shutter, or by nailing boards over the orifice. The more recently built fans are made as large as forty-five diameter, and have fire-proof casings of iron. COMPARATIVE ECONOMY OF FURNACE AND FAN VENTILATION. (FROM MR. R. HOWE'S PAper.) 43. Suppose we have two furnace-pits, the first 260 feet deep, the second 655 feet deep. To arrive at the horse-power of furnace-ventilation, we must find the pressure producing it. First, Barometer of the first pit, 30.3"; temperature of downcast, 55°; of upcast, 240°. The pressure, therefore, of the downcast in pounds per square foot, is 1.3253 x 30.3 x 260 20.312 pounds. 459 + 55 The pressure of the upcast is 20.312 14.936 5.376 pounds as the pressure per square foot for ventilating pressure, and this pressure will give us, say, 30,358 cubic feet of air, then the horse-power will be 30358 x 5.376 4.94 horse-power in the air. Second, Mean barometer, 30.6"; temperature of down |