Size of Chimneys for Steam-boilers. Formula, H.P. 3.33(A — 0.6 VA) WH. (Assuming 1 H. P. 5 lbs. of coal burned per hour.) 144 113.10 106.72 4352 4701 5026 5331 5618 6155 128 For pounds of coal burned per hour for any given size of chimney, multiply the figures in the table by 5. 1. The draught power of the chimney varies as the square root of the height. 2. The retarding of the ascending gases by friction may be considered as equivalent to a diminution of the area of the chimney, or to a lining of the chimney by a layer of gas which has no velocity. The thickness of this lining is assumed to be 2 inches for all chimneys, or the diminution of area equal to the perimeter x 2 inches (neglecting the overlapping of the corners of the lining). Let D diameter in feet, A area, and E effective area in square feet. For simplifying calculations, the coefficient of A may be taken as 0.6 for both square and round chimneys, and the formula becomes E = A-0.6 VA. 3. The power varies directly as this effective area E. 4. A chimney should be proportioned so as to be capable of giving sufficient draught to cause the boiler to develop much more than its rated power, in case of emergencies, or to cause the combustion of 5 lbs. of fuel per rated horse-power of boiler per hour. 5. The power of the chimney varying directly as the effective area, E, and as the square root of the height, H, the formula for horse-power of boiler for a given size of chimney will take the form H.P. CE VH, in which C is a constant, the average value of which, obtained by plotting the results obtained from numerous examples in practice, the author finds to be 3.33. The formula for horse-power then is H.P. = 3.33E √H, or H.P. = 3.33(A .6 VA) VH. If the horse-power of boiler is given, to find the size of chimney, the height being assumed, E = 0.3 H.P.÷H.; = A − 0.6☎, = diam. of E+4". WE+4". For round chimneys, diameter of chimney For square chimneys, side of chimney If effective area E is taken in square feet, the diameter in inches is d = 13.54 E+4", and the side of a square chimney in inches is s = 12 VĒ+4". 0.3 H. P.2 If horse-power is given and area assumed, the height H = E In proportioning chimneys the height is generally first assumed, with due consideration to the heights of surrounding buildings or hills near to the proposed chimney, the length of horizontal flues, the character of coal to be used, etc., and then the diameter required for the assumed height and horse-power is calculated by the formula or taken from the table. An approximate formula for chimneys above 1000 H.P. is H.P. == 22 D2VH. This gives the H.P. somewhat greater than the figures in the table. The Protection of Tall Chimney-shafts from Lightning. -C. Molyneux and J. M. Wood (Industries, March 28, 1890) recommend for tall chimneys the use of a coronal or heavy band at the top of the chimney, with copper points 1 ft. in height at intervals of 2 ft. throughout the circumference. The points should be gilded to prevent oxidation. The most approved form of conductor is a copper tape about 4 in. by in. thick, weighing 6 ozs. per ft. If iron is used it should weigh not less than 24 lbs. per ft. There must be no insulation, and the copper tape should be fastened to the chimney with holdfasts of the same material, to prevent voltaic action. An allowance for expansion and contraction should be made, say 1 in. in 40 ft. Slight bends in the tape, not too abrupt, answer the purpose. For an earth terminal a plate of metal at least 3 ft. sq. and 1/16 in. thick should be buried as deep as possible in a damp spot. The plate should be of the same metal as the conductor, to which it should be soldered. The best earth terminal is water, and when a deep well or other large body of water is at hand, the conductor should be carried down into it. Right-angled bends in the conductor should be avoided. No bend in it should be over 30°. NOTES ON THE ABOVE CHIMNEYS.-1. This chimney is situated near Freiberg, on the right bank of the Mulde, at an elevation of 219 feet above that of the foundry works, so that its total height above the sea will be 71134 feet. The works are situated on the bank of the river, and the furnacegases are conveyed across the river to the chimney on a bridge, through a pipe 3227 feet in length. It is built throughout of brick, and will cost about $40,000.-Mfr. and Bldr. 2. Owing to the fact that it was struck by lightning, and somewhat damaged, as a precautionary measure a copper extension subsequently was added to it, making its entire height 488 feet. 1, 2, 3, and 4 were built of these great heights to remove deleterious gases from the neighborhood, as well as for draught for boilers. 