STEAM BOILERS 159 



to the oil-storage tanks, so that,. if the oil in the tanks should 

 take fire, steam could quickly be run in to smother the blaze. 



Thermal Advantages of Oil. The calorific value of a pound 

 of oil fuel is about 30% higher than that of a high-class coal, 

 so that, by using oil instead of coal, the same amount of heat 

 may be obtained with a smaller weight of fuel. As has already 

 been shown, it is possible to obtain more nearly perfect com- 

 bustion, using less excess of air, with oil than with coal, which 

 increases the efficiency, Again, there is no repeated opening 

 and closing of the fire doors when oil is used, and this prevents 

 loss of heat and at the same time gives a better distribution 

 of heat in the combustion chamber. Also, there is less soot 

 deposited on the heating surfaces, in consequence of which the 

 transfer of heat is rapid and there is less heat lost up the chim- 

 ney. The capacity of the boiler may therefore be increased 

 from one-third to one-half by changing from coal to oil, while 

 for short periods the capacity may be doubled. Owing to the 

 uniformity of combustion of oil, the metal of the boiler is not 

 subjected to such severe conditions as when solid fuel is used. 



Rapidity of Regulation. Another great advantage of oil 

 fuel is the quickness and ease with which the intensity of com- 

 bustion may be altered. This is of particular importance in a 

 plant that may be subjected to quick increases in the load. 

 The fire in the furnace of an oil-burning boiler may be brought 

 very quickly from a moderate heat to a most intense heat, to 

 meet a sudden demand for more steam. Also, in case the load 

 falls off very suddenly, the burners may be adjusted rapidly to- 

 produce a correspondingly smaller quantity of heat. In emer- 

 gencies, the fires may be put out instantly, and may be almost 

 as quickly relighted when the danger is past. The closeness 

 with which the combustion may be made to follow the demand 

 for steam enables an almost uniform steam pressure to be 

 maintained by the use of automatic regulators for the oil pres- 

 sure and air supply. 



Economy of Storage and Handling. Oil fuel may be stored 

 and handled with less labor and at less cost than is possible 

 with coal. The volume occupied by a given weight of oil is 

 less than the volume of an equal weight of coal; also, because 

 of the greater calorific value of oil, it is possible to store 50% 



