STEAM BOILERS 



351 



regard to handling drafts, there is very seldom a loss of 

 over 5 to 8 per cent due to lack of air. 



On the other hand, if there be too great an excess of 

 air, it would not only furnish oxygen for combustion in 

 sufficient quantities, but the excess would be heated as it 

 passes through the boiler from a temperature of the 

 outside air to a temperature of the flue gases, thus taking 

 up part of the heat which would be transferred to the 

 water. This loss generally amounts to from 4 to 10 per 

 cent. 



481. Smoke prevention. Black smoke is caused by in- 

 complete combustion. It is generally noticed when start- 

 ing a fire or when fresh coal is put on. To avoid as 

 much of this as possible, keep the fire hot and feed the 

 coal in small quantities. Do not have the door open 

 longer than is absolutely necessary, as the excess of air 

 cools the fire and instead of burning the CO to CO 2 , it 

 passes off as CO or free carbon, which causes the smoke. 



HANDLING A BOILER 



482. The flues are made of a soft, tough iron or steel. 

 They are put in place, then expanded with a tube ex- 



FIG. 240 FLUE EXPANDED WITH 

 PROSSER EXPANDER 



FIG. 241 l-LUE EXPANDED WITH 

 DUDGEON EXPANDER 



