266 Great Blowing Engine erected at the Dowlais Iron Works. pass freely in and out. The cistern is filled with water about 5 feet deep, when the engine is at rest; and when it is set to work, the air which is forced into the inverted chest displaces the water therefrom until it stands only about 2 feet deep in the space beneath the inverted chest; the water so displaced rises in the space around the outside of the cistern, until it is about 9 feet deep therein; and then the surface of the water within the inverted chest will be depressed by the condensed air, to about 7 feet below the level of the surface of the water in the cistern at the outside of the chest. The pressure occasioned by a column of 7 feet of water is very nearly 3 pounds per square inch, and the compressed air within the air-chest must be thus much more elastic than the atmospheric air; or assuming the absolute elasticity of the atmospheric air to be 14·7 pounds per square inch, that of the compressed air will be 177 pounds per square inch; and the volume of the compressed will be 147 of the volume of the same absolute quantity of common air by weight. The area of the surface of the water in the inverted chest is (36 ft. x 14 ft. =) 504 square feet, and the area of the surface of the water which is raised in the space between the walls of the cistern and the outside of the chest, is nearly the same; consequently, the surface of the water at the outside of the inverted chest will be raised as much as the surface of the water within it is depressed by the compressed air, and vice versa. The area of the blowing cylinder (84 inches diameter) is 38-48 square feet, and the horizontal area of the inverted chest is 131 times the area of the cylinder. When the water is displaced from the inverted chest as above stated, so as to raise a column of water 7 feet high, that part which contains the compressed air above the water is 7 feet high, and its capacity is 3780 cubic feet, that is, about 12:3 times the capacity of the blowing cylinder. A blowing engine of the above dimensions is capable of blowing three furnaces for smelting iron from the ore, if it is worked at its full speed of 16 strokes per minute, which may be done when the ore is of such a quality that it can be reduced with a moderate blast; but when the operation of the furnaces requires the compression of the air to be 3 pounds per square inch, the engine usually makes 12 strokes per minute, and blows two furnaces. The piston is adapted to make a stroke of 8 feet, but it usually works with a stroke of 73 feet; and the blowing cylinder being 38 48 square feet in area, it takes in 298.2 cubic feet of common air at every half stroke. The vacant spaces left at the top and bottom of the cylinder into which the piston does not enter, are each about 12 cubic feet capacity, and when the air is compressed, an additional quantity of (ths of 12 =) 2·45 cubic feet of common air will be crowded into those spaces, so as to escape being displaced from the cylinder by the piston. Hence the quantity of common air discharged by the blowing cylinder would be (298.2 ·2·45 =) 295·75 cubic feet at each half stroke, from which, if we deduct th to allow for the loss by leakage through the piston and valves, it will be about 286 cubic feet actually discharged at each half stroke, or × 24 = 6864 cubic feet of common air blown per minute into the two furnaces. As 13-28 cubic feet of common air weigh 1 pound, the actual weight of 6864 cubic feet of common air is 517 pounds of air blown per minute by this engine. The volume of this air is reduced by the compression to (144ths of 6846) 5700 cubic feet per minute of air having an elasticity of 177 pounds per square inch. It is usual to blow the air into the great smelting furnaces through nose pipes of about 2 inches diameter; each furnace has two such pipes, and the area of the four apertures is 19.64 square inches. In many iron works a small furnace called a cupola for remelting cast-iron, is also blown during the daytime with two nose pipes of 14 inches diameter = 2:45 square inches area. We may conclude that 5700 cubic feet of compressed air is forced per minute through 22-09 square inches of aperture, and in that case the velocity with which the condensed air would issue from the orifices would be about 37.200 feet per minute, or 620 feet per second. The resistance or load on the steam piston is at the rate of 9.2 pounds per square inch, when the resistance to the blowing piston is 3 lbs. per square inch. The mechanical power exerted for this purpose may be thus computed. The motion of the piston is 12 double strokes per minute of 73 feet long = 186 feet motion per minute. The area of the blowing cylinder is 5542 square inches x 3 pounds pressure per square inch = 16.625 lbs. for the resistance to the motion when the blowing piston is driving the compressed air through the conduit-pipe; but when the piston first begins to act upon the air which is taken into the cylinder, that air not being compressed, it can only resist the piston by degrees, as it becomes compressed and increases in elasticity; the air does not acquire the elasticity of 17-7 lbs. per square inch, until the piston has moved through a space of (2,0% of 84 ft.