STEAM BOILERS 109 



Form of Chimney. The form of a chimney has a pronounced 

 effect on its capacity. A round chimney has a greater capacity 

 for a given area than a square one. If the flue is tapering, the 

 area for calculation is measured at its smallest section. The 

 flue through which the gases pass from the boilers to the. chim- 

 ney should have an area equal to, or a little larger than, the 

 area of the chimney. Abrupt turns in the flue or contractions 

 of its area should be carefully avoided. Where one chimney 

 serves several boilers, the branch flue from each furnace to 

 the main flue must be somewhat larger than its proportionate 

 part of the area of the main flue. 



Brick Chimneys. Chimneys are usually built of brick, 

 though concrete, iron, and steel are often used for those of 

 moderate height. Brick chimneys are usually built with a flue 

 having parallel sides and a taper on the outside of the chimney 

 of from ^ to j in. per ft. of height. A round chimney gives 

 greater draft area for the same amount of material in its 

 structure and exposes less surface to the wind than a square 

 chimney. Large brick stacks are usually made with an inner 

 core and an outer shell, with a space between them. Such 

 chimneys are usually constructed with a series of internal 

 pilasters, or vertical ribs, to give rigidity. The top of the 

 chimney should be protected by a coping of stone or a cast-iron 

 plate to prevent the destruction of the bricks by the weather. 



Iron and Steel Chimneys. Iron or steel stacks are made of 

 plates varying from to | in. thick. The larger stacks are 

 made in sections, the plates being about J in. thick at the top 

 and increasing to in. at the bottom; they are lined with fire- 

 brick about 18 in. thick at the bottom and 4 in. at the top. 

 Sometimes no lining is used on account of the likelihood of 

 corrosion and the difficulty of inspection, and also because 

 the inside of lined stacks cannot be painted. 



Chimney Foundations. On account of the great concentra- 

 tion of weight, the foundation for a chimney should be care- 

 fully designed. Good natural earth will support from 2,000 

 to 4,000 Ib. per sq. ft. The footing beneath the chimney 

 foundation should be made of large area, in order to reduce 

 the pressure due to the weight of the chimney and its founda- 

 tion to a safe limit. 



