! STEAM thus been obtained; and it is evident that by constantly and gradually diminishing the load on the piston, an additional effect may be always obtained from a given amount of evaporation, to an extent which is only limited by practical circumstances which restrain the application of this expansive principle. Since the cost of producing steam as a mechanical agent depends chiefly on the quantity of fuel necessary to effect the evaporation of a given volume of water, it follows that all the mechanical effect obtained by this principle of expansion is so much power added to the steam without further expense. Its importance, therefore, will be obvious in the economy of steam power. For the manner of rendering it available in steam machinery, see STEAM ENGINE. One of the most important courses of experiments which have been made upon this subject is that undertaken by a committee of the French Institute, consisting of MM. Prony, Arago, Gerard, and Dulong, in consequence of an application from the French government to the Academy to point out the best means of preventing accidents from the bursting of the boilers of steam engines. The experiments were conducted chiefly by Arago and Dulong, and were certainly not only extremely delicate as to their management, but the most hazardous which science and art owe to the courage and zeal of philosophers. Steam was produced of a sufficient pressure to force a column of mercury up a glass tube to the height of nearly 43 feet; an atmosphere being measured by a column of mercury measuring 29.922 inches. The following table exhibits the temperatures and corresponding pressures of steam, as determined by these experiments, up to fifty atmospheres. Pressure in Temperature Temperature Pressure in 1 2120 13 380-66° 234 14 386.94 2 250.5 15 392.86 The temperature and pressure of steam produced by immediate evaporation, when it has received no heat, save that which it takes from the water, have a fixed relation one to the other. If this relation were known, and expressed by a mathematical formula, the temperature might always be inferred from the pressure, or vice versa. But physical science has not yet supplied principles by which such a formula can be deduced from any known properties of liquids. In the absence, therefore, of any general relation established by direct reasoning, empirical formulæ have been proposed, which express, with more or less precision, this relation in different parts of the thermometric scale. These formulæ have been so constructed as to give results which nearly agree with the results obtained by experiment. One set of experiments to ascertain the relative bulks of steam and water was made by Watt at an early period of his career, and these experiments were subsequently repeated by his assistant Southern, with great care and skill. Southern's formula for determining the pressure of steam of any given temperature is probably more The last six temperatures in the above table widely identified than any other with engineer- are deduced by calculation from the formula ing practice, and it gives results sufficiently accurate for engineering purposes. This formula is as follows: If F represent the elastic force of the steam in inches of mercury, and t its corresponding temperature in degrees of Fahrenheit's thermometer, St+51-3) 135-767 J 12 M. Regnault has shown that the total amount of heat existing in a given weight of steam increases with the pressure. Thus, in steam Rule-To the given temperature in degrees with a pressure of 147 lbs. upon the square of Fahrenheit's thermometer add 513 degrees: inch, the sensible heat of the steam is 2120, then from the logarithm of the sum subtract the latent heat 966 60, and the sum of the 2-1327940, the logarithm of 135 767. Multiply latent and sensible heats 1,178.60; whereas, the remainder by the index 513, and to the in steam of 90 lbs. upon the square inch natural number answering to the sum add the sensible heat is 320-20, the latent heat the constant fraction 1-10th. The result will be the elastic force of the steam in inches of mrcury. 891-4°, and the sum of the latent and sensible heats 1,211.6°. There is, therefore, a difference of 33° in the total heat of a pound of water STEAM raised into steam of 147 lbs. pressure and that of a pound of water raised into steam of 90 lbs. pressure; so that the high steam if expanded into low will have an excess of temperature beyond that necessary for the maintenance of the vaporous form, or, in other words, will be in the state of surcharged steam. Surcharged steam, or steam to which more heat is imparted than is necessary for the maintenance of the vaporous form, comes under the same physical laws as air and other permanent gases. And with all gases, when the temperature is constant, the pressure varies simply as the density, or inversely as the volume. When the pressure is constant, the dilatation is uniform with uniform additions of heat, and is at the rate of of the volume at 32° for every additional degree of temperature. When the volume is constant, the increase of pressure is of the pressure, at 32° for each additional degree of temperature. In the Edinburgh New Philosophical Journal for July 1849, a formula