=) 1·4 feet; and during this motion the resistance to the motion of the piston increases regularly from nothing at the commencement to 3 lbs. per square inch at the conclusion. After the piston has moved thus far, the air will become sufficiently elastic to open the forcing valves, and pass away through the conduit-pipe to the furnaces. If we assume for an average, that the resistance during the compression of the air is only half as much as it is afterwards, then the resistance will be equal to (7·75 ft. + 6·35 ft. =) 14·1 feet at each stroke, or x 12 = 169.2 feet per minute × 16-625 lbs. = 2,813,000 pounds raised one foot per minute 33,000 = 85-2 horse power is exerted by the engine when it is blowing two smelting furnaces and a cupola.* DOWLAIS BLOWING ENGINE, 1851. The largest blowing engine hitherto erected is that constructed at Dowlais Iron Works in 1851. The blowing cylinder is 144 inches diameter with a stroke of 12 feet, making twenty double strokes per minute; the pressure of the blast being 34 lbs. per square inch. The discharge pipe is 5 feet diameter and about 140 yards long, thus answering the purpose of a regulator. The area of the entrance air valves is 56 square feet, and of the delivery air valves, 16 square feet. The quantity of air dis charged at the above pressure is about 44,000 cubic feet per minute. The steam cylinder is 55 inches diameter, and has a stroke of 13 feet, with a steam pressure of 60 lbs. per square inch, and working up to 650 horse power. The steam is cut off when the piston has made about one-third of its stroke by means of a common gridiron valve, near the back of the slide valve; there is also on one side of the nozzle a small separate slide valve for moving the engine by hand when starting. The cylinder ports are 24 inches wide by 5 inches long, and the slide valve has a stroke of 11 inches with a 4-inch lap. The engine is non-condensing, and the steam is discharged into a cylindrical heating tank, 7 feet diameter and 36 feet long, containing the feed water from which the boilers are supplied. Under the steam cylinder there are about 75 tons of cast iron framing, and 10,000 cubic feet of limestone walling in large blocks, some of them weighing several tons each. The beam is cast in two parts of about 16 tons each, the total weight upon the beam gudgeons being 44 tons; it is 40 feet 1 inch long from outside centre to outside centre, and is connected to the crank on the fly-wheel shaft by an oaken connecting rod, strengthened from end to end by wrought iron straps. The beam is supported by a wall across the house 7 feet thick, built of dressed limestone blocks, to which the pedestals are fastened down by twelve screw bolts of 3 inches diameter. The fly-wheel is 22 feet diameter, and weighs about 35 tons. Eight Cornish boilers are employed to supply the steam, each 42 feet long and 7 feet diameter, made of inches best Staffordshire plates, and having from end to end a single 4-feet tube, in which is the fire-grate, 9 feet long. For some time this engine supplied blast to 8 furnaces of large size, varying from 16 feet to 18 feet across the boshes; it is now blowing, with three other engines of small dimensions, 12 furnaces, some of which make upwards of 235 tons of good forge pig iron per week, the weekly make of the 12 furnaces being about 2,000 tons of forge pig iron. With the exception of the cylinders, made and fitted at the Perran Foundry, Truro, this engine and boilers were made at the Dowlais Iron Works, and erected according to the design, and under the superintendence, of Mr. Samuel Truran, the Company's engineer. Mr. Bourne many years ago proposed to apply his high speed engines, noticed in previous editions of this work, to the blowing of air for furnaces, and he contrived an annular valve, suitable either for air pumps or water pumps, which was to be moved by suitable Farey on the Steam Engine. Blowing Engines made with Slide Valves for the Admission and Emission of Air. mechanism. This valve encircled the pump barrel, and it might be made to move up or down on the barrel, round and back again, or round and round continuously. Some time afterwards this valve was patented by Mr. Slate, and it has been applied successfully in several cases, and among others to the blast engines constructed by Boulton and Watt for the Indian Iron Company, and of