Steam:
Its Generation and Use

CHIMNEYS. Chimneys are required for two pur- poses— ist,to carry off obnoxious gas- es; 2d, to produce a draught, and so facilitate combustion. The first re- quires size, the second height. Each pound of coal burned yields from 13 to 30 pounds of gas, the vol- ume of which varies with the temper- ature. 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 ab- solute 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 3OO°- Therefore, height and area are the only elements necessary to consider in an ordinary chimney. The intensity of draught is, how- ever, independent of the size, and de- pends upon the difference in weight of the outside and inside columns of air, which varies nearly as the product of the height into the difference of tem- perature. This is usually stated in an equivalent column of water, and may vary from 0 to possibly 2 inches. After a height has been reached to produce draught of sufficient intensity to burn fine, hard coal, provided the area of the chimney is large enough, there seems no good mechanical reason for adding further to the height, what- ever the size of the chimney requir- ed. Where cost is no consideration there is no objection to building as high as one pleases ; but for the purely utilitarian purpose of steam making equally good results, might be attained with a shorter chimney at much less cost. The intensity of draft required va- ries 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 1 % inch of water is nec- essary, which can be attained by a well- proportioned chimney 175 feet high. Generally a much less height than 100 feet can not be recommended for a boiler, as the lower grades of fuel can- not be burned as they should be with a shorter chimney. A round chimney is better than square, and a straight flue better than a 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 inte- rior surface, and that a commercial horse-power requires the consumption on an average of 5 pounds of coal per hour, we have the following formulae : (U H . , -r- ° A A . A 1 i8 33 E ;/ h . S =12 p E~r4 • D 13.54 1/E4-4 / 0.3 Hz2 ÖÜ N 0AT I <DN jkuane.—TZ,_x. -K; 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; S side of square chimney, anß D — dia. of round chimney in inches. The table 011 page is calculated by means of these formulae. To find the draft of a given chimney in inches of water : Divide 7.6 by the absolute temperature of the external air (ra — / + 460}; divide 7.9 by the absolute temperature of the gases in the chimney ("e — t' r 460); subtract the latter from the former, and multi- ply the remainder by the height of the chimney in feet. I his rule, ex- pressed in a formula, would be : 7-6 _ To find the height of a chimney, to give a specific draft power,express- ed in inches of water : Proceed as above, through the first tzvo steps, r1—STONÉ- p': . concrete ■ . (hen divide the given draft pozuer