CHIMNEYS. [From the "Building and Engineering Times."] The object of a chimney is to produce the draught necessary for the proper combustion of the fuel, as well as to furnish a means of discharging the noxious products of combustion into the atmosphere at such a height from the ground that they may not be considered a nuisance to people in the vicinity of the chimney. Regarding the second of the above purposes for which chimneys are built, it need only be said, that it is of secondary importance only, and that where due attention is given to the proper methods of setting boilers, and proportionating grate areas, furnaces, and rate of combustion, the smoke nuisance is comparatively unknown, and is of no practical importance whatever. The main points to be considered in designing chimneys are the right proportions to insure, first, a good and sufficient draught, and, second, stability. Without entering into any demonstration of the velocity of the flow of the heated gases through the furnace and flues leading into and up the chimney, we will briefly state a few of the principles governing the dimensions of chimneys. The motive power or force which produces the draught is the action of gravity upon the difference in the specific gravities of the heated column of the gases of combustion inside the chimney, and the atmosphere at its normal temperature outside of the chimney, by which the former is forced up the flue; and the laws governing its velocity are the same as those governing the velocity of a falling body; and it can be proved that its velocity, and, consequently, the amount or volume of air drawn into the furnace, and which constitutes the draught, is in proportion to the square root of the height of the chimney. It is a common error that the force of the draught is in direct proportion to the height; so that, with two chimneys of the same area of flue, one being twice the height of the other, the higher one would produce a draught twice as strong as the other. The intensity of draught under these circumstances would be in the proportion of the square root of 1 to the square root of 2, or as 1 to 1.42. To double the draught-power of any given chimney by adding to the height, it would be necessary to build it to four times the origi nal height. Practically there is a limit to the height of a chimney of any given area of flue, beyond which it is found that the additional height increases the resistance due to the velocity and friction more rapidly than it increases the flow of cold air into the furnace. For chimneys not over forty-two inches in diameter, the maximum admissible height is about three hundred feet. From an investigation of the same laws we find that the velocity of the flow of cold air into the furnace is in proportion to the square root of the ratio between the density of the outside air and the difference in the densities of the outside air and the heated gases in the chimney; from which we may deduce the fact that very little increase of draught is obtained by increasing the temperature of the gases in the chimney above 550 or 600 degrees F. By raising the temperature of the flue from 600 to 1200 degrees we would increase the draught less than twenty per cent, while the waste of heat would be very considerable. Conversely, we may reduce the temperature of the flue about one-half, when the temperature is as high as six hundred degrees, by means of an economizer or otherwise, and the reduction of draught-force would be only about twenty per cent, as before. It is found that the principal causes which act to impair the draught of a chimney, and which vary greatly with different types of boilers and settings, are the resistance to the passage of the air offered by the layer of fuel, bends, elbows, and changes in the dimensions of the flues, roughness of the masonry of brick flues, holes in the passages which allow the entrance of cold air, and, generally, any variation from a straight, air-tight passage of uniform size from combustion-chamber to chimney-flue; and the resistance to draught is in direct proportion to the magnitude and number of such variations. In designing a chimney, it is, therefore, always necessary to consider the type of boiler, method of setting, arrangement of boilers and flues, location of chimney, and every thing which will be likely to in any way interfere with its efficient performance. Much, of course, depends upon the judgment and experience of the designer, and it would be impossible to give any general rule which would cover all cases. When only one boiler discharges into a chimney, for instance, the chimney requires a much larger area per pound of fuel burned than when several similar boilers discharge into a chimney of the same height; and, taking all these varying circumstances into consideration, a great deal of judgment is, in many cases, required to determine the proper dimensions. It is a common idea that a "chimney cannot be too large:" in other words, the larger the area of the flues, the better the draught will be. But this is not always the case. In many cases where a chimney has been built large enough to serve for future additions to the boiler-power, the draught has been much