Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGDouble- Furnace BoilersDouble-furnace boilers (also called M-typeboilers) are installed on most older destroyersand on many other combatant ships. Theseboilers are designed to carry a steam drumpressure of approximately 615 psig and to generatesaturated steam at approximately 490° F.The saturated steam for auxiliaries goes directlyfrom the steam drum to the auxiliary steamsystem; all other steam goes through the superheater.Double-furnace boilers are designed invarious sizes and capacities to suit differentinstallations. They range in capacity from about100,000 to about 250,000 pounds of steam perhour at full power.Figure 10-17 shows the general arrangementof a double-furnace boiler. The same type ofboiler is shown in sectional view infigure 10-18and in cutaway view in figure 10-19.One of the two furnaces in this boiler is usedfor generating saturated steam; the other is usedfor superheating the saturated steam. Becauseeach of the two furnaces can be fired separately,thus allowing control of superheated steam temperatureover a wide range of operating conditions,the double-furnace boiler has long beencalled a "superheat control boiler." As notedpreviously, however, the control of superheat isnot necessarily related to the number of furnaces.Therefore we will refer to this boiler asWATER WALLHEADERMAIN STEAMWATERDRUM38,36Figure 10-17.— General arrangement of doublefurnaceboiler.a double-furnace boiler, rather than as a superheatcontrol boiler, even though the boiler showndoes in fact have controlled superheat.Since each furnace has its own burners, thedegree of superheat can be controlled by proportioningthe amount of fuel burned in thesuperheater-side furnace to the amount burnedin the saturated-side furnace. When burners arelighted only on the saturated side, saturatedsteam is generated; when burners are lightedon the superheater side as well as on the saturatedside, the saturated steam flowing throughthe superheater becomes superheated. The degreeof superheat depends primarily upon (1) thefiring rate on the superheater side, and (2) therate of steam flow through the superheater.However, the rate of steam flow through thesuperheater is basically dependent upon the firingrate on the saturated side. Therefore wecome back again to the idea that the degree ofsuperheat depends primarily upon the ratio ofthe amount of oil burned in the superheater sideto the amount burned in the saturated side.The flow of combustion gases in the doublefurnaceboiler is partly controlled by gas baffleson one row of water screen tubes and on onerow of division wall tubes, as shown in figure10-18. The gas baffles on the water screentubes direct the combustion gases toward thesuperheater tubes and also deflect the combustiongases away from the steam drum and thewater screen header. The baffles on the divisionwall tubes by the saturated-side furnacekeep the saturated-side combustion gases fromflowing toward the superheater tubes, thusprotecting the superheater when the superheaterside is not lighted off. In addition, the baffleson the division wall tubes deflect combustiongases from the superheater side up toward thetop of the saturated side, thus allowing thegases to pass toward the uptake without disturbingthe fires in the saturated- side furnace.The double-furnace boiler has a steam drum,one water drum, one water screen header, andone water wall header. All these drums andheaders run from the front of the boiler to therear of the boiler. Most of the saturated steamis generated in the main bank of generatingtubes on the uptake side of the boiler; most ofthese tubes are 1 inch in outside diameter, but afew rows of 2- inch tubes are installed on the sideof the tube bank nearest the furnace. The evaporationrate is much higher in the 1-inch tubesthan in the 2-inch tubes, since the ratio of heattransfersurface to the volume of contained water248