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deal in this place with the losses com- efficiency, for the lower limit cannot at mon to them.
the best differ much from the temperaThe first cause of loss, viz., by con- ture of surrounding bodies, but the upduction and radiation, is proportionately per is only limited by mechanical diffigreater for small than for large engines, culties; and though it may be shown not only from the fact that the heated that it is impossible to abstract all the agent is generally contained and con- imparted heat in the form of work, the veyed in cylindrical vessels and pipes, in proportion of heat so abstracted being, which the volume varies as the square of, in fact, never greater than one-seventh but the surface only directly as, the di- in the best steam engines of the day, ameter, and thus the radiation is propor- yet the efforts made in this direction tionately greater, but loss by leakage must be attended with success in proporand imperfect lagging is also generally tion as those difficulties are overcome. greater; furthermore, the loss of heat in In the discussion which followed the its transference from the fuel to the reading of Mr. Flannery's paper* on agent is greater, for, as will be shown by high-pressure steam boilers these diftithe results of experiments, the condi- culties seem to have formed the only tions of economical combustion are ground of objection to their use. Pressharder to maintain. Both these sources ures of from 300 to 500 pounds per of loes may, however, be considerably square inch have been successfully used modified by mechanical arrangements. by Mr. Perkins, and the application of
The second cause of loss, from the high pressures and speeds is greatly innecessary rejection of heat, is often over. creasing for small steam engines. looked. There is not space to examine The third cause of loss, viz., that from the question as fully as it deserves, but friction and imperfection of machinery, the conclusion to be drawn is that the has a special interest from the high agent should be used between as great speeds now used in some small engines. ranges of temperature as practicable. It may be regarded as certain that the This is an important point in discussing proportionate loss from these causes is their relative efficiency, and since the greater on a small than on a large scale; proportionate range for hot air is greater but, again, the proportionate waste of than for steam or gas, the efficiency of fuel is nearly always of less consequence. the former in this particular is the high- From the recently issued first reports on er; but at the same time this very quali-“ Friction at High Speeds,” of the Inty renders it less capable of developing stitution of Mechanical Engineers, it so much power, since only small differ- would appear that, at any rate up to the ences of pressure accompany this range, velocities at present used, the friction which is limited by practical considera- seems slightly to decrease as the velocity tions, such as lubrication, packing, &c. increases. There is, however, another and a more obvious way of regarding the action of
SECTION II.—STEAM. these agents so as to compare their effi Steam engines are at present by far ciencies; the heat rejected in the waste the most extensively used of small mosteam or gas has to be entirely restored tors, some on a very small scale being to the new portion of the agent, by the made to develop considerable power. expenditure of fuel, while in the air en The generation of steam on a small gine this is partly effected by compres- scale is almost entirely limited to vertision in the return stroke; and so, though cal boilers—at any rate, for purposes on the loss of fuel is not so great in the air land—though the horizontal form, as engine, the high-back pressure resulting well as sectional boilers, are also coming from compression makes the resultant into more general use, the portability of work also not so great, and leads to the the first being a strong recommendation, conclusion, which is verified in practice, Figs. 1, 2 and 3 are sections of vertical that air engines are not so powerful as boilers in common use, the general form steam or gas engines, though more eco- being a cylindrical wrought-iron shell nomical.
containing a furnace, also cylindrical The higher the initial temperature of
* Vide Minutes of Proceedings Inst. C.E. vol., liv. p. the agent the greater will be its possible 123.
