Proportions used in Practice. The mean value of the quantities in the last column is 10-23. Hence we have the following rule : RULE 1.- To find the area of chimney.-Multiply the number of nominal horse power by 10-23; the product is the area of chimney in square inches. Example.-Required the area of the chimney for an engine of 400 nominal horse power. In this example we have, according to the rule, area of chimney in square inches=400 × 1023=4092. We may also find a formula for connecting together the area of the chimney, the length of the stroke, and the diameter of the cylinder; thus 10.23 x d2 × 35 ď× ys. X area of chimney in square inches= 47 This formula, expressed in words, gives the following rule : — 5 To work this example according to the first rule, we find, by referring to the table, pages 96. and 97., that the nominal horse power of this engine is 104.6 hence, according to Rule 1., area of chimney in square inches=104·6 × 10·23=1070. The latter value is greater than the former one by 70 inches. This difference arises from our taking too great a divisor in Rule 2. Either of the values, however, are near enough for all practical purposes. VIII. Water in Boiler. The quantity of water in the boiler differs not only for different boilers, but differs even for the same boiler at different times. It may be useful, however, to know the average quantity of water in the boiler for an engine of a given horse power. Following out the same plan that we have followed with the others, we here subjoin some particular examples of particular engines. IX. Area of Water Level. We here subjoin some examples of the area of water level in the boilers of particular steam vessels. Vessel. Engines. Collective Power of Area of Water Level. Dee and Solway Sydenham Retribution The mean value of the numbers in the last column is a little more than 1. Hence we may take one square foot of water level for each horse power. We may put this result in the form of a rule. RULE 1.-To find the area of water level. The area of water level contains the same number of square feet as there are units in the number expressing the nominal horse power of the engine. Example.-Required the area of water level for an engine of 200 nominal horse power. According to the rule, the answer is 200 square feet. To proceed in the same manner as we have done with the others, we add a rule for finding the area of water level when the diameter of cylinder and the length of stroke is given. RULE 2.-To find the area of water level.—Multiply the square of the diameter in inches by the cube root of the stroke in feet; divide the product by 47; the quotient expresses the number of square feet in the area of water level. Example.-Required the area of the water level for an engine whose stroke is 8 feet, and diameter of cylinder 50 inches. In this case, according to the rule, area of water level in square feet In order to work this example by Rule 1., we must refer to the table of nominal horse power at pages 96 and 97; and we find that the nominal horse power is 106 4. Hence, according to the rule, the area of water level = 106.4 square feet. X. Steam Room. It is obvious that the steam room, like the quantity of water, is an extremely variable quantity, differing, not only for different boilers, but even in the same boiler at different times. It is desirable, however, to know the content of that part of the boiler usually filled with steam. Following the same method as previously, we here add some examples of the average quantity of steam room in the boilers of different steam vessels. This formula, when expressed in words, gives the following rule : RULE 2. To determine the cubic feet of water usually in the boiler.-Multiply together the cube root of the stroke in feet, the square of the diameter of the cylinder in inches, and the number 5; divide the continual product by 47 ; the quotient expresses the cubic feet of water usually in the boiler. Example.-Required the usual quantity of water in the boilers of an engine whose stroke=8 ft., and diameter of cylinder =50 in. Here we have from the rule, Sydenham Dee and Solway Thames and Medway The mean value of the quantities in the last column is about 3: hence we have the following rule for determining the average quantity of steam room. RULE 1.-To determine the average quantity of steam room.-Multiply the number expressing the nominal horse power by 3; the product expresses the average number of cubic feet of steam room. Example.-Required the average capacity of steam room for an engine of 460 nominal horse power. According to the rule, Average capacity of steam room=460 × 3 cubic feet=1380 cubic feet. This rule may be so modified as to apply when the length of stroke and diameter of cylinder is given; thus, cubic feet of steam room= This formula, when expressed in words, gives the following rule :RULE 2.