PRINCIPLES OF NAVAL ENGINEERING 



Double- Furnace Boilers 



Double-furnace boilers (also called M-type 

 boilers) are installed on most older destroyers 

 and on many other combatant ships. These 

 boilers are designed to carry a steam drum 

 pressure of approximately 615 psig and to gen- 

 erate saturated steam at approximately 490° F. 

 The saturated steam for auxiliaries goes direct- 

 ly from the steam drum to the auxiliary steam 

 system; all other steam goes through the super- 

 heater. Double-furnace boilers are designed in 

 various sizes and capacities to suit different 

 installations. They range in capacity from about 

 100,000 to about 250,000 pounds of steam per 

 hour at full power. 



Figure 10-17 shows the general arrangement 

 of a double-furnace boiler. The same type of 

 boiler is shown in sectional view infigure 10-18 

 and in cutaway view in figure 10-19. 



One of the two furnaces in this boiler is used 

 for generating saturated steam; the other is used 

 for superheating the saturated steam. Because 

 each of the two furnaces can be fired separately, 

 thus allowing control of superheated steam tem- 

 perature over a wide range of operating condi- 

 tions, the double-furnace boiler has long been 

 called a "superheat control boiler." As noted 

 previously, however, the control of superheat is 

 not necessarily related to the number of fur- 

 naces. Therefore we will refer to this boiler as 



WATER WALL 

 HEADER 



MAIN STEAM 



WATER 

 DRUM 



38,36 

 Figure 10-17.— General arrangement of double- 

 furnace boiler. 



a double-furnace boiler, rather than as a super- 

 heat control boiler, even though the boiler shown 

 does in fact have controlled superheat. 



Since each furnace has its own burners, the 

 degree of superheat can be controlled by pro- 

 portioning the amount of fuel burned in the 

 superheater-side furnace to the amount burned 

 in the saturated-side furnace. When burners are 

 lighted only on the saturated side, saturated 

 steam is generated; when burners are lighted 

 on the superheater side as well as on the satu- 

 rated side, the saturated steam flowing through 

 the superheater becomes superheated. The de- 

 gree of superheat depends primarily upon (1) the 

 firing rate on the superheater side, and (2) the 

 rate of steam flow through the superheater. 

 However, the rate of steam flow through the 

 superheater is basically dependent upon the fir- 

 ing rate on the saturated side. Therefore we 

 come back again to the idea that the degree of 

 superheat depends primarily upon the ratio of 

 the amount of oil burned in the superheater side 

 to the amount burned in the saturated side. 



The flow of combustion gases in the double- 

 furnace boiler is partly controlled by gas baffles 

 on one row of water screen tubes and on one 

 row of division wall tubes, as shown in figure 

 10-18. The gas baffles on the water screen 

 tubes direct the combustion gases toward the 

 superheater tubes and also deflect the combus- 

 tion gases away from the steam drum and the 

 water screen header. The baffles on the divi- 

 sion wall tubes by the saturated-side furnace 

 keep the saturated-side combustion gases from 

 flowing toward the superheater tubes, thus 

 protecting the superheater when the superheater 

 side is not lighted off. In addition, the baffles 

 on the division wall tubes deflect combustion 

 gases from the superheater side up toward the 

 top of the saturated side, thus allowing the 

 gases to pass toward the uptake without dis- 

 turbing the fires in the saturated- side furnace. 



The double-furnace boiler has a steam drum, 

 one water drum, one water screen header, and 

 one water wall header. All these drums and 

 headers run from the front of the boiler to the 

 rear of the boiler. Most of the saturated steam 

 is generated in the main bank of generating 

 tubes on the uptake side of the boiler; most of 

 these tubes are 1 inch in outside diameter, but a 

 few rows of 2- inch tubes are installed on the side 

 of the tube bank nearest the furnace. The evapo- 

 ration rate is much higher in the 1-inch tubes 

 than in the 2-inch tubes, since the ratio of heat- 

 transfer surface to the volume of contained water 



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