which a description was given by Mr. E. A. Cowper in 1856 to the Institution of Mechanical Engineers. These engines are six in number, two pairs of them being intended to blow air at 2 lbs. per square inch as a maximum pressure, and the other pair to blow air at 4 lbs. per square inch as a maximum pressure. The general form and construction of the engine is that of a "Pedestal or Table Engine;" the air-cylinder stands on a low pedestal, and itself forms the pedestal or table on which the steamcylinder stands. The foundation-plate is 6 feet square, and carries a wrought-iron crank-shaft in four plummer-blocks, having two light fly-wheels, one on each end of the shaft, and the two eccentrics for driving the air-valve, one on each side of the air-cylinder, and the eccentric for driving the steam-valve in the centre. The steam-piston has one piston-rod fixed in a short cross-head at the top, and this cross-head has two other piston-rods for driving the air-piston, which pass down outside the steam-cylinder through stuffing-boxes in the cover of the air-cylinder, and are attached to the air-piston. The long cross-head taking the connecting-rods to the cranks, is attached to the short cross-head by a pin, so as to allow a little freedom in case of unequal wear; the guides are attached to the steam-cylinder cover. The air-valve is made under Mr. Archibald Slate's patent, and is a ring or crown-valve entirely enclosing the air-cylinder, and is not self-acting by the pressure of the air in any way, but is moved by the pair of eccentrics at the proper times, so as to give ample passage for the air to move with the greatest freedom, and the valve has such a proportion of lap as to cause the air to be compressed up to the working pressure before it is delivered, thus giving the engine no more work to do than is necessary. The openings or passages for the air from the air-cylinder to the valve are extremely short, and the bars between the openings are made inclined, so as to cause a regular wear on the brass packingrings which form the rubbing face of the valve. The body of the air-valve is made of thin sheet-iron, neatly curved to two turned cast-iron rings, to which it is well secured by a great number of small bolts; these rings are bored out inside to receive the brass packingrings before mentioned, which are secured in their places by bolts. There are no springs to the brass packing-rings, but they are bored out to be a perfect fit to the outside of the air-cylinder, and are then cut into eight pieces; and should any wear take place they can be at once adjusted, by introducing a thin sheet of paper behind them and screwing them fast in their places again. It should, however, be remarked that this valve is. in under totally different circumstances from any that have hitherto been made, as it is perfectly in balance, or rather it is suspended perfectly free, and slides up and down a turned cylindrical surface, and therefore there is no tendency or power to cause wear under any variation in the pressure of the air. The mode in which the two eccentrics drive the air-valve is by means of a "Gimbal Ring;" that is to say, there is a wrought-iron ring encircling the air-valve, and attached to it by two pins opposite each other, and the eccentric-rods are attached to the ring at two other points at right angles with the first: thus the air-valve is perfectly free. The air-cylinder is 30 inches diameter and 2 feet 6 inches stroke, and the piston makes 80 strokes per minute. The air-piston is packed with hemp packing, and has a ring to screw it down; the screws are so arranged that they can be got at by simply unscrewing small plugs in the cylinder-cover, when a socket-spanner can be introduced to screw the ring down. The air passes into the air-cylinder beyond the end of the valve, first at one end and then at the other, and is delivered into the hollow part of the valve, from which it escapes through two light copper branch pipes placed opposite each other, and having turned joints, fitting turned collars, fixed on the valve. The other ends of the pipes rest on a small surface or shelf prepared for them, and on which they slide backwards and forwards about 1-8th inch; these ends of the pipes are curved in the same manner as the other ends, so that the faces are in one plane, and the air-main has the faces 267 of its branches surfaced to receive them; thus the air is taken equally from each side of the air-valve. The steam-valve has considerable lap, and is so proportioned as to cut off the steam just after the half stroke and have a very free exhaust. The boilers are on the Cornish plan, are fitted to be chiefly used with wood as fuel, and the furnaces are made proportionately large for this purpose. The boilers are fed by a donkey-engine, entirely