STEAM BOILERS 157 



cold air will increase the noise greatly and at the same time 

 will cause loss of heat; hence, as a general rule, it is safe to 

 assume that, the quieter the fire, the more nearly perfect is the 

 combustion. This is further corroborated by the fact that 

 preheating of the air, which is conducive to better combustion, 

 reduces the roaring. Sputtering of the flame from the burner 

 may be due to the heating of oil to such a point as to cause it 

 to be vaporized. 



Draft Required for Oil Burning. The draft of the chimney 

 performs the function of drawing air into the furnace and of 

 pushing the gaseous products of combustion through the boiler 

 and out into the air. When solid fuel is used, a large part of 

 the draft pressure is required to force the air through the layer 

 of fuel on the grates, the remainder serving to overcome the 

 resistance to the flow of the hot gases through the furnace, 

 tubes, flues, and chimney. When oil is used, there is no resist- 

 ance due to a fuel bed on a grate, and the draft pressure sim- 

 ply overcomes the resistance due to the flow of the air and hot 

 gases; consequently, the draft pressure required is much less 

 than for solid fuel. It follows, then, that when a boiler is 

 changed to use liquid fuel instead of solid fuel, the chimney 

 "that was satisfactory for the latter gives too strong a draft for 

 the former, and care must be exercised to prevent the admis- 

 sion of an excess of air. The draft pressure necessary for the 

 burning of oil fuel ranges from .1 to .5 in. of water, the former 

 corresponding to economical firing and the latter to firing with 

 a large excess of air. If the boiler is overloaded, the draft 

 must be increased above that required for normal working. 



Formation of Soot. An insufficient supply of air or an exces- 

 sive feeding of oil will result in the formation of soot, which will 

 by deposited on the heating surfaces and will reduce the effi- 

 ciency of heat transmission from the hot gases to the water in 

 the boiler, and may result in the overheating of some parts. 

 If such accumulations occur, they must be cleaned away, to 

 maintain the evaporative efficiency of the boiler. With care- 

 ful management of fires, boilers burning oil have been run at 

 full capacity for weeks without the formation of a troublesome 

 amount of soot. On the other hand, with coal as a fuel, it 

 would have been necessary to clean the tubes daily. 

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