5. The structure rests on a solid granite foundation, 55 x 30 feet, and 16 feet deep. In its construction there were used 1,700,000 bricks, 2000 tons of stone, 2000 barrels of mortar, 1000 loads of sand, 1000 barrels of Portland cement, and the estimated cost is $40,000. It is arranged for two flues, 9 feet 6 inches by 6 feet, connecting with 40 boilers, which are to be run in connection with four triple-expansion engines of 1350 horse-power each. 6. It has a uniform batter of 2.85 inches to every 10 feet. Designed for 21 boilers of 200 H. P. each. It is surmounted by a cast-iron coping which weighs six tons, and is composed of thirty-two sections, which are bolted together by inside flanges, so as to present a smooth exterior. The foundation is in concrete, composed of crushed limestone 6 parts, sand 3 parts, and Portland cement 1 part. It is 40 feet square and 5 feet deep. Two qualities of brick were used; the outer portions were of the first quality North River, and the backing up was of good quality New Jersey brick. Every twenty feet in vertical measurement an iron ring, 4 inches wide and 34 to 1⁄2 inch thick, placed edgewise, was built into the walls about 8 inches from the outer circle. As the chimney starts from the base it is double. The outer wall is 5 feet 2 inches in thickness, and inside of this is a second wall 20 inches thick and spaced off about 20 inches from main wall. From the interior surface of the main wall eight buttresses are carried, nearly touching this inner or main flue wall in order to keep it in line should it tend to sag. The interior wall, starting with the thickness described, is gradually reduced until a height of about 90 feet is reached, when it is diminished to 8 inches. At 165 feet it ceases, and the rest of the chimney is without lining. The total weight of the chim ney and foundation is 5000 tons. It was completed in September, 1888. 7. Connected to 12 boilers, with 1200 square feet of grate-surface. Draughtgauge 1 9/16 inches. 8. Connected to 8 boilers, 6' 8" diameter x 18 feet. square feet. Grate-surface 448 9. Connected to 64 Manning vertical boilers, total grate surface 1810 sq. ft. Designed to burn 18,000 lbs. anthracite per hour. 10. Designed for 12.000 H.P. of engines; (compound condensing). 11. Grate-surface 434 square feet; H.P. of boilers (Galloway) about 2500. 13. Eight boilers (water-tube) each 450 H.P.; 12 engines, each 300 H.P. Plant designed for 36,000 incandescent lights. For the first 60 feet the exterior wall is 28 inches thick, then 24 inches for 20 feet, 20 inches for 30 feet, 16 inches for 20 feet, and 12 inches for 20 feet. The interior wall is 9 inches thick of fire-brick for 50 feet, and then 8 inches thick of red brick for the next 30 feet. Illustrated in Iron Age, January 2, 1890. A number of the above chimneys are illustrated in Power, Dec., 1890. Chimney at Knoxville, Tenn., illustrated in Eng'g News, Nov. 2, 1893. 6 feet diameter, 120 feet high, double wall: Exterior wall, height 20 feet, 30 feet, 30 feet, 40 feet; 66 66 thickness 21% in., 17 in., 13 in,, 8% in.; Interior wall, height 85 ft., 35 ft., 29 ft., 21 ft.; 66 66 thickness 131⁄2 in., 81⁄2 in., 4 in., 0. Exterior diameter, 15' 6" at bottom; batter, 7/16 inch in 12 inches from bot. tom to 8 feet from top. Interior diameter of inside wall, 6 feet uniform to top of interior wall. Space between walls, 16 inches at bottom, diminishing to 0 at top of interior wall. The interior wall is of red brick except a lining of 4 inches of fire-brick for 20 feet from bottom. Stability of Chimneys.-Chimneys must be designed to resist the maximum force of the wind in the locality in which they are built, (see Weak Chimneys, below). A general rule for diameter of base, of brick chimneys, approved by many years of practice in England and the United States, is to make the diameter of the base one tenth of the height. If the chimney is square or rectangular, make the diameter of the inscribed circle of the base one tenth of the height. The "batter" or taper of a chimney should be from 1/16 to 14 inch to the foot on each side. The brickwork should be one brick (8 or 9 inches) thick for the first 25 feet from the top, increasing brick (4 or 41⁄2 inches) for each 25 feet from the top downwards. If the inside diameter exceed 5 feet, the top length should be 11⁄2 bricks; and if under 3 feet, it may be brick for ten feet. (From The Locomotive, 1884 and 1886.) For chimneys of four feet in diameter and one hundred feet high, and upwards, the best form is circular, with a straight batter on the outside. A circular chimney of this size, in addition to being cheaper than any other form, is lighter, stronger, and looks much better and more shapely. Chimneys of any considerable height are not built up of uniform thickness from top to bottom, nor with a uniformly varying thickness of wall, but the wall, heaviest of course at the base, is reduced by a series of steps. Where practicable the load on a chimney foundation should not exceed two tons per square foot in compact sand, gravel, or loam. Where a solid rockbottom is available for foundation, the load may be greatly increased. If the rock is sloping, all unsound portions should be removed, and the face dressed to a series of horizontal steps, so that there shall be no tendency to slide after the structure is finished. All boiler-chimneys of any considerable size should consist of an outer stack of sufficient strength to give stability to the structure, and an inner stack or core independent of the outer one. This core is by many engineers extended up to a height of but 50 or 60 feet