Principles of naval engineering - Historic Naval Ships Association

Chapter 10- PROPULSION BOILERSno useful purpose in combustion and is a directsource of heat loss.Calculations will show that approximately14 pounds of air will furnish the oxygen theoreticallyrequired for the complete combustionof 1 pound of fuel. In actual practice, of course,the amount of air necessary to ensure completecombustion must be somewhat in excess of thattheoretically required. About 10 to 15 percentexcess air is usually sufficient to ensure propercombustion. Too much excess air serves nouseful purpose, but merely absorbs and carriesoff heat.When fuel is burned in the boiler furnace,the difference between the HEAT INPUT and theHEAT ABSORBED represents the HEAT LOSS.Heat losses may be unavoidable, avoidable, or—in some cases— avoidable only to a limited extent.Most heat losses may be accounted for,but some losses cannot normally be accountedfor.All fuel contains a small amount of moisturewhich must be evaporated and superheatedto the furnace temperature. Since the expenditureof heat for this purpose constitutes a heatloss in terms of boiler efficiency, every precautionshould be taken to prevent contaminationof the fuel oil with water.All fuel contains some hydrogen which, whencombined with oxygen by the process of combustion,forms water vapor. This water vapormust be evaporated and superheated, and in bothprocesses it absorbs heat. Consequently, althoughthe heat of combustion of hydrogen isvery great, a small heat loss occurs becausethe water vapor formed as a result of the combustionof hydrogen must be evaporated andsuperheated.Since atmospheric air is the source of theoxygen utilized for combustion in the boilerfurnace, there is bound to be some moisture inthe combustion air. This moisture must beevaporated and superheated, and therefore constitutesa heat loss.The heat loss due to heat being carried awayby combustion gases is the greatest of all theheat losses that occur in a boiler. Althoughmuch of this heat loss is unavoidable, somemay be prevented by keeping all heat-transfersurfaces clean and by using no more excess airthan is actually required for combustion.Another heat loss occurs because of incompletecombustion of the fuel. When the carbonin the fuel is burned to carbon monoxide, insteadof carbon dioxide, there is a tremendous heatloss of 10,100 Btu per pound. This should beconsidered an avoidable loss, since the admissionof a sufficient amount of excess air willensure complete combustion.Heat losses that cannot be measured orthat are impracticable to measure are (1) lossesdue to unburned hydrocarbons, gaseous or solid;(2) losses due to radiation; and (3) other lossesnot normally accounted for.FIREROOM OPERATIONSAlthough a complete discussion of fireroomoperations is beyond the scope of this text,some understandingof the major factors involvedin boiler operation may be useful.Basically, the fireroom force must controlthree inputs—feed water, fuel, and combustionair— in order to provide one output, steam. Understeady steaming conditions, when steam demandsare relatively constant for long periods of time,there is no great difficulty about providing auniform flow of steam to the propulsion turbines.But one of the special requirements of navalships is that they must be able to maneuver andto change speed quickly, and this requirementimposes upon the fireroom force the responsibilityfor making very rapid increases and decreasesin the amount of steam furnished to theengineroom. Under conditions of rapid change,boiler operation is a teamwork job that requiresgreat skill and alertness and smooth coordinationof efforts by several men.For manual operationof the boilers, a normalfireroom watch consists of one petty officer incharge of the watch; one checkman for eachoperating boiler; one burnerman for each operatingboiler front; one blowerman for eachoperating boiler; and one or more men to actas messengers and to check the operation ofthe auxiliary machinery. When automatic boilercontrols are installed, boilers may be operatedwith fewer men on watch when the controls arebeing used.When a boiler is being operated manually,the checkman controls the water level in theboiler by manual operation of the feed stop andcheck valves. The checkman stands at the upperlevel, near the feed stop and check valves, andnear the boiler gage glass. The checkman admitswater to the boiler as necessary to maintain thewater at or very near the designed water level.The check watch requires the utmost vigilanceand reliability; if any one job in the fireroom267