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DESIGN AND CONSTRUCTION OF CHIMNEY, OR STACK
Peculiarities of chimney design. Brick construction. Foundations. The various types of
chimneys compared. Steel chimneys. The proportions of the chimney. Height of chimney. Its dimensions to suit certain boiler capacity. Methods of calculation. Limits to the heights of single wall chimneys. Vertical and cross-sections. The outer walls. Octagonal versus square forms of construction. Relative numbers of bricks used. The chimney base and foundation. Quality of mortar. The central core. Care in construction. Form of central flue. The cap. Provisions for reaching the top. Lightning rods. The chimney of strength and the chimney of expediency.
Thus far our methods of construction and the necessary materials for them have been such as are encountered daily by the architect and the builder. We now come to the erection of the chimney or stack, which has many peculiarities and restrictions on its design and construction, resulting from its narrow foundation, great height, and the necessity of its resisting not only the high-wind pressures and great changes in temperature at different seasons, but also the great difference of temperature on the inside and on the outside. It seems necessary, therefore, to treat this subject of chimney construction in a separate chapter wherein we will consider the respective merits of and the objections to chimneys of the more common forms and materials.
Regarding the chimney built of brick, the principal objections would appear to be its first cost, which is considerable, and the fact that owing to its narrow base and great height very firm and solid foundations must be prepared. This, of course, becomes more difficult and expensive where the ground is soft and excavations must be made at great depth, or where piles have to be driven to build the foundation upon.
At the present time many sheet iron or steel chimneys are erected, and it is the prevailing idea that they are the more economical. About the only advantage they seem to possess, however, is that owing to their comparatively light weight they may be erected on superstructures upon which a brick chimney could not. Then, too, their first cost is much less than for a brick chimney of equal capacity.
Some of their disadvantages are, that they are very liable to rust at the scams and rivets, owing to the impossibility of keeping these points properly protected from water. Therefore they are comparatively short lived.
Since the above paragraph was written the author saw one of these steel chimneys which had been built by a most reliable firm, erected with a great deal of care, painted with the best materials to be had, that in a little over a year became a total wreck, owing to the rusting of the material around the joints and rivets. A new stack had to be erected. The combined cost of the two would have built a good and substantial brick chimney that would have endured for many years.
Again, in the effort to protect the metal they must be frequently painted, or coated with some of the numerous “cure-all” paints, “warranted to protect them perfectly inside and out”; and the use of any protective covering is a continual expense for maintenance to which the brick chimney is not subject.
The conclusion, therefore, must be that, if the life of the chimney is of less consideration than its first cost, we would adopt that constructed of sheet iron or steel; but if we regard permanency and the ultimate outlay, both for construction and maintenance and all the advantages derived, brick is evidently the material to be chosen.
The height of the chimney will depend somewhat upon surrounding hills, high buildings, and similar obstructions to the free course of the wind, but should never be less than the diameter of the internal flue multiplied by twenty. The diameter of the internal flue will depend on the aggregate areas of the smoke flues or "up-takes” leading from the boilers, and these necessarily depend upon the grate surface, allowing about 4.5 square feet per horse-power.
There are many methods of calculating the diameter of chimney flues, some of which are very complex and depend upon many assumed conditions at each step, which ofttimes have hardly more practical value than guesses. Others assume to calculate the volume of gases, the speed of their flow, the area of grate openings, etc., all of which might be changed with each sample of coal, or according to the condition of the weather.
Practical engineers will probably favor the following simple method, even with its arbitrary assumptions, and will be quite successful in the practical application of it as it is the result of much actual experience. The horsepower being given say in this case 470 — and allowing 4.5 square feet of grate surface
per horse-power, we have 104.4. At 5 pounds of coal per horsepower, which is quite liberal, we will burn 2,350 pounds of coal per hour. Our chimney is 100 feet high. We divide the pounds of coal burned per hour by the square root of the height multiplied by 12 (10 X 12=120) and we have 19.58 as the area of the chimney flue, in square feet. Divide this by -7854 and extract the square root and we have the diameter, slightly less than
Having the diameter of flue and height given we may by inverse methods
obtain the horse-power, grate surface, etc. In making these calculations we should be sure to get capacity enough; for if the chimney is a little too large no harm is done, while if a little too small a serious expense is incurred for a supplementary one.
All chimneys over 75 feet high should be built with a central “core," or fue, preferably of circular form, surrounded by an outside casing sufficiently strong to properly support the inner core and to resist the pressure of the strongest winds.
The thickness of the walls of both the outer portion and the inner core should be sufficient to be very rigid near the ground and gradually thinner as the walls rise, the “breaks” being, of course, on the inside of the outer portion and the outside of the inner core. These breaks are usually four inches, or the width of a brick at each step.
The “batter,” or inclination of the outside face of the main structure should be a quarter of an inch per foot. In our case we are supposed to require a chimney 100 feet high, with a circular flue 5 feet in diameter.
In the illustrations, Fig. 21 shows a vertical section through the center of the chimney and its foundation. It shows the thickness of walls, special form of central flue, retaining caps at the top, and special arrangements for increasing the draft. Fig. 22 is an elevation of the exterior, showing its general form and appearance when completed. Fig. 23 shows a horizontal section through the octagonal portion on the line AA, Fig. 21. Fig. 24 shows a horizontal section through the square base, on the line BB, Fig. 21. Fig. 25 illustrates, on an enlarged scale, the number of bricks necessary for a course, if the square form were to be continued to the top. Fig. 26 shows the economy of adopting the octagonal form, as saving material and labor, and offering considerably less surface to wind pressure from certain directions.
In the Figs. 25 and 26 the upper half shows the laying of a “header” course and the lower half the laying of a "straight” course. In these two sketches it will be seen that in a header course the octagonal form contains 52 bricks less than the square form; and in a straight course 32 bricks are saved. Assuming five courses per foot in height, and that in each foot we have one header course, we save by the octagonal form 180 bricks. This, of course, is less as we approach the top, but the average saving will be considerably over 100 bricks per foot, or over 8,000 for the whole work.
Actual experience shows that the extra labor cost of making the many corners is more than balanced by the smaller number of bricks laid in the octagonal form than in the square form. The appearance is much enhanced and the wind pressure is considerably diminished by getting rid of the projecting corners.
The base of the chimney is of square form, this being more convenient