from the base of the chimney, but the better practice is to run it up the whole height of the chimney; it may be stopped off, say, a couple feet below the top, and the outer shell con tracted to the area of the core, but the better way is to run it up to about 8 or 12 inches of the top and not contract the outer shell. But under no cir cumstances should the core at its upper end be built into or connected with the outer stack. This has been done in several instances by bricklayers, and the result has been the expansion of the inner core which lifted the top of the outer stack squarely up and crecked the brickwork. For a height of 100 feet we would make the outer shell in three steps, the first 20 feet high, 16 inches thick, the second 30 feet high, 12 inches thick, the third 50 feet high and 8 inches thick. These are the minimum thicknesses admissible for chimneys of this height, and the batter should be not less than 1 in 36 to give stability. The core should also be built in three steps, each of which may be about one-third the height of the chimney, the lowest 12 inches, the middle 8 inches, and the upper step 4 inches thick. This will insure a good sound core. The top of a chimney may be protected by a cast-iron cap; or perhaps a cheaper and equally good plan is to lay the ornamental part in some good cement, and plaster the top with the same material. Weak Chimneys.-James B. Francis, in a report to the Lawrence Mfg. Co. in 1873 (Eng'g News, Aug. 28, 1880), gives some calculations concerning the probable effects of wind on that company's chimney as then constructed. Its outer shell is octagonal. The inner shell is cylindrical, with an air-space between it and the outer shell; the two shells not being bonded together, except at the openings at the base, but with projections in the brickwork, at intervals of about 20 ft. in height, to afford lateral support by contact of the two shells. The principal dimensions of the chimney are as follows: Height above the surface of the ground... .211 ft. 23 in. Diameter of the inscribed circle of the octagon near the ground. 15" ........... 11% 66 66 334" One tenth of the height for the diameter of the base is the rule commonly adopted. The diameter of the inscribed circle of the base of the Lawrence Manufacturing Company's chimney being 15 ft., it is evidently much less than is usual in a chimney of that height. Soon after the chimney was built, and before the mortar had hardened, it was found that the top had swayed over about 29 in. toward the east. This was evidently due to a strong westerly wind which occurred at that time. It was soon brought back to the perpendicular by sawing into some of the joints, and other means. The stability of the chimney to resist the force of the wind depends mainly on the weight of its outer shell, and the width of its base. The cohesion of the mortar may add considerably to its strength; but it is too uncertain to be relied upon. The inner shell will add a little to the stability, but it may be cracked by the heat, and its beneficial effect, if any, is too uncertain to be taken into account. The effect of the joint action of the vertical pressure due to the weight of the chimney, and the horizontal pressure due to the force of the wind is to shift the centre of pressure at the base of the chimney, from the axis toward one side, the extent of the shifting depending on the relative magnitude of the two forces. If the centre of pressure is brought too near the side of the chimney, it will crush the brickwork on that side, and the chimney will fall. A line drawn through the centre of pressure, perpendicular to the direction of the wind, must leave an area of brickwork between it and the side of the chimney, sufficient to support half the weight of the chim. ney; the other half of the weight being supported by the brickwork on the windward side of the line. Different experimenters on the strength of brickwork give very different results. Kirkaldy found the weights which caused several kinds of bricks, laid in hydraulic lime mortar and in Roman and Portland cements, to fail slightly, to vary from 19 to 60 tons (of 2000 lbs.) per sq. ft. If we take in this case 25 tons per sq. ft., as the weight that would cause it to begin to fail, we shall not err greatly. To support half the weight of the outer shell of the chimney, or 322 tons, at this rate, requires an area of 12.88 sq. ft. of brickwork. From these data and the drawings of the chimney, Mr. Francis calculates that the area of 12.88 sq. ft. is contained in a portion of the chimney extending 2.428 ft. from one of its octagonal sides, and that the limit to which the centre of pressure may be shifted is therefore 5.072 ft. from the axis. If shifted beyond this, he says, on the assumption of the strength of the brickwork, it will crush and the chimney will fall. Calculating that the wind-pressure can affect only the upper 141 ft. of the chimney, the lower 70 ft. being protected by buildings, he calculates that a wind-pressure of 44 02 lbs. per sq. ft. would blow the chimney down. Rankine, in a paper printed in the transactions of the Institution of Engi |