Consumption of Fuel and Prevention of Smoke, READ BEFORE THE BRITISH ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE. 1851. ADVERTISEMENT. The Two valuable Lectures, on "The Construction of the Boilers of Steam Engines," and "The Causes and Prevention of Explosions," were kindly delivered by their scientific Author, before the Leeds Mechanics' Institution, at the request of the Committee of the "Yorkshire Union of Mechanics' Institutes," in consequence of several disastrous accidents which had recently occurred. They are now published, by the Committee, in a cheap form, by the permission of the Author, along with a paper read by him before the "British Association for the Advancement of Science," on "The Consumption of Fuel and the Prevention of Smoke," in the hope that much practical benefit may result from the publication in this manufacturing district. The best thanks of the Mechanics' Institutions are due to Mr. Fairbairn for his public-spirited conduct. THE COMMITTEE OF THE YORKSHIRE UNION LEEDS, May, 1851. THN ON THE CONSTRUCTION OF BOILERS. LECTURE I. The modifications of the Steam Engine which have been adopted since its introduction by Watt, three-quarters of a century ago, have been very numerous and varied; and although the progression in its applications and improvements has been most rapid and wonderful, we are still undecided as to the best form of its construction. Sound principles scientifically applied, and the gradually increasing excellence of our mechanical workshop, have enabled us to attain the great perfection which characterises the working parts of the modern steam engine. The steam engine itself may be regarded as a comparatively perfect machine, and I shall, therefore, confine my observations almost exclusively to that very important and necessary adjunct-the Boilerwhich is the source of its power. With this limitation a very wide field of inquiry is opened out, and in the earliest steps of the investigation we become perplexed with the endless variety of forms and constructions which at different periods have been adopted by engineers, and which have never, unfortunately, received the same judicious attention that was paid, as I have already remarked, to the steam engine. This is an anomalous and much to be regretted fact, for the boiler, being the source of the motive power, is undoubtedly one of the most important parts of the whole machine. Upon its proper proportions and arrangements for the generation of steam depend the economy and regularity with which the engine can be worked, and upon its strength and excellence of workmanship depends the safety of the lives and property of those who come in contact with it. Regarding the steam engine as one of the most active agents in the extension of our prosperity, and in the civilization of the world, and seeing how it is mixed up with the daily duties and workings of society, the safety and efficiency of every part, and more especially the boiler, are subjects of national importance; and I feel gratified by being called upon to lay before you such knowledge and experience on this subject of deep interest as I myself possess. I propose to consider the Boiler in its Construction, Management, Security, and Economy. 1st. As to the Construction. Here I shall have to go a little into detail in order to show, in construction, the absolute necessity there exists for adhering to form and other considerations, essential in the practice of mechanical engi neers, in effecting the maximum of strength with the minimum of material. In boilers this is the more important, as any increase in the thickness of the plates obstructs the transmission of heat, and exposes the rivets as well as the plates to injury on the side exposed to the action of the furnace. It has generally been supposed that the rolling of boiler plate iron gives to the sheets greater tenacity in the direction of their length than in that of their breadth; this is, however, not correct; as a series of experiments which I made some years since fully proves that there is no difference in the tensile strength of boiler plates whether torn asunder in the direction of the fibre, or across it. From five different sorts of iron the following results were obtained : From this it appears that we may safely use iron plates in the construction of boilers in whatever direction may best suit the convenience of the maker. Next to the tenacity of the plates, comes the question of riveting, or the best and surest means of securing them together. On this part of the subject we have been widely astray, and it required some skill, and no inconsiderable attention, in conducting the experiments, to convince the unreflecting portion of the public, and even some of our boiler makers, that the riveted joints were not stronger than the plate itself. At first sight this would appear to be the case, but a moment's reflection will soon convince us to the contrary, as in punching holes along the edge of a plate it is obvious that the plate must be weakened to the extent of the sectional areas punched out, and that it is next to impossible, under the circumstances, to retain the same strength in the material after such