110 STEAM BOILERS 



Height of Chimney. The relation between the height of 

 the chimney and the pressure of the draft, in inches of water, is 

 given by the formula 



_./7.6 7.9^ 

 P = - 



in which P = draft pressure, in inches of water; 



H = height of chimney, in feet; 

 T a and T c = absolute temperatures of the outside air and of 



the chimney gases, respectively. 



EXAMPLE. What draft pressure will be produced by a 

 chimney 120 ft. high, the temperature of the chimney gases 

 being 600 P., and of the external air 60 F.? 

 SOLUTION. Substituting in the formula, 



/ 7.6 7.9 \ 



^ = 120 ( I =.859 in. 



\460+60 460+600/ 



To find the height of chimney to give a specified draft pres- 

 sure, the preceding formula may be transformed. Thus, 

 P 



7.G 7.9 



U 



EXAMPLE. Required, the height of the chimney to produce 

 a draft of \\ in. of water, the temperature of the gases and of 

 the external air being, respectively, 550 and 62 F. 



SOLUTION. Substituting in the formula, 



, 



7.6 7.9 

 522 ~ 1,010 



In determining the height of a chimney in cities, it should 

 be borne in mind that it must almost always be carried to a 

 height above the roofs of surrounding buildings, partly in 

 order to prevent a nullification of the draft by opposing air- 

 currents and partly to prevent the commission of a nuisance. 



Area of Chimney. The height of the chimney being decided 

 on, its cross-sectional area must be sufficient to carry off 

 readily the products of combustion. The following for- 

 mulas for rinding the dimensions of chimneys are in common 

 use: 



