TABLE XIX. Modes of applying the compressed air in consumers' engine, ... ... ... CASE 1.-Where air at 45 lbs. pressure is reheated to 320° ... ... ... ... Quantity of air at 45 lbs. pressure (that is, 4 atmospheres absolute) required per ind. H.P. per hour. Cubic feet. 125 4 145'4 188'4 240'6 258'0 331.8 The next important question is the diameter of the pipe required for the transmission of a given volume of air. A good deal of attention has been given to this question, especially by continental engineers, and the general conclusion seems to be that the friction of air is governed by the same rules as those which govern the friction of water; and, in fact, that the friction of all fluids is governed by the following rules : The friction is in direct proportion to the density of the fluid. Thus, water being eight hundred times as heavy as air at atmospheric pressure, the friction at equal velocities of water will be eight hundred times greater than that of air. If, however, the air is compressed to eight atmospheres, thereby increasing its density eightfold, the friction of water will only be one hundred times as great as that of the compressed air at equal velocities. Following the same rule, it is found that the friction of steam is less than that of air, because it is lighter, the density of steam at equal pressures being approximately half that of air. It is also generally accepted that the friction is in direct proportion to the length of the pipe, the friction for 200 feet being twice as great as that for 100 feet. The friction is also in proportion to the internal surface of the pipe. Thus a pipe 6 inches in diameter will have twice as much internal surface as a pipe 3 inches in diameter. On the other hand, the 6-inch pipe has four times the sectional area of the 3-inch pipe; so that the interior surface of the 6-inch pipe is proportionally only half as great as in a 6-inch pipe the loss half that due to friction in a 3-inch pipe. friction the internal surface of the 3-inch pipe. Therefore, other things will be only being equal, due to 45 50 VERTICAL 06 08 FIG. 4836.-Friction of air in pipes. Horizontal lines, ; vertical lines, || . HORIZONTAL LINES REPRESENT LOSS OF PRESSURE IN LBS PER SQUARE INCH FOR EACH FOST IN LENGTH DIAGRAM PIPES OF THE DIAMETERS WRITTEN AGAINST THEM. of Loss of pressure by Friction in compressed Air Pipes Calentated by Thrig's Forfidy and verified by some obspratiqu 400 009 feet per secon Decimals of a pound per. be in the ratio of 22 to 42, or as 4: 16. Thus for a double velocity the friction is quadrupled, and for a treble velocity the friction is ninefold. It is, however, held by some who have investigated the subject, that the friction does not vary as the square. Mr. Edgar C. Thrupp, as already mentioned in p. 241, as the results of experiments on water, has the following formula :Flow of air in wrought-iron pipes : If the head is given (as it usually is) in pounds pressure per square inch, this can be turned into head in feet by multiplying the head in pounds by the number of cubic feet of air of the given pressure required to weigh 144 lbs. (that is, equal to 1 lb. per square inch for 1 square foot). Thus, if the pressure is 46 lbs. per square inch above the atmosphere, then 456 cubic feet at 60° will weigh 144 lbs., and if the given head is o'0002 lb. per square inch, the head in feet = 456 X 0'0002 = o'0912 feet. Example.-Air at 46 lbs. pressure. Loss of pressure per foot run = 0'0002 lb. per square inch. By means of this formula the loss of pressure in air-pipes for air at a pressure of about four atmospheres has been calculated for various velocities, the results being shown in Fig. 4836. The student will readily understand the use of this table. Suppose, for instance, that it were required to transmit the compressed air to a distance of 5000 feet, with a loss of pressure of 5 lbs. per square inch to overcome the friction of the pipes. Then the required size of the pipe will be found on the line opposite the decimal figure o‘001, which means that the friction per foot of pipe is part of a pound, and therefore for 5000 feet will be 5 lbs. If the quantity of air is sufficient for say 50 H.P., using 300 cubic feet per H.P. per hour, or 5 cubic feet per H.P. per minute, or a total of 250 cubic feet per minute for the 50 H.P., or rather more than 4 cubic feet per second, it will be found, on reference to the figure, that a 5-inch pipe will take the air at a velocity of about 32 feet per second with the given head, and, the area of this pipe being rather less than of a square foot, the volume delivered will be rather more than 4 cubic feet per second. This table may be used for air at greater or less densities by altering the right-hand column of figures in proportion. It may also be used for steam by reducing the figures in the right-hand column one half. A further allowance, however, must be made for bends, as per the following table, which shows the loss in pressure for each right-angle bend for air at 46 lbs. pressure :TABLE XX.-ADDITIONAL LOSS OF PRESSURE, IN DECIMALS OF A POUND, IN BENDS AND KNEES OF 90° DUE ΤΟ CHANGE OF DIRECTION. From experiments recently made by the author, he is inclined to doubt whether the friction in smooth, straight tubes increases so much with increase of velocity as is generally supposed. Nevertheless, the usually accepted rule is supported by a great body of practical experience. Hydraulic Power.-In the same way as power is transmitted by pumping air, so it may be transmitted by pumping water. Water, however, is generally used at very high pressures, from 600 to 1000 lbs. pressure per square inch, 800 lbs. being a common pressure. This system is largely adopted on surface works, and to some extent in mines. A Hydraulic pumps in the dip-workings of a mine may be driven by the pressure from the rising main at the pumping-shaft. Water is not so generally convenient for a mine as compressed air. Return-pipes are usually necessary for the exhaust-water. leakage may cause expense; and great care is required in its use, because of the great momentum of the water moving in the pipes, which causes it to burst the pipes if a valve is suddenly closed, unless there are sufficient safety-valves. The great strength required for the pipes also makes it expensive, except in the case where small pipes, say not exceeding 3 inches in diameter, suffice. In these small pipes there is generally a great margin of strength, which is utilized in the case of hydraulic pressure. The reason why high pressure is used is because water is incompressible, and therefore its density is not increased by increase of pressure, and the friction is generally believed to be proportional to the density. So that there is no more friction in water at 1000 lbs. pressure than at 10 lbs. pressure, whilst the power transmitted by a given weight is, at the higher pressure, one hundred times as great as at the lower pressure. When the transmission of water-power is direct from the generating engine to the motor, say an underground pump, the loss of power may be moderate, say 15 per cent. for the generating engine, and 10 per cent. friction in pipes, leaving 75 per cent. to be utilized by the motor. When, however, the power is used for winding and hauling and various machines, there is great loss, because the pressure of the water is constant, and the power required variable, so that probably only about 25 per cent. is utilized. Electric Transmission.—Within the last fifteen years the use of electricity has been applied on a considerable scale for the transmission of power in mines. The use of this wonderful agent is one of the greatest triumphs of scientific engineering. Some instances may be given of what has been actually accomplished. At the St. John's Colliery, Normanton, two coupled engines on the surface, having 22-inch cylinders and 4-feet stroke, running at fifty revolutions per minute, by means of gearing, belts, and pulleys, give movement to three series-wound Immisch dynamos (see Fig. 484), each about 50 H.P. and capable of giving a current of 60 ampères at 600 volts, and also one small compound dynamo giving a current at 155 volts. The conductors |