Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGthe floor, front wall, side walls, and rear wallwith refractory material. The refractory liningserves to protect the furnace casing and to preventloss of heat from the furnace. Refractoriesretain heat for a relatively long time and thushelp to maintain the high furnace temperaturesrequired for complete and efficient combustionof the fuel. Refractories are also used to formbaffles which direct the flow of combustiongases and protect drums, headers, and tubesfrom excessive heat.There are many different kinds of refractorymaterials. The particular use of each type isdetermined by the chemical and physical characteristicsof the material in relation to therequired conditions of service. Refractoriescommonly used in the furnaces of naval propulsionboilers include firebrick, insulatingbrick, insulating block, plastic fireclay, plasticchrome ore, chrome castable refractory, hightemperature castable refractory, air-settingmortar, and burner refractory tile.Casings, Uptakes, and SmokepipesIn modern boiler installations, each boileris enclosed in two steel casings. The inner casingis lined with refractory materials, and theenclosed space constitutes the furnace. Theouter casing extends around most of the innercasing, with an air space in between. Air fromthe forced draft blowers is forced into the spacebetween the inner and the outer casings, andfrom there it flows through the air registersand into the furnace.The inner casing encloses most of the boilerup to the uptakes. The uptakes join the boiler tothe smokepipe. As a rule, the uptakes from twoor more boilers connect with one smokepipe.Both the inner and the outer casings of boilersare made of steel panels. The panels maybe flanged and bolted together, with gasketsbeing used at the joints to make an airtightseal, or they may be welded together. The casingsare made in small sections so they can beremoved for the inspection and repair of boilerparts.Saddles and SupportsEach water drum and water header restsupon two saddles, one at the front of the drumor header and one at the rear. The upper flangesof the saddle are curved to fit the curvature ofthe drum or header, and are welded to the drumor header. The bottom flanges, which are flat,rest on huge beams built up from the ship'sstructure. The bottom flange of one saddle isbolted rigidly to its support. The bottom flangeof the other saddle is also bolted to its support,but the bolt holes are elongated in a fore-andaftdirection. As the drum expands or contractsbecause of temperature changes, the saddlewhich is not rigidly fastened to the supportaccommodates to the changing length of thedrum by sliding backward or forward over thesupport. The flanges which are not rigidly fastenedare known as boiler sliding feet .AirheatersSome boilers of recent design have steamcoilairheaters to preheat the combustion airbefore it enters the furnace. A typical steamcoilairheater consists of two coil blocks, eachcoil block having three sections of heating coilsin a single casing. Each individual section hasrows of copper-nickel alloy tubes, helicallywound with copper fins. Airheaters used in thepast on some older naval ships were installedin the uptakes and the combustion air was preheatedby the combustion gases; these airheatersthus utilized heat which would otherwise havebeen wasted. The use of these older airheaterswas discontinued in naval ships because the savingof heat was not considered sufficient to justifythe added space and weight requirements.The new steam-coil airheaters use auxiliaryexhaust steam as the heating agent; they are installednear the point where the combustion airenters the double casing.Fittings, Instruments, and ControlsThe major boiler components just describedcould not function without a number of fittings,instruments, and control devices. These additionalboiler parts are merely mentioned herefor the sake of completeness; they are taken upin detail in chapter 11 of this text.Internal fittings installed in the steam drummay include equipment for distributing the incomingfeed water, for separating and drying thesteam, for giving surface blows to remove solidmatter from the water, for directing the flow ofsteam and water within the steam drum, and forinjecting chemicals for boiler water treatment.In addition, many boilers have desuperheatersfor desuperheating the steam needed for auxiliarypurposes.244