diminution has been effected, as existed in the previously solid plate. This was clearly demonstrated by a series of experiments which took place some years since, and in which the strength of almost every description of riveted joints was determined by tearing them directly asunder. The results obtained from these experiments were conclusive as regards the relative strength of riveted joints and the solid plates. In two different kinds of joints-double and single riveted the strengths were found to be, in the ratio of the plate, as the numbers 100, 70, and 56. Assuming the strength of the plate to be The strength of a double riveted joint would be, after allow- 100 70 56 These proportions of the relative strengths of plates and joints may therefore, in practice, be safely taken as the standard value, in the construction of vessels required to be steam and water tight, and subjected to pressure varying from 10 lbs. to 100 lbs. on the square inch. In the construction of boilers exposed to severe internal pressure, it is desirable to establish such forms, and so to dispose the material, as to apply the greatest strength in the direction of the greatest strain; and in order to accomplish this, On It will be necessary to consider whether the same arrangement be required for all diameters, or whether the form as well as the disposition of the plates should not be changed. To determine these questions in cylindrical boilers, recourse must be had to experiment, or such deduction as may apply to any given case, and such as is founded upon unerring data derived from experimental research. this head I am fortunate in having before me the calculations of Professor W. R. Johnson, of the Franklin Institute of America, whose inquiries into the strength of cylindrical boilers are of great value, and from which the following short abstract may be useful. "1st. To know the force which tends to burst a cylindrical vessel in the longitudinal direction, or, in other words, to separate the head from the curved sides; we have only to consider the actual area of the head, and to multiply the units of surface by the number of units of force applied to each superficial unit. This will give the total divellent force in that direction. "To counteract this, we have, or may be conceived to have, the tenacity of as many longitudinal bars as there are lineal units in the circumference of the cylinder. The united strength of these bars constitutes the total retaining or quiescent force, and at the moment when rupture is about to take place, the divellent and the quiescent forces must obviously be equal. "2nd. To ascertain the amount of force which tends to rupture the cylinder along the curved side, or rather along the opposite sides, we may regard the pressure as applied through the whole breadth of the cylinder upon each lineal unit of the diameter. Hence the total amount of force which would tend to divide the cylinder in halves, by separating it along two lines, on opposite sides, would be represented by multiplying the diameter by the force exerted on each unit of surface, and this product by the length of the cylinder. But even without regarding the length, we may consider the force requisite to rupture a single band in the direction now supposed, and of one lineal unit in breadth; since it obviously makes no difference whether the cylinder be long or short, in respect to the ease or difficulty of separating the sides. The divellent force in this direction is therefore truly represented by the diameter multiplied by the pressure per unit of surface. The retaining or quiescent force, in the same direction, is only the strength or tenacity of the two opposite sides of the supposed bond. Here also at the moment when a rupture is about to occur, the divellent force must exactly equal the quiescent force." Mr. Johnson then goes on to show, that as the diameter is increased, the product of the diameter and the force or pressure per unit of surface are increased in the same ratio. This truism I shall endeavour to prove ; as also, that as the diameter of any cylindrical vessel is increased, the thickness of the metal must also be increased in the exact ratio of the increase of the diameter: the pressure, or, as Mr. Johnson calls it, the divellent force, being the same, when the diameter of a boiler is increased, it must be borne in mind that the area of the ends is also increased, not in the ratio of the diameter, but in the ratio of the square of the diameter; and it will be seen that instead of the force being doubled, as is the case in the direction of the diameter and circumference, it is quadrupled upon the ends, or, what is the same thing, a cylinder double the diameter of another cylinder, has to sustain four times the pressure in the longitudinal direction. The retaining force or the thickness of the metal of a cylindrical boiler does not, however, increase in the same ratio as the area of the circle, but simply in the ratio of the diameter; consequently the thickness of the metal will require to be increased in the same ratio as the diameter is increased. From this it appears that the tendency to rupture by blowing out the ends of a cylindrical boiler will |