independent of the blast-engines, so that they are complete in themselves, and there is no fear of getting short of water whilst the blastengines stand for "tapping," at which time, indeed, the boiler should always be fed, if only to keep the steam down a little. The engines having to be transported some distance up the country, a limit of weight was given, viz., one ton for any one part of the engine; and, in accordance with this limitation, the total weight of a pair of these engines is only 11 tons as compared with 25 tons, the weight of an ordinary blast-engine of equal power; and the weight of the heaviest single piece of an ordinary engine is 4 tons as compared with 1 ton, the weight of the heaviest piece in the new engines. The pair of engines are arranged to blow 3600 cubic feet per minute, and are speeded to 80 revolutions per minute, which, with 2 feet 6 inches stroke, makes 400 feet per minute. In Hackworth's direct-action blowing engine the cylinder is moved instead of the annular valve, which remains stationary. ON CENTRIFUGAL PUMPS FOR WATER. In the early editions of this work the advantages of the centrifugal pump for many purposes were pointed out, and the speedy introduction of that species of pump was predicted. Shortly afterwards, namely, in 1848, Mr. Appold exhibited a model of a rotatory pump as a convenient one for draining purposes, and made experiments on it with 6, 24, and 48 arms or vanes. A pump of this description was shown at the Great Exhibition of 1851, and experimented upon by the jury. In this pump the fan revolving vertically was 1 ft. diameter, and 3 in. wide, having an opening one half the total diameter in the centre of each side for the admission of the water, and a central division plate extending to the circumference, to give a direction to the two streams of water, and convenient for fixing on the shaft: the 6 arms curved backwards, terminating nearly tangentially to the circumference. The revolving fan was fixed on the end of the horizontal driving shaft, passing through a stuffing-box in the side of the casing, and it worked between two circular cheeks, running close without actually touching, by which the outer revolving surfaces were shielded from the water, but a free ingress was allowed for the water, and a large space left all round the periphery of the fan facilitated the discharge of the water. In the experiments instituted by the jury, the power employed was measured with great care by means of Morin's dynamometer, and the following results were obtained : Height of Lift. Appold's Centrifugal Pump. Discharge per Revolutions per Minute. Velocity of Per centage of effect to Power. Minute. WITH STRAIGHT RADIAL ARMS. ft. 18.0 gallons. 720 ft. per minute. 24 WITH STRAIGHT INCLINED ARMS. 18.0 268 Centrifugal Pumps for Water by Appold, Whitelaw, Bessemer, Gwynne, and Thomson. pump is A large pump constructed on this plan, erected at Whittlesea Mere, for the purpose of draining, was reported, in July 1852, to have been then working for nearly a year with complete success. This 4 ft. diameter, with an average velocity of 90 revolutions, or 1250 ft. per minute, and is driven by a double cylinder steam engine, with steam 40 lbs. per inch, and vacuum 134 lbs. per inch; it raises about 15,000 gallons of water per minute, an average height of 4 or 5 ft. The cost of the engine and pump was about 1,600l. The following experiments were tried to ascertain the per-centage of effect obtained from the pump; the power employed being measured by taking indicator figures from the engine, deducting in each case the power that was indicated when the engine was working at the same speed without the pump, which was found to take 10-6-horse power. The quantity of water discharged was measured by calculating the overflow from an opening 6 ft. wide in each case. The centrifugal pump is by no means a modern machine, though efficient pumps upon this principle are only of recent introduction. Demour, in 1732, proposed to raise water by putting into rapid rotation pipes spreading at the top like a V. The inverted Barker's mill, or a revolving T, was a very early suggestion, and the best form of this species of pump was investigated by Euler. Jorge, in 1816, submitted to the French Academy of Sciences a species of centrifugal pump of the same construction as the ordinary fans for blowing air. A similar species of pump, under the title of the Massachusetts pump, was introduced in America, and was re-invented by Andrews and various other persons. In 1831, Blake introduced a disc pump, with vanes on the one side of the disc, while the other side was smooth. The vanes gave rotatory motion to the entering water, which drove it to the periphery of the case, where it overflowed over the edge of the disc to the smooth side, and