Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGgive it the proper viscosity for atomization. Itshould be NOTED, however, that with the conversionto the new distillate fuel (NSDF), thefuel will not need to be heated as the viscosityis much lower than the fuel oil (NSFO) now beingused. The fuel must be forced into the furnaceunder pressure through the atomizers whichdivide the fuel into very fine particles. Meanwhile,combustion air must be forced into thefurnace and admitted in such a way that the airwill mix thoroughly with the finely divided fuel.And finally, it is necessary to supply enoughheat so that the fuel will ignite and continue toburn.Combustion is a chemical process whichresults in the rapid release of energy in theform of heat and light. When a fuel burns, thechemical reactions between the combustibleelements in the fuel and the oxygen in the airresult in new compounds. The combustible componentsof fuel are mainly carbon and hydrogen,which are present largely in the form of hydrocarbons.Sulfur, oxygen, nitrogen, and a smallamount of moisture are also present in fuel.In almost all burning processes, the principalreactions are the combination of the carbonand the hydrogen in the fuel with the oxygenin the air to form carbon dioxide and a relativelysmall amount of water vapor. In the absenceof sufficient air to form carbon dioxide, carbonmonoxide will be formed. A reaction of lesserimportance is the combination of sulfur andoxygen to form sulfur dioxide.Atmospheric air is the source of oxygen forthe combustion reactions occurring in a boilerfurnace. Air is a mixture of oxygen, nitrogen,and small amounts of carbon dioxide, watervapor, and inert gases. The approximate compositionof air, by weight and by volume, is asfollows:ElementOxygenNitrogen, etc.Weight(Percent)23,1576.85Volume(Percent)20.9179.09At the proper temperature, the oxygen inthe air combines chemically with the combustiblesubstances in the fuel. The nitrogen,which is 76.85 percent by weight of all air enteringthe furnace, serves no useful purpose incombustion but is rather a direct source of heatloss, since it absorbs heat in passing throughthe furnance and carries off a considerableamount of heat as it goes out the stack.When a combustion reaction occurs, a definiteamount of heat is liberated. The totalamount of heat released by the combustion of afuel is the sum of the heat released by eachelement in the fuel. The amount of heat liberatedin the burning of each of the principal elementsin fuel oil is as follows:Chem- Heat ReleasedElement ical By CombustionSymbol (BTUperlb)Hydrogen (to water). .Carbon (to carbonmonoxide)Carbon (to carbondioxide)Sulfur (to sulfurdioxide)H^CCS62,0004,44014,5404,050Notice that much more heat is liberated whencarbon is burned to carbon dioxide than when itis burned to carbon monoxide, the differencebeing 10,100 Btu per pound. In burning to carbonmonoxide, the carbon is not completelyoxidized; in burning to carbon dioxide, the carboncombines with all the oxygen possible, andthus oxidation is complete.Thus far in this discussion, we have assumedthat the oxygen necessary for combustion waspresent in the exact amount required for thecomplete combustion of all the combustible elementsin the fuel. However, it is not a simplematter to introduce just exactly the requiredamount of oxygen— no more, no less— into theboiler furnace.Since atmospheric air is the source of oxygenfor the combustion process that occurs in theboiler furnace, let us first calculate the amount ofair that would be needed to furnish 1 pound ofoxygen. By weight, the composition of air is23.15 percent oxygen and 76.85 percent nitrogen(disregarding the very small quantities of othergases present in air). To supply 1 pound of oxygenfor combustion, therefore, it is necessary tosupply 1/0.2315 or 4.32 pounds of air.Since nitrogen constitutes 76.85 percent of theair (by weight), the amount of nitrogen in this4.32 pounds of air will be 0.7685 x 4.32 or 3.32pounds. As mentioned before, the nitrogen serves266