Chapter 10- PROPULSION BOILERS 



AIR 

 INLET 



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GENERATING 

 TUBES 



OUTER^ 

 CASING 



INNER . 

 CASING 



■*- ECONOMIZER 



AIR 

 INLET 



OUTER 

 'CASING 



INNER 

 CASING 



WATER WALL 

 HEADER 



SUPERHEATER- 

 SIDE BURNERS 



WATER 



SCREEN 



HEADER 



SATURATED- 

 SIDE 

 BURNERS 



SUPERHEATER TUBES 

 (CROSS SECTION) 



38.37 



Figure 10-18.— Double-furnace boiler (sectional view, looking toward rear wall). 



is much greater in the smaller tubes. The larger 

 tubes are used in the rows next to the furnace 

 because it is necessary at this point to provide 

 a flow of cooling water and steam sufficient to 

 protect the smaller tubes from the intense ra- 

 diant heat of the furnaces. 



Double-furnace boilers have anywhere from 

 15 to 50 downcomers, which vary in size from 

 about 3 inches in outside diameter to about 7 

 inches OD. The downcomers are installed be- 

 tween the inner and the outer casings, as may be 

 seen in figure 10-19. 



The use of large-tube downcomers and small 

 generating tubes results in extremely rapid cir- 

 culation of water. Only a few seconds are re- 

 quired for the water to enter the steam drum as 

 feed water, flow through the downcomers, circu- 

 late through the water drum or header, rise in 

 the generating tubes, and return to the steam 



drum as a mixture of water and steam. Some 

 notion of the extreme rapidity of circulation may 

 be obtained from the fact that water in the down- 

 comers may flow at velocities of from 3 to 7 

 feet per second. 



The economizer on a double-furnace boiler 

 is usually larger than the economizer on a 

 header-type boiler. As a rule, the economizer 

 on a double-furnace boiler has about 60 U-shaped 

 economizer tubes. 



On the basis of the classification methods 

 given earlier in this chapter, we may consider 

 the double-furnace boiler as one which has the 

 following characteristics: It is a water-tube 

 boiler with natural circulation of the accelerated 

 type. It is a drum-type (rather than a header- 

 type) boiler. It has tubes which are arranged 

 roughly in the shape of the letter M— hence it is 

 often called an M-type boiler. It has two 



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