BOILERS, STEAM. During the last ten years no special improvements have been made in the construction of steam-boilers in the direction of Improving their economy of fuel; in fact, further progress in this direction is scarcely possible in boilers fired with anthracite coal, since many years ago boilers were made which have given results equal to about 80 per cent of the theoretical efficiency of the fuel. As the chimney gases carry off as a minimum about 15 per cent of the heat of the fuel, and losses due are generally not less than 5 per cent, it is readily seen that the margin left for further saving is extremely slight. As a lb. of pure carbon is capable of generating 14,500 thermal units, equivalent to an evaporation of 15 lbs. of water from and at 212° per lb. of carbon, an efficiency of 80 per cent is equal to an evapo ration of 12 lbs. of water from and at 212° per lb. of combustible. Now nearly this result has been reached in actual test is shown by the results of the boilers tested at the Centennial Ex hibition at Philadelphia, in 1876, Out of fourteen boilers tested, the five highest in the lint, in order of economy, gave results as follows (Reports of the Judges of Group XX. Centennial Exhibition Reports): These boilers were of different types, as shown in Vol. I of this work.
The Firmenich, Root, and Babcock & Wilcox boilers were of the water-tube type. The Lowe boiler was an externally tired, horizontal tubular boiler of peculiar design. The Smith boiler was a horizontal tubular boiler with a set of NWater4 ulie appendages in the furnace, and the Galloway boiler was an internally fired shell-boiler with conical-shaped water-tubes cross ing the large internal line. Results with anthracite coal exceeding 12 lbs. evaporation from and at 212' per lb. of combustible have been frequently reported, but they are scarcely credi ble, since they would require an efficiency of over 80 per cent, and an allowance for the heat carried off in the chimney gases less than the actual and necessary loss. With semi-bitumi nous coal, however. containing less than 20 per cent volatile matter, the theoretical heating value being greater than 14,500 heat units, an evaporation of even 11 lbs. from and at 212° is not impossible; but this assumes a perfect combustion of the coal in the furnace, which can scarcely be reached in practice with ordinary boiler-furnaces on account of sonic of the gases evolved from the coal being chilled by the iron surfaces of the boiler, and therefore escaping unburned. A result of 12-5 lbs. with Cumberland coal is. however, frequently obtained, and this with quite a variety of types of boiler. It may be stated as a general proposition that any boiler, of whatever type (1), in the furnace of which the coal is thoroughly burned with no greater excess of air than is necessary to effect complete combustion, giving consequently the highest practically attainable temperature in the furnace (2). which has its heating snr face in a clean condition, so placed as to be completely and uniformly passed over by the currents of heated gases, and (3) sufficient extent of heating surface SO that it will absorb all the available heat in the gases above the temperature of the steam, is capable of giving the maximum economical result which can be obtained in the best type of boiler.
This conclusion is also derived from the results of numerous practical tests, as shown in the tests reported by Mr. C. 11. Barrus, hereafter referred to. .Nevertheless, the average steam boiler usually gives an economical result far below the maximum, so that possibly 60 per cent of the theoretical efficiency is nearer the average result than 80. This is accounted for by im proper construction of the boiler or setting, by unclean surfaces inside and out, by insufficient obstruction in the boiler-tubes and fines to the passages of gas, whereby the latter is circuited," or selects some passages rather than others, as in the horizontal tubular boiler, in which the tendency of the gases is to flow through the upper rows of tubes rather than through the lower, by improper proportions of grate and heating surface for the character of the coal used and for the draft pressure by improper firing, or by leaks of air through the setting. With bituminous coal the difficulty of obtaining maximum economy is greatly increased, on account of the fact that the right kind of furnace for burning such coal under a steam-boiler has not yet been invented. In all parts of the United States west of the Alleghany Mountains there is an enormous waste of fuel constantly going on for this reason. Economy of fuel therefore being independent of the type of boiler, the desirable qualities of boiler which arc to be sought for, and which depend largely upon the type, are : safety, low first cost, low cost of maintenance, accessibility for cleaning and for repairs, non-liability to destruction from expansion and contraction and from external corrosion, simplicity of construction, and small space occupied. It is not possible to combine all these desirable qualities in a single boiler ; for instance, the boiler of lowest first cost is generally high in cost of maintenance and repairs. and unsafe. In many boilers several desirable qualities are sacrificed to one pre requisite, as portability. A locomotive-boiler is one of the worst possible forms where the water is impure, but no other boiler can be used on a locomotive. In the attempt to combine as many as possible of these good qualities in a single boiler, and in the fallacious hope of im proving on the economy of established types, hundreds of new boilers have been invented during the last ten years, and many put on the market, in which the first principles of good construction are violated. These new boilers, however, generally disappear from the market in a few years, and they do not prevent the course of progress toward the use of a few standard types only, each adapted to certain locations. In these types there is nothing new in general principles of construction, and such improvements as have been made are confined to details.