through a pipe attached to the case on that side of the disc the water escaped. Whitelaw, in 1841, and Bessemer in 1845, introduced centrifugal pumps of various forms; but it was Appold's pump, with curved vanes, which attracted so much attention at the Great Exhibition in 1851, and gave such satisfactory results that first brought the centrifugal pump into notice and favour. In Gwynne's centrifugal pump, the water is drawn into a chamber in which revolve two discs, with a single arm between them. The arm is called an impeller, as it communicates the rotatory motion to the water. It is narrowest at the periphery of the discs, and it becomes gradually broader until its edge intersects the opening in the centre of one of the discs through which the water is drawn. The discs are made convergent at the outer edge to maintain the same area at all parts for the flow of the water. The escape-pipe from the water passes off as a tangent from the case. In D. Thomson's centrifugal pump, the blades, which are curved as in Appold's pump, are continued in a screw form round the axis, so as to put the water into gradual motion as it enters the case. In Professor James Thomson's centrifugal pump, the vanes are curved as in Appold's pump, but a large exterior annulus is provided, round which the water may freely circulate; and by the addition of this chamber, the efficiency of the pump is said to be increased. Professor Thomson's description of this pump is as follows: "In centrifugul pumps, when doing actual work in raising water or forcing it against a pressure, the water necessarily has a considerable tangential velocity on leaving the circumference of the wheel. This velocity in wheels in which the vanes or blades are straight and radial, is the same as that of the circumference of the wheel, in others, in which the vanes are curved backwards, it is somewhat less; but in all cases it is so great that the water on leaving the wheel carries away, in its energy of motion, a large and important part of the work applied to the wheel by the steam-engine or other prime mover. This energy of motion in centrifugal pumps and centrifugal fans, as ordinarily constructed, is mainly consumed in friction, and eddies in the discharge-pipe, which receives the water or air directly from the circumference of the wheel. In the improved centrifugal pump there is provided, around the circumference of the wheel, an exterior chamber, in which the water continues some time revolving in consequence of the rotatory motion it has on leaving the wheel. This chamber is called the exterior whirlpool chamber, and is ordinarily about double the size of the wheel in diameter. The water revolving in this chamber is in the same condition as water revolving in the whirlpool, which I have called the Whirlpool of Equal Energies, or Free Mobility. In this whirlpool (when some slightly modifying causes, such as the fluid friction, are neglected) the velocity of the water is inversely proportional to its distance from the centre, and the sum of the accumulated work or energy of motion and the work in the condition of water pressure of two equal masses of water in the same horizontal plane, is the same, so that when the velocity diminishes, the pressure increases; the energy of motion given up in the diminution of velocity being converted into water pressure. It is by this conversion of energy of motion into water pressure, through the medium of the exterior whirlpool, that a decided increase in the working efficiency of the centrifugal pump is attained; the work contained in the rapid motion of the water leaving the wheel, which in centrifugal pumps as ordinarily constructed is wasted, being in the improved pump usefully employed in increasing the pumping power of the machine." ON CENTRIFUGAL PUMPS FOR AIR. Centrifugal pumps for air are usually called fans, and the merit of their practical introduction belongs to Messrs. Carmichael of Dundee. Air, like water, has a certain height or head corresponding to every different pressure, and the velocity of efflux will be equal to that of a heavy body falling from the height of the head or column necessary to produce the pressure, or the velocity of rotation in a fan must be such that the centrifugal force of the revolving air will balance this pressure. The most elaborate experiments which have been made upon fans are those of Mr. Buckle, late of Soho, and we shall here recapitulate the main particulars of his researches :— An ordinary eccentrically placed fan, 4 feet diameter-the blades 10 inches wide and 14 inches long-and making 870 revolutions per minute, will supply air at a density of 4 ounces per square inch, to 40 tuyeres, each being 14 inch diameter, without any falling off in density. The experiments herein detailed were made