88 COMBUSTION AND FUELS 



exceeded, the oil will be more troublesome to use and should 

 be purchasable at a correspondingly lower price. 



Firebrick Lining of Furnaces. It is possible to burn oil fuel 

 by spraying the oil directly into the metallic firebox of an 

 internally fired boiler or the ordinary furnace of an externally 

 fired boiler; but, although it may be done and .occasionally is 

 done, it is not good practice and is not recommended. The 

 spray of oil issuing from the burner should not strike the tubes 

 or the comparatively cold metal surfaces of the boiler, but 

 should first be completely burned in a combustion chamber of 

 ample size, after which the hot gases may be led into contact 

 with the heating surface of the boiler. A carefully designed 

 furnace is either partly or wholly lined with firebrick, which 

 protects the boiler from the direct action of the flames, prevents 

 the hot gases from being chilled before combustion is complete, 

 and tends to produce a more uniform transmission of heat to 

 the boiler. 



Effect of Firebrick Lining on Combustion. Under the effect 

 of the high temperature of combustion of oil fuel, the firebrick 

 lining of the furnace is maintained in an incandescent state, 

 which is of advantage in that it tends to promote a more nearly 

 uniform flow of heat to the boiler. If there is dirt or water in 

 the oil supply, or if the oil pumps do not act properly, so that 

 the oil supply is variable in pressure or not continuous, the 

 burners will act in a gusty, erratic manner. Under such cir- 

 cumstances, with an unlined firebox or furnace, it would be 

 difficult and troublesome, if not impossible, to maintain com- 

 bustion; but with a lining of incandescent firebrick there is a 

 reserve of heat in the furnace, so that combustion will be 

 restarted in case the fuel supply is momentarily interrupted 

 by dirt or water. Moreover, the firebrick, acting as a heat 

 reservoir, makes the flow of heat to the boiler more regular, 

 without seriously reducing the heat-transmitting efficiency of 

 the plates or tubes that it covers. 



Furnace Proportions for Oil Fuel. It is not necessary to 

 observe any definite ratio of length to breadth or length 

 to depth of the combustion space. The important point is to 

 provide ample volume, and then to insure that the gases fill it in 

 all parts and have the same velocity of flow throughout it. 