-Multiply the square of the diameter of the cylinder in inches by the cube root of the stroke in feet; divide the product by 15; the quotient expresses the number of cubic feet of steam room. T In order to work this example by the first rule, we must refer to the table of nominal horse-power. We find that the nominal horse power of this engine is 1064; hence, according to Rule 1., average stream room in cubic feet = 106 4 x 3=320 nearly. Before leaving these rules, we would again repeat that they ought not to be considered as rules founded upon considerations for giving the maximum effect from the combustion of a given amount of fuel; and consequently the engineer ought not to consider them as invariable, but merely to be followed as far as circumstances will permit. We give them, indeed, as the medium value of the very variable practice of several well-known constructors; consequently, although the proportions given by the rules may not be the best possible for producing the most useful effect, still the engineer who is guided by them is sure not to be very far from the common practice of most of our best engineers. It has often been lamented that the methods used by different engine makers for estimating the nominal powers of their engines has been so various that we can form no real estimate of the dimensions of the engine, from its reputed nominal horse power, unless we know its maker; but the same confusion exists, also, to some extent, in the construction of boilers. Indeed many things may be mentioned, which have hitherto operated as a barrier to the practical application of any standard of engine power for proportioning the different parts of the boiler and furnace. The magnitude of furnace and the extent of heating surface necessary to produce any required rate of evaporation in the boiler are indeed known, yet each engine maker has his own rule in these matters, and which he seems to think preferable to all others, and there are various circumstances influencing the result which render facts incomparable unless those circumstances are the same. Thus the circumstances that govern the rate of evaporation, as influenced by different degrees of draught, may be regarded as but imperfectly known. And, supposing the difficulty of ascertaining this rate of evaporation were surmounted, there would still remain some difficulty in ascertaining the amount of power absorbed by the condensation of the steam on its passage to the cylinder-the imperfect condensation of the same steam after it has worked the piston-the friction of the various moving parts of the machinery-and, especially, the difference of effect of these losses of power in engines constructed on different scales of magnitude. Practice must often vary, to a certain extent, in the construction of the different parts of the boiler and furnace of an engine; for, independently of the difficulty of solving the general problem in engineering, the determination of the maximum effect with the minimum of means, practice would still require to vary according as in any particular case the desired minimum of means was that of weight, or bulk, or expense of material. Again in estimating the proper proportions for a boiler and its appendages, reference ought to be made to the distinction between the power" or "effect" of the boiler, and its "duty." This is a distinction to be considered also in the engine itself. The power of an engine has reference to the time it takes to produce a certain mechanical effect without reference to the amount of fuel consumed; and, on the other hand, the duty of an engine has reference to the amount of mechanical effect produced by a certain consumption of fuel, and is independent of the time it takes to produce that effect. In expressing the duty of engines, it would have prevented much needless confusion if the duty of the boiler had been entirely separated from that of the engine, as, indeed, they are two very distinct things. The duty performed by ordinary land rotative steam engines is 66 One horse power exerted by 10 lbs. of fuel an hour; or, Quarter of a million of lbs. raised one foot high by 1 lb. of coal; or, Twenty millions of lbs. raised one foot by each bushel of coals. Though in the best class of rotative engines the consumption is not above half of this amount. The constant aim of different engine makers is to increase the amount of the duty; that is, to make 10 lbs. of fuel exert a greater effect than one horse power; or, in other words, to make 1 lb. of coal raise more than a quarter of a million of lbs. one foot high. To a great extent they have been successful in this. They have caused 5 lbs. of coal to exert the force of one horse power, and even in some cases as little as 3 lbs. ; but in these latter cases the economy is due chiefly to expansive action. In some of the engines, however, working with a consumption of 10 lbs. of coal per nominal horse power