BOILERS AND GENERATION OF STEAM 323 



outside. To illustrate: The air entering the furnace may 

 have an average temperature of 62 F., while that in chimneys 

 often has a temperature of 500 F. A cubic foot of air at 62 

 weighs .0761 Ibs., and at 500 it weighs .0413 Ibs. The heated 

 air is therefore .0348 Ibs. lighter than the average air. 

 Hence its rapid passage to the smoke-stack and the conse- 

 quent draught. The length of stack or passageway has much 

 to do with the rapidity with which the smoke travels. On 

 every square foot of the cross-section of a 100-ft. stack, there 

 is at the bottom an upward pressure of 100 times .0348 Ibs., 

 or 3.48 Ibs. 



Induced draught is obtained by placing a fan-blower at or 

 above the boilers. The uptake from the boiler is connected 

 to the inlet of the blower and the outlet is carried to the chim- 

 ney, discharging the gases and heated air into the chimney. 



Forced draught is obtained by conducting the discharge of a 

 powerful blower to the ash pit, the air being forced through 

 the fire. 



364. Theory of Combustion and Smoke. Smoke is a by- 

 product of the combustion of fuel, and is invariably the result 

 of incomplete combustion. It is composed chiefly of minute 

 particles of carbon and steam, and is due largely to an excess 

 of air admitted to the fire, although in a few cases the produc- 

 tion of smoke is due to an insufficient supply of air. If the 

 boiler is not crowded and the draught is good, the volume of 

 smoke will be reduced by first allowing the coal to coke in 

 front of the grates and by then pushing it back over the bright 

 coals. The hollow bridge wall, with suitable means for regu- 

 lating the supply of air, also gives good results where there 

 is a strong draught. A small grate area and a very hot fire will 

 reduce the volume of smoke, as will a very large grate area 



