CONDITION OF THE BOILER. The sweeping of the boiler tubes, when in regular service, was done once in eight days. Three days before the date of the experiment they had been swept, so that they were in an average condition as regards soot. The other fire surfaces had not been swept for seven months, and must have been thickly coated with the soot due to the combustion of about 450 tons of coal in that time. The water heating surfaces, on the water side, had not been cleaned for seven months, during which time about 3370 tons of water had been vaporized from them. This water, taken from the Vesdre river, was very pure and left no scale. An analysis of the feed-water, previous to its entrance into the boiler, was made; and another analysis was made of the water in the boiler, which, as none of it had been blown out or removed, except by vaporization and foaming, for seven months, contained in solution nearly all the mineral matter entering with the water during that time, with the exception of what may have been precipitated as scale or sludge. The following are the results of these two very interesting analyses, and show the centessimal composition of the waters: In comparing the above two analyses, there must be recollected: 1st. That the feed-water being very pure, some of the substances in solution, and notably the nitric acid, could not be determined with absolute precision. 2d. That the feed-water, as drawn from the river Vesdre, necessarily varies slightly in composition from day to day; hence the single specimen whose analysis is first given in the above table may not, and probably does not, correspond exactly to the water whose analysis is last given, representing a mean of the feed-water during seven months. 3d. That a portion of the sulphuric acid and of the lime has disappeared to form scale and sludge. Notwithstanding the above uncertainties, these analyses are of much value from their rarity, and for the information they give in relation to the persistence of the chlorides of calcium and sodium, and of organic matter, in the fresh water of land boilers. EXPERIMENT TO ASCERTAIN THE POTENTIAL AND ECONOMIC VAPORIZATION OF WATER BY THE BOILER WITH SEMI-BITUMINOUS COAL. The experiment continued one working day of 13 consecutive hours,, during which the quantity of water fed into the boiler was previously measured in a tank with great accuracy, the temperature being noted every half hour. The steam, as fast as generated, was worked off by the engine, the boiler pressure being maintained nearly uniform, and noted every half hour, also. The coal thrown into the furnaces, and the refuse therefrom, in ash, clinker, etc., were carefully weighed. The fires, at the commencement, were started with just wood. enough to light the coal, and at the end of the experiment they were burnt completely out, so that the weight of coal consumed was determined exactly. The water in the boiler at the commencement of the · experiment had a less temperature than at its end, and there was, likewise, a slight variation in the quantity; these differences were carefully ascertained, and the weight of water vaporized, per tank, corrected for them. As the day on which the experiment was made was a regular working day, intervening between two other working days, and as the fires were banked at night, the brick masonry was at about the average temperature at the commencement of the experiment, and did not require the expenditure of fuel to heat it up. The coal was burnt at its maximum rate of combustion. The draught of the chimney was poor. The following are the data and results of the experiment: Square feet of grate surface per square foot of draught grate surface, 11.0714 square per foot of 48.5897 13. Duration of the experiment in consecutive hours, Total number of pounds of semi-bituminous coal con- 40398. 5203 Total number of pounds of refuse from the coal, in ash, 1, 617- Total number of pounds of combustible, or gasifiable Per centum of the coal in refuse, Number of pounds of coal consumed per hour, Number of pounds of combustible consumed per hour, Pounds of combustible consumed per hour per square foot of grate, Pounds of coal consumed per hour per square foot of heating surface, Pounds of combustible consumed per hour per square Number of pounds of water that would have been vapo- Pounds of water vaporized from 100° Fahrenheit by Pounds of water vaporized from 100° Fahrenheit by Pounds of water vaporized' from 212° Fahrenheit by Pounds of water vaporized from 212° Fahrenheit by 4586' 11.859 400-2300 352.7700 12.9106 11.3800 0.2657 0.2342 43501.7525 48584.5248 8.3609 9.4858 * 9.3378 10.5941 In the above experiment all the conditions were combined that should have given a maximum economic result. There was a very small calorimeter above the bridge-walls, whereby the air dilution of the gases of combustion was kept at a minimum. There was only a medium rate of combustion per square foot of grate per hour, and a very slow rate of combustion per square foot of heating-surface. The ratio of water-heating surface to grate surface was very large, and the direction of the gases of combustion in their route from furnace to chimney was several times