PRINCIPLES OF NAVAL ENGINEERING 



give it the proper viscosity for atomization. It 

 should be NOTED, however, that with the con- 

 version to the new distillate fuel (NSDF), the 

 fuel will not need to be heated as the viscosity 

 is much lower than the fuel oil (NSFO) now being 

 used. The fuel must be forced into the furnace 

 under pressure through the atomizers which 

 divide the fuel into very fine particles. Mean- 

 while, combustion air must be forced into the 

 furnace and admitted in such a way that the air 

 will mix thoroughly with the finely divided fuel. 

 And finally, it is necessary to supply enough 

 heat so that the fuel will ignite and continue to 

 burn. 



Combustion is a chemical process which 

 results in the rapid release of energy in the 

 form of heat and light. When a fuel burns, the 

 chemical reactions between the combustible 

 elements in the fuel and the oxygen in the air 

 result in new compounds. The combustible com- 

 ponents of fuel are mainly carbon and hydrogen, 

 which are present largely in the form of hydro- 

 carbons. Sulfur, oxygen, nitrogen, and a small 

 amount of moisture are also present in fuel. 



In almost all burning processes, the prin- 

 cipal reactions are the combination of the car- 

 bon and the hydrogen in the fuel with the oxygen 

 in the air to form carbon dioxide and a relative- 

 ly small amount of water vapor. In the absence 

 of sufficient air to form carbon dioxide, carbon 

 monoxide will be formed. A reaction of lesser 

 importance is the combination of sulfur and 

 oxygen to form sulfur dioxide. 



Atmospheric air is the source of oxygen for 

 the combustion reactions occurring in a boiler 

 furnace. Air is a mixture of oxygen, nitrogen, 

 and small amounts of carbon dioxide, water 

 vapor, and inert gases. The approximate com- 

 position of air, by weight and by volume, is as 

 follows: 



Element 



Oxygen 



Nitrogen, etc. 



Weight 

 (Percent) 



23,15 



76.85 



Volume 

 (Percent) 



20.91 



79.09 



At the proper temperature, the oxygen in 

 the air combines chemically with the combus- 

 tible substances in the fuel. The nitrogen, 

 which is 76.85 percent by weight of all air en- 

 tering the furnace, serves no useful purpose in 

 combustion but is rather a direct source of heat 

 loss, since it absorbs heat in passing through 



the furnance and carries off a considerable 

 amount of heat as it goes out the stack. 



When a combustion reaction occurs, a defi- 

 nite amount of heat is liberated. The total 

 amount of heat released by the combustion of a 

 fuel is the sum of the heat released by each 

 element in the fuel. The amount of heat liber- 

 ated in the burning of each of the principal ele- 

 ments in fuel oil is as follows: 



Chem- Heat Released 

 Element ical By Combustion 



Symbol (BTUperlb) 



Hydrogen (to water). . H^ 



Carbon (to carbon 



monoxide) C 



Carbon (to carbon 



dioxide) C 



Sulfur (to sulfur 



dioxide) S 



62,000 



4,440 



14,540 



4,050 



Notice that much more heat is liberated when 

 carbon is burned to carbon dioxide than when it 

 is burned to carbon monoxide, the difference 

 being 10,100 Btu per pound. In burning to car- 

 bon monoxide, the carbon is not completely 

 oxidized; in burning to carbon dioxide, the car- 

 bon combines with all the oxygen possible, and 

 thus oxidation is complete. 



Thus far in this discussion, we have assumed 

 that the oxygen necessary for combustion was 

 present in the exact amount required for the 

 complete combustion of all the combustible ele- 

 ments in the fuel. However, it is not a simple 

 matter to introduce just exactly the required 

 amount of oxygen— no more, no less— into the 

 boiler furnace. 



Since atmospheric air is the source of oxygen 

 for the combustion process that occurs in the 

 boiler furnace, let us first calculate the amount of 

 air that would be needed to furnish 1 pound of 

 oxygen. By weight, the composition of air is 

 23.15 percent oxygen and 76.85 percent nitrogen 

 (disregarding the very small quantities of other 

 gases present in air). To supply 1 pound of oxy- 

 gen for combustion, therefore, it is necessary to 

 supply 1/0.2315 or 4.32 pounds of air. 



Since nitrogen constitutes 76.85 percent of the 

 air (by weight), the amount of nitrogen in this 

 4.32 pounds of air will be 0.7685 x 4.32 or 3.32 

 pounds. As mentioned before, the nitrogen serves 



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