with a fan 3 feet 1013 inches diameter, the width of the vanes being 104 and the length 14 inches; the eccentricity of the fan 1 inch, with reference to the fan case, the number of vanes being 5, and placed at an angle of 6° to the plane of the diameter; the inlet openings on the sides of the fan chest 17 inches diameter the outlet opening 12 inches squarc; the Experiments upon the Performance of Centrifugal Fans for Blowing Air. space between the tips of the blades and the chest increasing from inch on the exit pipe to 3 at the bottom, in a line perpendicular with the centre. To the blast pipe leading to the tuyeres a slide valve was attached, by means of which the area of the discharge was accurately adjusted to suit the required density. The gauge to indicate the density of the air, was a glass graduated tube, primed with water, it being more sensitive and having a greater range than the mercurial one. 269 The horse-power was ascertained by an indicator, the friction of the engine and gearing being deducted in each experiment. The first column of the table is the number of the experiment. The second is the number of revolutions of the fan per minute. The third is the velocity of the tips of the vanes in feet per second. The fourth is the pressure of the air in ounces per square inch, as indicated by the gauge. The fifth is the height of a column of mercury in inches such as would balance the pressure of the air. The sixth is the height of a column of air in feet such as would produce the The seventh is the area of the discharge pipe in square inpressure. ches. The eighth is the indicated horse-power. The ninth is the theoretical velocity of the air in feet per second. The tenth is of the theoretical velocity of the air in feet per second. The eleventh is the theoretical quantity of air discharged in cubic feet per second. The twelfth is the centrifugal force of the air in ounces per square inch, computed from the theoretical velocity. If water be taken as 827 times heavier than air, and mercury as 13.5 times heavier than water; then mercury will be 11,164 heavier than air, which for the purposes of this computation we may reckon it to be, though, according to M. Regnault's experiments, it is only 10,513 5 times heavier. A column of mercury, one inch in height, will on this supposition balance a column of air 11,164 inches or 930·3 feet in height. Let A be a column of mercury equal in height to any given density, and let B represent 930 3, and C 64*; then (A x B × C)=V, or the velocity that a body would acquire in falling the height of a column of air equivalent to the density. The centrifugal force of the air coincides with the results obtained by the laws of falling bodies, that is, when the velocity is the same as the velocity which a body will acquire in falling through the height. of a homogeneous column of air of any given density, as is shown in the table, column 12. Here we have taken the velocity as obtained from the laws of falling bodies (shown in column 9). To obtain the centrifugal force or density of air, we may apply the following general rule: : HAVING GIVEN THE VELOCITY OF THE AIR, AND THE DIAMETer of the RULE.-Divide the velocity of the air in feet per second by 4·01, and Or, D = N x 000034 V, where D is the pressure of the air in ounces per square inch, N the number of revolutions of fan per minute, and V the velocity of the tips of the fan in feet per second. square Let us now compare the results given in the foregoing table. To do this, we shall first take the velocity of the tips of vanes per second, and the power necessary to drive the fan. Taking Nos. 1, 2, 3, 4, 5, and 6 we shall find, by inspecting the table, that the velocities corresponding to these numbers are 236 8, 220 8, 202∙1, 185·2, 171·5, and 1441 feet per second, and the corresponding pressures of air per inch are 9.4, 7·9, 6·9, 5·6, 4·5, and 3.5 ounces. The fan, it must be understood, is discharging no air: the velocity of the fan is merely keeping the air at a certain density or pressure per square inch. Under these circumstances, it requires a certain velocity of the tips of the fan to maintain a given density of air: and Mr. Buckle states that the law which should govern the velocity of the tips of the fan appears from these experiments to be that this velocity shall be of the velocity which a body would acquire in falling through the height of a homogeneous column of air such as would produce the pressure, or, in other words, that the velocity of the tips shall be of the theoretical velocity, and by comparing Nos. 1, 2, 3, 4, 5, and 6 experiments as above-that is, by comparing the velocity of the tips of the fan in feet per second with of the theoretical velocity—we shall find them to agree tolerably nearly. Thus, if the The space which a gravitating body will pass through in one second is 16 1-12th feet; but by the principle of accelerating forces the velocity of a falling body in any given time is equal to twice the space through which it has passed in that time, or the velocity is equal to the square root of the number obtained by multiplying sixty-four by the height in feet. 7 9 ... 