per hour, the power really exerted amounts to much more than that represented by 33,000 lbs. lifted one foot high in the minute for each horse power. Some engines lift 56,000 lbs. one foot high in the minute by each horse power, with a consumption of 10 lbs. of coal per horse power per hour; and even this performance has been somewhat exceeded without a recourse to expansive action. In all modern engines the actual performance much exceeds the nominal power; and reference must be had to this circumstance in contrasting the duty of different engines. We may here give the following table, taken from the experiments or collected data of Parkes, Wicksteed, and Boulton and Watt, and which is supposed to contrast the respective distinguishing features, and the comparative economy, of the two systems of generating steam practised in the Boulton and Watt boiler, and the Trevithick or Cornish boiler. Boulton and Watt boiler, 16 to 18 feet long, weighing 7 tons. Mean Cornish boiler, cylindrical, 36 to 40 feet long, weighing 12 to 13 tons each. Mean By this Table, it appears that, in the Cornish boiler, 1. The ratio of the area of the heating surface to that of the fire grate is more than double what it is in the common boiler. 2. The proportion of heating surface to the quantity of water evaporated, or of fuel consumed, is about ten times as great. 3. The ratio of combustion is slower with the Cornish boiler than with the common one, in the ratio of 1 to 4. 4. There is an economy of about 27 per cent. in fuel by the use of the Cornish system of boilers, as compared with ordinary land boilers; though they are less economical, as has been shown by the trials at the Blackwall Railway, than marine boilers, having the same amount of surface. Although a great economy of fuel is gained by the Cornish boiler, it is important to observe that this advantage is attended with a loss of time." Thus, one waggon-shaped boiler at the Albion Mills, weighing only 7 tons, evaporated 55 cubic feet of water per hour; while three cylindrical or Cornish boilers, weighing 48 tons, evaporate only 48 cubic feet. This is chiefly the result of the system of firing practised with the one boiler and not with the other. Again, in estimating the different qualities of the two classes of boilers from a table containing practical data, as the one we have given, it ought to be remembered that the average pressure of the steam in the Cornish boiler is greater than the average pressure of steam in the waggon boiler. The high pressure of the steam in the Cornish boiler requires the maintenance of a higher temperature in and about the boiler, and consequently there would be greater losses from dispersion, but for the effectual kind of clothing employed in the Cornish engines and boilers, and to which no inconsiderable part of their economy is to be attributed. It appears probable to us that the marine tubular boiler, or the tubular boiler with upright tubes, will supersede the Cornish boiler; and the exchange, it can hardly be doubted, would be an improvement; for with the same economy there would be a saving of weight and a saving also of the expense of setting. Indeed in the case of all land boilers, this expense will probably be hereafter saved by the substitution of the marine form of boiler for that which has heretofore been adopted. With these remarks we leave the subject of boilers, where we have perhaps lingered too long. We may mention that, although a great deal of attention and engineering ingenuity has been bestowed upon the improvement of boilers, there still remains great scope for improvement. Indeed, although some few engineers have spent a good deal of attention and ingenuity upon the subject of boilers and furnaces, still it is much to be lamented that the aim of so many others is no higher than to follow closely in the footsteps of their predecessors. The rule of the great majority of constructors seems to be to follow as closely as circumstances will permit that which has been found to answer. The blind submission which they display to the maxims of their predecessors, and their servile imitation of their maxims, has hitherto operated as an almost impenetrable barrier against extensive improvement in this most important department of engineering. Indeed we ought not to expect any other result. No improvement can reasonably be expected in any art or science, unless some judicious departures be made from the common methods. The construction of the boiler of a locomotive engine affords a striking example of the amount of improvement which may sometimes be made by judicious experiment, and the extension of this principle to marine boilers, which has now been accomplished, is an important step in the right direction. Nevertheless much remains to be done both as regards the distribution of heat-receiving surface in the best manner, and the introduction of