is given by Mr. Rankine, which educes results very nearly corresponding with those which M. Regnault obtained by experiment; and from this formula, therefore, the most material of M. Regnault's results may be obtained. Mr. Rankine assumes a point of temperature t, which is 462-28° of Fahrenheit's scale below the ordinary zero of that scale, as a new absolute zero, and he supposes the boiling point of the water to have been adjusted under a pressure of 29-922 inches of mercury; so that 180° of Fahrenheit may be exactly equal to 100° of the centigrade thermometer. The formula is applicable for finding the elasticity of other vapours besides that of water; but three constants, a, B, y, have to be determined for each fluid by experiment. If P be the pressure of the steam and t the point of absolute zero, as explained above, then the formula for calculating the pressure from the temperature is The value of a, other things being the same, depends upon the measure of elasticity adopted. If it be inches of mercury, the value of a will be 6426421; if it be pounds avoirdupois on the square inch, the value of a will be 6.117817. The following table is derived from M. Regnault's experiments, with the addition of a column showing the volume of the steam relatively with the volume of the water from which it is generated, computed by Mr. D. K. Clark, and given in his work on Railway Machinery. In this table we have the total pressure of the steam in lbs. per square inch, its relative volume as compared with that of the water from which it is produced, its temperature, its total heat, or, in other words, the sum of its latent and sensible heats, and finally the weight of a cubic foot of the steam at the several pressures or densities enumerated. The steam is supposed in every case to be saturated with water." 128 219 351-8 1221 2 2846 140 216 352.9 1221.5 *2885 142 213 354-0 1221.9 *2922 144 210 *2406 2524 *2566 2662 *2771 3623 3800 *3970 D The proper level of the water within this boiler is maintained by means of a balanced buoy or float communicating with the rod N, which is attached to a lever set on the top of the stand-pipe P. The top part of this pipe is widened out so as to form a small cistern into which the water for replenishing the boiler is pumped by the engine; and a valve in the bottom of the cistern, when opened by the lever communicating with the rod N as the float subsides from the falling of the waterlevel, admits a sufficiency of feed water to replace the water removed by evaporation. When there is already a sufficiency of water in the boiler the valve in the feeding cistern remains closed, and the excess of water in such a case runs to waste through an overflow pipe provided for that purpose. In marine and locomotive boilers the use of a float for regulating the admission of the feed water is inapplicable, and there the attendant has from time to time to adjust a cock in the feed-pipe, so as to admit the proper quantity of water. To enable him to know at what level the water stands within the boiler a succession of cocks, called gauge-cocks, is attached to the boiler, rising one above the other; and the highest of these cocks when turned should always let out steam, and the lowest water. A glass tube is also affixed perpendicularly to the outside of the boiler in such a manner, that its upper extremity communicates with Waggon Boiler.-One of the oldest forms of the steam within the boiler, and its lower The water conboiler used for land engines, is that called the extremity with the water. waggon boiler, an isometric view of which is sequently stands in the tube at the same level given in fig. 1; but this boiler has now almost as in the boiler, and the height of the water in entirely disappeared, owing to its small evapo- the boiler is thus rendered visible. Cocks rative power compared with the consumption are provided at the top and bottom of the of fuel, and to its incapacity to resist the tube, so that if the tube happens to be Steam Boiler. A vessel in which water is converted into steam for the purpose of supplying steam engines, or for any other purposes for which steam is used in the arts, or in domestic economy. STEAM BOILER broken, the issue of the steam or water may boiler bottom, before they escape to the chim be prevented. The draught through the furnace of land boilers is regulated by a plate of metal or damper, as it is called, which closes, to a greater or less extent, the opening of the flue, in the manner of a sluice. A damper of this kind is seen at O, and it is counterpoised by a weight in the stand-pipe P, by means of a chain passing over pulleys. When the pressure of the steam in the boiler rises beyond the desired point, the water is forced up by it into the pipe P to a more than usual height. The weight in P being thus floated up, the damper at O preponderates, and partly closes the flue, whereby the intensity of the draught through the furnace is diminished, and less steam is raised. To provide an escape for the superfluous steam, which if suffered to accumulate would burst the boiler, one or more valves, opening upwards and loaded by a weight or spring to a sufficient degree to balance the pressure of the steam, must be applied. This species of valve is called the safety-valve, and a valve of this kind is seen in the figure affixed to the top of the boiler near N: a pipe proceeds from the safety-valve, which conducts the waste steam into the atmosphere. Cylindrical Boilers.