1 .........1' 1.028 1. 1. 12 ...... 1.35 1.06 The mean 1.008 But we shall not only find that the proportion of of the theoretical velocity applies to the fan when it is not discharging air, but that it applies also when the outlet pipe is open; that is, that the maximum effect of the fan is when the vanes move with of the theoretical velocity due to the density of the air, and that the greatest quantity of air is discharged by the fan under these conditions with the least expenditure of power. To illustrate this doctrine more fully, let us refer to the table of experiments, and we may take Nos. 9, 10, and 11. Here the pressure in each case is six ounces per square inch. In No. 10 the velocity of the tips of the vanes is 213.3 feet per second, while the theoretical velocity is 211 feet per second, being nearly the same. The quantity of air discharged is 77-9 cubic feet per second, and the power employed in this case amounts to 12.5 horses. We take now No. 11 experiment. Here the velocity of the tips of the fan is 192⚫ feet per second, and of the theoretical velocity is 190 feet per second. Now these two experiments are in proportion to each other nearly; viz. in No. 11 the quantity of air discharged amounts to 35.7 cubic feet per second, and takes 6.4 horse-power; while in No. 10 the fan discharges 77·9 cubic feet per second, and takes 12.5 horsepower. Thus the discharge of air is nearly 2 to 1, and the horse-power employed in the same proportion. We now take No. 9 experiment. Here the velocity of the tips of the fan is 221 feet per second, being 10 feet per second more than the theoretical velocity-the cubic feet discharged per second being 71.5, and the power 13-8 horses. Now, if we compare this with No. 10 experiment, we shall find that the velocity is 10 feet per second more, and the cubic feet discharged 6 less, and the horse-power 1.3 more. In the following examples, beginning with No. 15, we shall call the theoretical velocity per second unity. In this first example we shall also call the quantity of air discharged in cubic feet per second, unity, and also the horse-power. The pressure of the air in the four following experiments is 5 ounces per square inch. Nearly all the preceding examples justify the conclusion, that the greatest results are obtained when the theoretical velocity of the air and the velocity of the tips of the vanes are nearly equal. To deduce from these experiments rules of general application in practice: Let D denote the pressure of the air in ounces per square inch, and A a column of mercury of a height sufficient to produce that pressure. Then by the laws of falling bodies (A × 930·3 × 64) =V, the velocity acquired by a body falling through the height of a column of air of equal weight. RULE. Multiply the pressure of air in inches of mercury by 930-3 (which will give the height in feet of the column of air required to produce the pressure), and multiply the product by the constant number 64, which will give the velocity of the tips of the vanes in feet per second required to produce the pressure given. Example.-Let the pressure be 9.4 ounces per square inch, = 1·175 inches of mercury. Then ✔ (930-3 × 64 × 1·175) = 2644, the theoretical velocity, of which is = 237·96 = V, or velocity of tips of vanes per second. TO DETERMINE the powER NECESSARY TO DRIVE A FAN AT ANY GIVEN VELOCITY WHEN NOT DISCHARGING AIR. RULE.-Multiply the force in pounds resisting the vanes of the fan by the velocity of the tips of the vanes in feet per second, and by the constant number 60, and divide by 33,000; the quotient will be the force necessary to drive the fan at the given speed when not discharging air. Example.-Taking the same pressure and velocity as before, 38 X 9.4 =22:32 P, or pounds acting on the vanes of fans. Then 16 237.96 × 60 × 22.32 9.6, the horse power required. 33000 HAVING GIVEN THE VELOCITY OF THE AIR IN FEET PER SECOND (OR, AS IT HAS BEEN TERMED, THE THEORETICAL VELOCITY) TO DETERMINE THE DENSITY OF THE AIR IN ACCORDANCE WITH THE LAWS OF CENTRIFUGAL FORCE. Example. Let the velocity be 2644 feet per second, and the diameter of the fan 3·9 feet. Then by former rules we have 11169 x 1.209 144 264.4 4:01 = 66.2 and 66-22 3.9 =11169 and 9.3 ounces density, the answer required. Or by the second rule, take the velocity of the fan in feet per second, multiplied by the number of revolutions of the fan per minute, the product multiplied by '000034 the density required. |