changed, so as to thoroughly mix and com bine them. The coal was of excellent steam-producing quality. The experiment was of such short duration that, in connection with the small per centum of refuse from the coal, there could have been but little loss from the cleaning of fires; and the radiation from the thick walls of brick masonry must have been insignificant. Yet, notwithstanding all these favorable circumstances, the economic vaporization under the atmospheric pressure was only 10-5941 pounds of water from the temperature of 212 degrees Fahrenheit per pound of the combustible or gasifiable portion of the coal. This vaporization should have been not less than 12 pounds of water, which is what has been obtained in ordinary marine boilers during much longer experiments, with poorer proportions for economy of fuel, and not better coal, the rate of combustion per hour per square foot of heating surface being about the same. Here is a loss of economic effect of not less than 11 per centum, and to what is it to be attributed? Probably to the infiltration of air through the masonry enclosing the boiler shells. The comparatively cold external air so entering mixes with the hot gases of combustion and cools them by extracting a portion of their heat. The result is a less difference between the temperature of the mixture and that of the boiler water than would have been between the latter and the gases of combustion if unmixed, accompanied by a correspondingly less extraction of heat in equal time. Another injurious result is that the cool infiltered air, after absorbing heat from the gases of combustion, passes out with them at the same chimney temperature; the loss of heat thus caused being measured by the weight of air entering, its specific heat, and the difference of its temperature when entering and leaving the boiler. The cooling effect of the infiltering air may even check the combustion of any uncombined coal gases in the tubes and connections. The greater the rate of combustion in a boiler set in masonry the greater will be the absolute quantity of air inleaked, because the greater rate of combustion is necessarily associated with a less pressure inside the masonry, that of the external air remaining constant. In fact, it is this less pressure which gives the greater draught, producing the greater rate of combustion. But, as this greater rate of combustion causes a correspondingly greater quantity of hot gases to pass through the masonry flues in equal times, the per centum of loss by the inleaked air, that is, the loss relative to the absolute quantity of heat in the hot gases, will not be materially changed by change in the rate of combustion, within the limits of ordinary practice. During the many experiments on boiler vaporization made or investigated by the writer an inferior result was always found with boilers of the same type, having the same proportions of heating to grate surface, and of calorimeter; burning the same coal at the same rate of combustion per hour per square foot of grate surface; but having their furnaces and flues in brick masonry instead of in iron shells; and just in proportion to the extent of the brick surface with which the gases. of combustion were in contact. Whenever the highest vaporization is desired from the fuel consumed at a given rate of combustion, brick settings should be wholly discarded and all parts of the boiler arranged within an iron shell; and the difference will be the greater with boilers having the least ratio of water-heating to grate surface, because in them the gases of combustion enter the chimney with the highest temperature, and the infiltered air carries off a correspondingly greater quantity of heat. Were it possible for the gases of combustion to leave the boiler with the temperature of the atmosphere, there could, of course, be no economic loss. by any amount of air leaked in, for whatever heat was imparted to it on entering would be yielded up before leaving; the loss will be just in proportion to the temperature of the chimney, other things equal. When it is considered that walls of brick masonry are easily pervious to water, there can be no difficulty in believing they are still more easily permeable by air. If brick-work be adopted for the setting of boilers, an air space of about 2 inches width should be left in the masonry, whose entire exterior should be well coated with pitch, or with a cement of mastic, to stop the inleakage of air as much as possible. This, however, the writer has never known done; and he has always observed that, after a short period of use, the masonry was more or less warped by heat, and cracked, the inleakage of air becoming correspondingly increased. When a boiler set in brick-work is used during only a portion of the day, as is generally the case, a serious loss of fuel is experienced in heating the mass of masonry from the temperature it has when the boiler isout of use to its temperature when the boiler is in operation. This loss is measured by the weight of the masonry into the product of its specific heat by the difference of the temperatures stated. A boiler set in brick-work will also lose more heat by radiation. |