and concentric with it, and fired through number of different devices for robbing an opening in one side. In other par- them of their heat before this takes ticulars there is considerable difference. place; the oldest and most usual is that In Fig. 1, which, though extensively of the cross tubes c, c, whose section is, used, is deservedly falling into disfavor, however, generally circular; these have the gases pass upward to the funnel the advantage of adding to the strength through a number of vertical tubes, of the furnace; the furnace, besides beeither a cast-iron top or an enlarged ing subjected to external pressure, is the crown being necessary to allow their vulnerable part in the, by no means unescape to the funnel; this latter device common, event of shortness of water. is shown in the figure, and also an inge- Another plan is to have pendent tubes P, nious arrangement for preventing the P, P, of which the "Field" type is a good earlier destruction of the central tubes, example, as it allows a complete circulaby the tendency of the flames to ascend tion of water by separating the ascend
through them, which is especially the ing and descending columns, by an incase when the fire is low. This consists serted tube shown in the diagram. The of a concave wrought-iron plate (D), by success of this plan is shown from the whose under side the gases are reflected fact that no less than 134,700 are now and are compelled to ascend round its in use. Another good device is applied edge, where combustion of some portion in the “Davy-Paxman” boiler (Fig. 4), of them takes place, and the side of the in which the tubes, instead of ending in furnace is acted on by the flame instead the furnace, pass down and communiof by the current of cold air, which cate with the water in the lower part of otherwise would be drawn up and be in the boiler, and so establish complete circontact with it. Fig. 2 is a still more culation, which is prevented from acting common form, the gases passing up a too violently by bafflers placed at their central flue; but there are a considerable upper ends. These and various other
arrangements, though adding to the at d. Another form of sectional boiler complication of the boiler, are very de- may be briefly described as a square box sirable if effective in action, for the loss of tubes, which run from side to side in of heat in them, which experiments indi- two directions, and through which the cate, is thereby much diminished. Thus flames are made to pass successfully. the boiler of Latham and Bradley, 10 Yet another form, which is used in feet high and 4 feet in diameter, has 132 America, consists of a series of cast-iron square feet of heating surface. Fig. 3 spheres with necks, which are faced, and is an excellent design known as the so connection is made by means of long “Talbot,” which presents considerable bolts. The advantages of sectional boilheating surface; the furnace is of cast ers are summarized in the paper of Mr. iron, which in a recent modification is Flannery,* where there is also a quotalined with firebrick, and the ash box acts tion from the third report of the Admias a water heater. These boilers have ralty on the subject. There is a large
given good results in practice, and the number of inventions of both vertical advantage of a non-tubulous boiler with and sectional boilers, but the foregoing large heating surface is realized by those examples are typical of the forms genwho have experienced the cost and trou- erally used. A small vertical boiler comble which tubes often entail. Fig. 5 is a monly has the engine attached to it, and view of an ordinary sectional boiler, and forms its support; this may be effected it will be seen that the amount of heat- by bolting the engine either to its side, ing surface exposed must cause it to be or by means of a cast-iron bedplate or a a rapid steam generator. The arrange wrought-iron ring securing it upon its ment a, a, a of firebrick and the iron top. Portability is thus secured, and plates b, b, b divert the flames, and com- the evils arising from alternate pel them to pass well round the tubes, pansion and contraction of the boiler, after which they escape by the fue c. A portion of the steam space is shown | 128.
* Vide Minutes of Proceedings Inst. C.E., vol. liv. p.