improved means of feeding the fire with coals. The modes of feeding the fire at present in use are far from satisfactory, and in steam vessels particularly are the occasion of great labour and expense, all of which could be obviated by the introduction of some automatic machinery, by which the engine would supply the furnaces with the fuel they require. The revolving grate described in page 53, with its accompanying apparatus, is the self-acting firefeeder to which we think a preference should be given; and in the boiler with upright tubes described at page 69 the revolving grate is of easy application. Experiments with these upright tubes has shown that they are considerably more efficacious than tubes lying horizontally, while their use is attended with various collateral advantages. For locomotives, boilers with short upright tubes would be preferable to the ordinary plan of locomotive boilers: a large fire surface would then be obtainable, and but little power would require to be wasted upon the blast. TABLE OF DIMENSIONS OF THE PRINCIPAL PARTS OF MESSRS. MAUDSLAY, SONS, AND FIELD'S MARINE ENGINES. TABLE OF THE DIMENSIONS OF MARINE STEAM ENGINES OF MESSRS. SEAWARD AND Co.'s MANUFACTURE. 1447 28 53 26 22 680 30 10 11 11 12 33 TABLE OF CIRCUMFERENCES FOR ANGLED IRON HOOPS. ANGLE OUTSIDE. Diam. Circum. Diam. Circum. Diam. Circum. Diam. Circum. Diam. Circum. Diam. Circum. Diam. Circum. Diam. Circum. Diam. Circum. Diam. Circum. ft. in. ft. in. ft. in. ft. in. ft. in. 11001 11 9 135 15 2 16 10 4 3 3 3 3 37 7 3 41 81 73 4 1 81 2 3 1 3 51 3 5 6 1 3 3 7 8 1 11 1 11 38 2 0 38 2 0 3 9 2 07 5 5 21 5 2 31 15 7 5 11 17 157 17 7 12 101 10 6 15 81 5 72 15 71 5 8 7 37 3 7 41 7 4 7 7 51 7 5 7 61 7 61 Having thus far disposed of the subject of boilers, there may be no great impropriety, before entering at length upon the subject of engines, to set down some of the chief dimensions of marine and locomotive engines, the locomotive boiler and engine being so identified, that it is difficult to separate the consideration of them. We first give a table of the dimensions of Messrs. Maudslay and Co's side lever marine engines, and another of the dimensions of Messrs. Seaward and Co's. We do not attach any great importance to these dimensions, yet it is satisfactory to be in possession of the dimensions used by makers of some reputation, though not the best absolutely. We add some rules and tables for facilitating the construction of boilers, &c., by determining the lengths of plate or angle-iron requisite for the formation of hoops of different diameters. For plate or flat bar : 1 RULE. Add the thickness of the bar to the required diameter, and the corresponding circumference in the table of circumferences of circles given at page 24. is the length of the bar. If the iron be bent edgewise the breadth of the bar must be added to the diameter; for it is the thickness of the bar measured radially that is to be taken into consideration. In such pieces of work as the tires of railway wheels, which have a flange on one edge, it is necessary to add, not only the thickness of the tire, but also two thirds of the thickness of the flange. Generally, however, the tire bars are sent from the iron works so curved that the plain edge of the side tire is concave, and the flange edge convex; while the which is afterwards to be bent into contact with the cylindrical surface of the wheel is a plane. By this means the addition to the diameter of two thirds the thickness of the flange is unnecessary: for the curving of the flange edge has the effect of increasing the real length when only the chord of the arc is measured for the circumference. The radius of the curve in which the tires are first bent may be safely taken as four times the circumference of the hoop or wheel. In the form of rules, these results will be, first:- when the tire is straight BELE.-Add the thickness of the hoop, and two-thirds the thickness of the flange to the diameter, and find the corresponding circumference. And when the tire is curved it will be: RULE. Add the thickness of the hoop to the diameter, and find the circumference. In calculating the length of angle iron required for a given diameter the tables given above, obtained from a work by Mr. Foden, which is little known, are to be used instead of the table of circumferences of circles. The first of these is for iron, in which the angle or flange is outside, and the second for iron, in which the angle or flange is inside. When the angle is outside, its total breadth must be added to the internal diameter, and the circumference found in the table marked, " angle outside." And when the angle is inside, the breadth of the angle must be subtracted from the diameter, which, in this case, is the external diameter, and the circumference found in the table marked, " angle inside." We here add a table of the dimensions of the parts of Locomotives |