The increase of speed is pressure gains simply in the ratio of the diameter, or speed, provided there is the same number of revolutions. in the ratio of the increase of the radius. is produced by centrifugal force. The atoms of air, after being whirled round by the wings, are thrown out at their periphery by a force equal to the centrifugal force resulting from the speed of the C2 wings. This centrifugal force may be simply expressed by 2gr c is the speed in feet per second; g the speed of gravitation in the first second; and r the radius of the fan. According to this, the effects of a fan ought to be far greater than they actually are; therefore, a remarkable loss of power must take place in these machines. It is thus very clear that the increase of diameter augments the effect of the machine in a numerical proportion, while an increase of revolutions adds to the effect in the proportion of the square. It is also very clear that an increased diameter greatly increases the friction, while the increase of speed does not augment it in the least. The friction, in these machines, is the greatest objection to their use; therefore, the movable parts should be as light as possible. Friction increases in the ratio of the weight, where the materials are the same, but not with an augmentation of speed, at least, not in the same ratio. From practical observation, the following formula has been deduced: in which a is the speed of the fan, that is to say, it represents the number of feet which the wings make in a second; b, the surface of the nozzle; e, the surface of a wing; and d, the velocity of the escaping blast. This formula we conceive to be the proper dimensions of a fan : VII. Receivers, or Regulators of Blast. Cylinder blast machines, as well as those of the common bellows form, make an irregular blast. The back and forward motion of the piston, which, when it arrives at the culminating points, ceases, for a few moments, to make any blast at all, of course causes an interruption of supply to the nozzles, and a consequent waving, sinking, and falling, in the pressure of the blast. Uniformity of pressure is so important an object at the blast furnace, that too much attention cannot be paid to it; but an attention commensurate with its im portance this subject has never received. If there were no other argument to convince the skeptical; if we had no facts to prove directly the great value, of a uniform pressure, the consideration that different pressures are necessary in blowing different furnaces ought to settle the question conclusively. We know that one kind of charcoal will permit a pressure of but half a pound, while another kind requires a pressure of one pound and more. In each case, too great or too little pressure is injurious. Where the coal is of such a nature as to require but three-quarters of a pound pressure, we are obliged to secure that amount. If, as not unfrequently happens, we make, in the same stroke of the cylinder, a difference of between half a pound and a pound, we destroy fuel uselessly, for that blast which is below three-quarters of a pound, as well as that which is above it, is only a waste. Regulators of blast, commonly called receivers, are of various forms. Scientifically, they may be divided into three classes: the wet receiver, or water regulator; the dry receiver, with movable piston; and the air chamber, of constant capacity. The wet receiver is not to be recommended, on account of its water. Though this water should be covered with floating oil, or other matter, as has been suggested, the receiver would not be adapted for use. a. The second class, or the dry receiver with movable piston, is generally employed, in blast machines, with two wooden cylinders; and in some machines, with iron cylinders. The top of a common blacksmith's bellows acts on the same principle, and belongs to this class. This receiver is more perfect than the first kind, but it is far from producing a uniform pressure, or at least that uniformity the blast furnace requires. For forges, such receivers answer very well. In Fig. 132, a receiver with a movable piston is shown. Its dimensions should be very nearly those of one of the cylinders. If the diameter is increased, the play of the piston, and consequently the resistance, are diminished; the latter is the cause of the irregularities in the blast. If the dead points, caused by the raising and falling of the piston, could be obviated, this receiver would be useful; but, as that is not likely ever to be the case, there is little probability that this apparatus will ever become a favorite among blast furnace owners. b. The air chamber of constant capacity is coming more and more into general use; this is unquestionably the best of all regulators. Air chambers of various forms have been tried: that the best form is the sheet iron cylinder may now be considered a set tled question. In order to make a good, uniform blast, this receiver should be of great capacity; if sufficiently large, the blast is perfect. Partly from considerations of economy, less frequently from those of expediency, wooden, stone, or brick chambers have been used. Vaults cut into rock, or native caves, have also been used as blast regulators. All these experiments have furnished no inducements for imitation, because the difficulty of keeping such chambers air-tight was too great to be overcome. Consequently, most of the stone and wooden chambers have been abandoned, and iron ones constructed. At the present time, air chambers are made of sheet iron one-eighth of an inch thick, and of greater or less capacity, according to the number of strokes of the piston of the blast machine, and the capacity of the cylinders. It would lead us too far to enter upon a thorough investigation concerning the capacity of a dry receiver; but we shall point out such facts as have a bearing upon the question. The dimensions of the air chamber have, in no respect, any relation to the capacity of the blast cylinder. This is influenced by other circumstances. The irregularities of pressure are less where two blast cylinders are working than where but one is employed; the result is still better where three are used. In the latter case, the blast is generally so uniform that no receiver is needed, and the employment of large pipes to conduct the blast to the furnace is all that is required. With