Principles of naval engineering - Historic Naval Ships Association

Chapter 10- PROPULSION BOILERSAIRINLET@@®@®@@@@@@®®® @®@®®@@®@@@@® ®@@@@@@@@@@(-/@ @@@@®@@@@@®@fc@@@®® ®®@@@@@C@@@@@@@@@@@@@®@® @®®@@®®@®@@®@@@®®@@®@@®@@©@©@@®@®@®GENERATINGTUBES*- ECONOMIZERAIRINLETOUTER^CASINGOUTER'CASINGINNER .CASINGINNERCASINGWATER WALLHEADERSUPERHEATER-SIDE BURNERSWATERSCREENHEADERSUPERHEATER TUBES(CROSS SECTION)SATURATED-SIDEBURNERSFigure 10-18.— Double-furnace boiler (sectional view, looking toward rear wall).38.37is much greater in the smaller tubes. The largertubes are used in the rows next to the furnacebecause it is necessary at this point to providea flow of cooling water and steam sufficient toprotect the smaller tubes from the intense radiantheat of the furnaces.Double-furnace boilers have anywhere from15 to 50 downcomers, which vary in size fromabout 3 inches in outside diameter to about 7inches OD. The downcomers are installed betweenthe inner and the outer casings, as may beseen in figure 10-19.The use of large-tube downcomers and smallgenerating tubes results in extremely rapid circulationof water. Only a few seconds are requiredfor the water to enter the steam drum asfeed water, flow through the downcomers, circulatethrough the water drum or header, rise inthe generating tubes, and return to the steamdrum as a mixture of water and steam. Somenotion of the extreme rapidity of circulation maybe obtained from the fact that water in the downcomersmay flow at velocities of from 3 to 7feet per second.The economizer on a double-furnace boileris usually larger than the economizer on aheader-type boiler. As a rule, the economizeron a double-furnace boiler has about 60 U-shapedeconomizer tubes.On the basis of the classification methodsgiven earlier in this chapter, we may considerthe double-furnace boiler as one which has thefollowing characteristics: It is a water-tubeboiler with natural circulation of the acceleratedtype. It is a drum-type (rather than a headertype)boiler. It has tubes which are arrangedroughly in the shape of the letter M— hence it isoften called an M-type boiler. It has two249