BOILER ACCESSORIES 137
It will be noted also that the possible H.P. of a chimney of a given sectional area increases with the height, a chimney of 18 inches diameter, for instance, when 50 feet high, being equal to the service of 23 H.P.; at 60 feet high of 25 H.P., and so on. The reason for this is, the increased height furnishes the increased motive column, and therefore the increased velocity of the gases, and the increased passage of air through the furnace, and therefore increased com- bustion. It should be mentioned that what is called the intensity of the draught, or the pressure available for driving the air and gases through the furnace in the chimney, varies as the square root of the height of the chimney, and this is because the height of the chimney varies with the velocity of the gases.
Construction of Boiler Chimneys
Boiler chimneys may be constructed of iron or steel, or of brick- work, and in modern plant are often constructed of the two combined, the chimney being built of steel, lined for the whole, or more fre- quently a portion, of its length with brickwork. For small boilers, such as those of portable engines and small boilers in positions where the smoke is not of consequence, simple iron and steel cylinders, formed from iron plates bent round and riveted together, and riveted to the shell of the boiler where the flue ends, are sufficient. For larger boiler plants, and particularly where the flue gases have to be delivered at a height where they will not be a nuisance, up till recently the common plan was to build brick chimneys, which were sometimes circular in section, sometimes octagonal, and sometimes square. The circular section is undoubtedly the best form, because the whole of the inside of the chimney is employed in carrying the gases; while with the square and octagonal forms the corners often give rise to eddies, which reduce the draught, and would also form pockets for the passage of the soot. The form, however, has also an important bearing upon the question of wind pressure. One of the problems in connection with the building of chimneys is providing sufficient strength for them to withstand the pressure under all con- ditions, and for this the circular chimney, for the same weight of material, exposes only half the surface to any particular wind that may be blowing; or put it in another way, a chimney that is to be of a given height and a given sectional area may be half the weight if it is circular in section of that required if it is square, and the pro- portion between the circular and octagonal chimneys is as 4 to 5 for the same conditions.
Another very important point in connection with the building of