which would be felt in a large engine, are so different in these latter, that no are not in this case appreciable, chiefly comparison can fairly be drawn. This because when used on a small scale the information being required, and also engine is only connected with a fixed data concerning the working of small external object by a belt or bearing at a steam engines of the ordinary type, little distance from it; but it has been through the kindness of Mr. Fry, Manremarked that with small marine en aging Director of the Bristol Wagon gines, where the case is different, the ob- Works, and of Mr. Arrowsmith, Steam jection is considerable. One excellent Printing Works, Bristol, two series of example of the latter method is afforded experiments were made upon a 2-HP. by the design shown in Fig. 3, in which vertical engine and boiler, manufactured the cylinder is let into the boiler, and, by Dodman, of King's Lynn, belonging besides the benefit of short steam con- to the former, and a 4-HP. horizontal nections, a perfect steam jacket is formed. engine called the Soho, and a vertical
The efficiency of small boilers is a boiler made by Tangye, of Birmingham, most important thing to consider, and, belonging to the latter. The data now so largely are they at present in use in given will refer only to the boilers, that
in conjunction with small engines, of the engines being introduced herethat it would be impossible to form a after. During the two days' trial of the satisfactory comparison between small former, which will be called No. 1, and motors without some reliable data on the one day's trial of the latter, No. 2, the subject. Now the paper of Mr. Flan- difficulty was experienced of keeping nery indicates a high efficiency tor sec. uniform the quantity of water and the tional boilers, but these are not used to pressure of steam; but during a run of anything like the same extent as the ver- three hours on No. 1, and of four hours tical form; and on turning to different with No. 2, by careful stoking and atsources for information, the author was tention, and by taking observations unable to obtain independent results. every quarter of an hour of revoluFor instance, in the comprehensive man- tions, pressure of steam, and supply of ual of Mr. D. K. Clark, † M. Inst. C.E., feeding water—a close approximation to although boilers of marine, locomotive, regularity was obtained. Both the dystationary, and portable engines are fully namometer and the indicator were used, treated, yet the class in question is not and the quantity of feed water and coals mentioned; and the ratio of heating sur- carefully weighed, and, as will be shown face to grate area and other conditions in No. 2, the volumes of steam used + A manual of, "Rules, Tables, and Data for Mechan the cylinder and ports obtained by actu
agreed fairly well with the volume of
ally filling these spaces with water after- been said it is seen that these boilers are warıls, so that little, if any, priming being superseded by more efficient ones, could have taken place in No. 2, while with larger heating surface, as Latham's in No. 1 the difference must not be over- and Bradley's, which, though little larlooked, and the boiler efficiency in this ger than No. 2, has five times as much case can only be approximate. No. 1 heating surface, and the field, of which was of the form shown in Fig. 1, and there are 1,380 in use; but the introducwas well lagged all over; No. 2 was of tion of more heating surface by tubes, the form represented in Fig. 2, but was &c., offers a larger surface for corrosion deficient in heating surface, having no and leakage, and therefore involves more pendent tubes and but one cross tube, repairs; and the general conclusion is circular in section.
that the necessity for small boilers is the
TABLE OF RESULTS OF EXPERIMENTS ON SMALL VERTICAL BOILERS.
The feed water was warmed to about | weak point in the use of steam on a 100° Fahrenheit. The coal used with small scale. No. I was Parkfield; an analysis, with There are four ways of using steam on which the author was kindly furnished a small scale, which seem to lead to the by Mr. Handal Cossham, shows 82 per following classification: cent. of carbon, 5.6 per cent. of hydro (1.) Single-acting engines using high gen, and 6.1 percent. of oxygen, and pressures and speeds. has a heating power of 14.3 pounds of (2.) Double-acting engines using modwater, at 2120 Fahrenheit, converted erate pressures and speeds. into steam; so that, theoretically, the (3.) Rotary engines. evaporation should have been from 100° (4.) Apparatus for causing the pressFahrenheit, about 12.5, instead of 3.3... lure or impulse of steam on fluid sur3.3
faces to directly raise or force that fluid. Efficiency= = 0.26. With No. 2 the The first three are always non-con12.5
densing engines; the last kind can hardevaporation should have been about the ly be called engines, being apparatus of same, making its efficiency=0.39. Thus, the injector or pulsometer type; but as although the ratio of heating surface of they are used principally as a substitute No. 2 was considerably less than No. 1, for other kinds of steam pumps, or manthe evaporative power was greater, ual operations, they must certainly be which seems only to be accounted for by considered as included by the term the fruct that most of the heating sur. Small Motive Power. Thus it may be face of the latter was vertical. If the said that all small steam engines are nonaverage evaporative power of large condensing. This is obviously because boilers from 100° Farhenheit for this the gain due to the partial absence of kind of coal be taken at 0.65, it is seen back pressure, which condensation sehow much inferior these small boilers are cures, would not be sufficient to compento larger ones, although from what has sate for the increased complication and