-The usual pressure of steam employed in waggon boilers is from 3 to 5 lbs. per square inch. In many engines, however, and especially in those which work expansively, it is found expedient to use steam of a higher pressure. To sustain this pressure waggon boilers are not well adapted, and hence a new class of boilers has been introduced of a cylindrical form, and which are therefore termed cylindrical boilers. These boilers can withstand a considerable pressure without danger. Their construction in the subordinate features is very various, but a form much approved is that known as the Cornish boiler. Cornish Boiler. The boilers used at the mines in Cornwall, have long been celebrated for great economy of fuel and other distinguishing circumstances. Fig. 2. ney. A brick wall is built at the back end of the fire bars, to form the termination of the furnace. This wall is called the furnace bridge. Behind the bridge in some Cornish boilers a pipe containing water extends horizontally within the flue, communicating at the one end with the bottom part of the boiler, and at the other end with the top part of the boiler. The hot air impinging upon this pipe causes the water within it to boil, and a constant circulation of the water is maintained within it. In modern boilers, however, this pipe is omitted. This boiler, like the waggon boiler, is set in brickwork, and it is also covered over with a brick arch, for the purpose of retaining the heat. But a vacant space is left between this brick arch and the top of the boiler for the purpose of enabling the boiler to expand without disturbing the brickwork. One of the most remarkable peculiarities of the Cornish boilers is the slowness of the combustion in the furnaces, and the large amount of heating surface allowed for the evaporation of the water. Thus, in certain experiments upon Cornish, waggon, and locomotive boilers, recorded in Mr. Bourne's work upon the steam engine, it was found that the number of pounds of fuel burned upon each square foot of fire-grate in the hour was, in the Cornish boiler, 3:46 lbs.; in the waggon boiler, 10.75 lbs.; and in the locomotive boiler, 79-33 lbs. The number of square feet of heating surface of the boiler employed to evaporate a cubic foot of water in the hour was, in the Cornish boiler, 69-58 square feet; in the waggon boiler, 9.96 square feet; and in the locomotive boiler, 6:06 square feet. The number of cubic feet of water evaporated by 112 lbs. of fuel was, in the Cornish boiler, 18-87 lbs.; in the waggon boiler, 13:91 lbs.; and in the locomotive boiler, 11-14 lbs. Marine Boilers.-Boilers set in brickwork being ineligible in steam vessels, a distinct class Fig. 4. 888 886 of boilers has been called into existence to meet the exigencies of steam navigation. In these boilers the fire and smoke come into contact only with metallic surfaces surrounded by water. Marine boilers consist of a series of large iron vessels, in which are set a number of metallic furnace chambers, connected with metallic flues winding within the boiler, or with a number of short metallic tubes of small diameter which deliver into the chimney. STEAM BOILER Boilers with large flues winding within them | chimney in the centre of the boiler, represented are termed flue boilers, and boilers in which the by the dotted circle in fig. 5. heat is communicated by means of a faggot of small tubular flues passing through the water are called tubular boilers. In figs. 4, 5, Fig. 5. Fig. 8. fig. 7. The flue then ascends perpendicularly upwards, and winds in a similar manner through an upper tier of flues until it finally reaches the Fig 9. Figs. 8 and 9 are representations of a tubular boiler; fig. 8 being a longitudinal section made perpendicularly through the boiler, and fig. 9 an end view, with one half cut transversely through the furnace and boiler. There are two furnaces in the boiler, and the smoke and flame, after passing over the furnace bridge, ascend into a bundle of small brass tubes, and pass through them on their way to the chimney. Upon the top of the boiler a chest is placed to serve as a receptacle for the steam, which there deposits any particles of water that may happen to be mixed with it. Iron rods pass perpendicularly and horizontally within the boiler to strengthen it, and to enable it to withstand a considerable pressure. Nearly all marine tubular boilers are formed on this general type. Sometimes the boiler shell is square instead of waggon-formed, and has more furnaces set within it; but these differences are not very material. Tubular boilers are now very largely employed in steam vessels. They were patented by Mr. Bourne in 1838, but first came into general use about 1844. They are lighter than flue boilers, and occupy less space; and if they are properly constructed they are not difficult to keep in repair. The consumption of fuel per horse-power in marine boilers is very extravagant, and attempts have recently been made to economise |