two cylinders and double stroke, the blast ought to be nearly uniform, according to theoretical calculations, if the weight of the piston is counterbalanced. This is generally the case at the Western establishments, and is exhibited in Fig. 133. Nevertheless, in practice, we find that these machines answer excellently for fineries and hard coal, while they are insufficient for soft charcoal, and for well-regulated smelting operations. Blast machines, with two double stroke cylinders, but without beams and counterbalance, must be adjusted by direct balance weight, because the united weight of the two pistons is too great even for a large air chamber. To set the cranks opposite each other in such a machine is not advisable, for the difference in pressure is so great that it cannot be effectually overcome by a chamber. Two cylinders and single stroke require opposite cranks. A blast machine with a single cylinder requires the largest possible air chamber, particularly where a waterwheel or an expansion steamengine is the motive power. The dimensions of the air chamber in a double cylinder machine and double stroke is sufficiently large if made of ten times the capacity of one of the blast cylinders. With one cylinder, or two single stroke cylinders, from twenty to thirty times the capacity of the cylinder is required. If large air-pipes are employed, and if the distance from the blast machine to the furnace is considerable, the capacity of the pipes may be taken into account. Where the pipes are narrow, they do nothing toward equalizing the blast; on the contrary, they cause a loss in power by friction. The form of an air-chamber is generally that of the cylinder, like a steam boiler, and varies from four to eight feet in diameter. It sometimes has straight, sometimes convex heads. The globular form has, in some places, been adopted, but we do not think that this form will ever be used extensively in this part of the world. The thickness or strength of the sheet iron for an air chamber is made to vary according to the pressure of the blast; but, as the strongest pressure would hardly tear iron one-eighth of an inch thick, and as that thickness is required to give stability to the form of the chamber, the question is one of slight practical interest. An air chamber should be provided with a safety-valve, to guard against accidents, as well as with a manhole, to afford an opportunity of getting into the interior, if that is found to be necessary. The air chamber, unless too small, is the best of all regulators. If we have any doubt in relation to what should be its capacity, it is always better to make the chamber too large than too small. VIII. Blast Pipes. It is seldom or never in our power to bring the blast generator close to the tuyere. Conductors are generally required to lead the blast from the blast machine to the furnace. Various forms of conductors have been invented, such as wooden and iron pipes, of a round, square, and polygonal section; but at present, scarcely any other than sheet or cast iron pipes are employed. At forge fires, and small blast, puddling, and reheating furnaces, or at those places where but little pressure is required, pipes of tin plate are used; but where stronger pressure is needed, as at charcoal blast furnaces working hard coal, and at anthracite and coke furnaces, pipes of sheet iron one-eighth of an inch thick, or of cast iron, are used. Cast iron would be preferable, in many respects, to sheet iron pipes, but, in consequence of their weight, the latter are coming more and more into general use. Sheet iron pipe can be made of almost any length, and it has an advantage in the small number of its joints. The diameter of the pipes varies according to the amount of air which is to pass through them. Where 1000 cubic feet of air per minute are to pass through one of medium length, the diameter should be at least ten inches. Each additional 1000 feet should have the same space, so that 4000 feet per minute require a diameter of twenty inches. If the distance from the blast machine to the furnace is more than 100 feet, the diameter of the pipe is to be increased; and it may be doubled with each additional 100 feet, in consequence of the friction of the air. A very appreciable loss of blast results from narrow pipes. Cast iron pipes require many joints; they are liable to leak, in consequence of the destruction of the cement in the joints, caused by contraction and expansion; this is particularly the case in long pipes. For cold blast pipes, the best joint we can use is the leaden one commonly employed in light gas pipes. Where hot air is to be conducted, as in the use of hot blast, the lead is liable to melt by the heat of the air. In this case, the joints must be cemented by a fire proof material. A cement which resists the influence of hot air is composed of iron filings, turnings, or borings, worked through a riddle or a coarse sieve, to make it uniform. Seventy-five pounds of sifted filings are to be mixed with one pound of powdered sal ammoniac and one ounce of flowers of sulphur, to which two pounds of clay in dry powder must be added. The whole of these ingredients must be well mixed together, and kept in a dry place for use. Whenever any cement is wanted, some of the dry and prepared material is moistened, and used immediately; for, as it very soon oxidizes, it is adapted to make good joints only when it is fresh. A few days are required to harden this cement; but, when thoroughly indurated, it is almost as durable as the iron itself. If pipes with muffs are used, which are preferable to those with flanches, care must be taken that the space between the muff and the pipe is not too great; one-quarter of an inch all around is sufficient. Where the space is excessive, the expansion of the cement, occasioned by the oxidation of the iron, is very apt to break the muff. Long, straight pipes are very liable to leak, because, from their length, they are stretched by a high heat. By reason of their weight, expansion and contraction break the cement of the joints. Such pipes should be laid upon a well-leveled and paved foundation, and rested upon rollers, which may be either short pieces of round bar iron, or short pieces of two inch cast iron pipes. In long conductors, the expansion and contraction of the pipes are frequently neutralized by sliding |