Principles of naval engineering - Historic Naval Ships Association

Chapter 10- PROPULSION BOILERSenormous heat losses. The blowerman mustlearn by experience how much air pressureshould be shown on the air pressure gage forall the various combinations of different numbersof burners, different sizes of sprayerplates, and different fuel pressures.The number of men assigned to operate thefireroom auxiliary machinery varies from oneship to another, depending upon the size of theship and the number of men available. Someships may have two or more men assigned tothis duty; on other ships, the work may be doneby the petty officer in charge of the watch orby the messenger. The burnerman and theblowerman may also take care of some of theauxiliaries. The checkman must never be givenany duties other than his primary ones of watchingand maintaining the water level.All fireroom operations are supervised andcoordinated by the petty officer in charge of thewatch. The petty officer in charge of the watchsupervises all lighting off, operating, and securingprocedures. He keeps the engineroomand the engineering officer of the watch informedof operating conditions when necessary.He must be constantly alert to the slightestindication of trouble and must be constantlyprepared to deal with any casualty that mayoccur. The petty officer in charge of the watchis responsible for making sure that all safetyprecautions are being observed and that unsafeoperating conditions are not allowed to exist.FIREROOM EFFICIENCYThe military value of a naval vessel dependsin large measure upon her cruising radius,which, in turn, depends upon the efficiency withwhich the engineering plant is operated. Perhapsthe largest single factor in determining theefficiency of the engineering plant is the efficiencywith which the boilers are operated.Greater savings in fuel, with consequent increasein steaming radius of the ship, may often bemade in the fireroom than in all the rest of theengineering plant put together.The capacity of a boiler is defined as themaximum rate at which the boiler can generatesteam. The rate of steam generation is usuallyexpressed in terms of pounds of water evaporatedper hour. You should know something ofthe limitations upon boiler capacity, the significanceof full-power and overload ratings,and the procedure for checking on boiler loads.The capacity of any boiler is limited bythree factors that have to do both with the designof the boiler and with its operation. These limitations,which are known as end points , are(1) the end point for combustion, (2) the endpoint for moisture carryover , and (3) the endpoint for water circulation.Boilers are so designed that the end pointfor combustion should occur at a lower rate ofsteam generation than the end point for moisturecarryover, and the end point for moisturecarryover at a lower rate than the end point forwater circulation. Since the end point for combustionoccurs first, it is the only end point thatis likely to be reached in a properly designedand properly operated boiler. However, it shouldbe understood that it is quite possible to reachthe end points for moisture carryover and watercirculation before reaching the end point forcombustion, by using larger sprayer plates thanthose recommended by the manufacturer or bythe Bureau of Ships. In such a case, the boilermight suffer great damage before the end pointfor combustion was reached.End Point for CombustionThe process of burning fuel in a boilerfurnace involves forcing the fuel into the furnaceat the proper viscosity through atomizers whichbreak up the oil into a foglike spray, and forcingair into the furnace in such a way that it mixesthoroughly with the oil spray. The amount of fuelthat can be burned is limited primarily by theactual capacity of the equipment that suppliesthe fuel (including the capacity of the sprayerplates), by the amount of air that can be forcedinto the furnace, and by the ability of the burnerapparatus to mix this air with the fuel. Thevolume and shape of the furnace are also limitingfactors.The end point for combustion for a boiler isreached when the capacity of the sprayer plates,at the designed pressure for the system, isreached or when the maximum amount of airthat can be forced into the furnace is insufficientfor complete combustion of the fuel. If the endpoint for combustion is actually reached becauseof insufficient air, the smoke in the uptakes willbe black because it will contain particles ofunburned fuel. However, this condition should berare, since the end point for combustion isartificially limited by sprayer plate capacitywhen the fuel is supplied at the burner manifoldat designed operating pressure. As noted before,269