improved as additional boilers have been set at work. The cause of this is to be found in the increased steadiness of draught where several boilers are at work and are fired successively, as well also as in the better maintenance of the temperature of the flue; as the velocity of the gases necessarily increases with the increased amount required to be discharged, and they do not have time to cool off to so great an extent as when they move more slowly. RULES FOR PROPORTIONING CHIMNEYS. [Published by the Babcock & Wilcox Co., of New York.] Chimneys are required for two purposes-1st, to carry off obnoxious gases; 2d, to produce a draught, and so facilitate combustion. The first requires size, the second height. Each pound of coal burned yields from 13 to 30 pounds of gas, the volume of which varies with the temperature. The weight of gas carried off by a chimney in a given time depends upon three things-size of chimney, velocity of flow, and density of gas. But as the density decreases directly as the absolute temperature, while the velocity increases, with a given height, nearly as the square root of the temperature, it follows that there is a temperature at which the weight of gas delivered is a maximum. This is about 550° above the surrounding air. Temperature, however, makes so little difference, that at 550° above, the quantity is only four per cent. greater than at 300°. Therefore, height and area are the only elements necessary to consider in an ordinary chimney. The intensity of draught is, however, independent of the size, and depends upon the difference in weight of the outside and inside columns of air, which varies directly with the product of the height into the difference of temperature. This is usually stated in an equivalent column of water, and may vary from 0 to possibly 2 inches. To find the maximum draught for any given chimney, the heated column being 612° F., and the external air 62° : Multiply the height above grate in feet by .0075, and the product is the draught power in inches of water. The intensity of draught required varies with the kind and condition of the fuel, and the thickness of the fires. Wood requires the least, and fine coal or slack the most. To burn anthracite slack to advantage, a draught of 11 inch of water is necessary, which can be attained by a well-proportioned chimney 175 ft. high. A round chimney is better than square, and a straight flue better than tapering, though it may be either larger or smaller at top without detriment. The effective area of a chimney, for a given power, varies inversely as the square root of the height. The actual area, in practice, should be greater, because of retardation of velocity due to friction against the walls. On the basis that this is equal to a layer of air two inches thick over the whole interior surface, and that a commercial horse-power requires the consumption of an average of 5 pounds of coal per hour, we have the following formulæ : = (1) (2) (3) (4) (5) In which H= horse-power; h height of chimney in feet; E = effective area, and A actual area in square feet; 8= side of square chimney, and D = dia. of round chimney in inches. The following table is calculated by means of these formulæ, by MR. Wм. Kent: SIZES OF CHIMNEYS WITH APPROPRIATE HEIGHT OF CHIMNEYS, AND COMMERCIAL HORSE-POWER. Side of Actual area, 88723887** Diameter in inches. 18 30 24 49 54 58 62 36 216 231 83 113 141 536 565 593 632 692 748 :: 1496 1639 1770 1893 80 5.47 7.07 35 6.57 8.30 38 7.76 9.62 10.44 12.57 43 48 13.51 15.90 54 16.98 19.64 40.19 44.18 46.01 50.27 The external diameter at the base should be one-tenth the height, unless it be supported by some other structure. The "batter" or taper of a chimney should be from 3 to 4 inch to the foot cn each side. Thickness of brick work: one brick (8 or 9 inches) for 25 feet from the top, increasing brick (4 or 4 inches) for each 25 feet from the top downward If the inside diameter exceed 5 feet the top length should be 11 bricks, and if under 3 feet it may be brick for ten feet. EXAMPLES OF LARGE CHIMNEYS.1 The Townsend Chimney, Port Dundas, Glasgow. -This is one of the tallest, if not the tallest chimney in the world. It was designed by Mr. Robert Corbett, of Glasgow, for Mr. Joseph Townsend, of the Crawford Street Chemical Works. It rests on blue clay, "solid as a rock." The foundation consists of thirty courses of bricks on edge, the lowest course being 50 feet and the top course 32 feet in diameter. The inside lining, or cone, is of 9-inch fire-brick and 60 feet in height, built distinct from the chimney proper, with air space between and covered on top to prevent dust from falling in, but built with open work in the upper four courses, so as to allow of air passing into the chimney. The chimney is 454 feet high above the ground level, and is built of brick, the thickness of the wall varying as follows: 1st section, 30 feet in height, 5 feet 7 inches thick. Iron hoops were built in the chimney at a distance of 9 inches from the surface at the bottom and 4 inches at the top, and at in 1 The best modern work on Tall Chimney Construction is by R. M. and F. J. Bancroft, published in England, for sale by W. T. Comstock, New York. |