PRINCIPLES OF NAVAL ENGINEERING 



The generating surface is that part of the 

 total heating surface in which water is being 

 heated and steam is being generated. The gen- 

 erating surface includes the generating tubes, 

 the water wall tubes, the water screen tubes, 

 and any water floor tubes that are not covered 

 by refractory material. 



The superheater surface is that part of the 

 total heating surface in which the steam is 

 superheated after leaving the boiler steam drum. 



The economizer surface is that portion of the 

 total heating surface in which the feed water is 

 heated before it enters the generatingpartof the 

 boiler. 



DESUPERHEATERS.— On boilers with non- 

 controlled superheaters, all steam is super- 

 heated but a small amount is redirected through 

 a desuperheater line. The desuperheater can be 

 located in either the water drum or the steam 

 drum; most generally, the desuperheater will be 

 found in the steam drum below the normal water 

 level. The purpose of the desuperheater is to 

 lower the superheated steam temperature back 

 to or close to saturated steam temperature for 

 the proper steam lubrication of the auxiliary 

 machinery. The desuperheater is most generally 

 an "S" shaped tube bundle that is flanged to the 

 superheater outlet on the inlet side and the 

 auxiliary steam stop on the outlet side. 



BOILER CLASSIFICATION 



Although boilers vary considerably in details 

 of design, most boilers may be classified and 

 described in terms of a few basic features or 

 characteristics. Some knowledge of these meth- 

 ods of classification provides a useful basis for 

 understanding the design and construction of the 

 various types of modern naval boilers. 



Location of Fire and Water Spaces 



One basic classification of boilers is made 

 according to the relative location of the fire and 

 water spaces. By this method of classification, 

 all boilers may be divided into two classes: fire- 

 tube boilers and water-tube boilers. In fire-tube 

 boilers, the gases of combustion flow through the 

 tubes and thereby heat the water which sur- 

 rounds the tubes. In water-tube boilers, the 

 water flows through the tubes and is heated by 

 the gases of combustion that fill the furnace and 

 heat the outside metal surfaces of the tubes. 



All boilers used in the propulsion plants of 

 modern naval ships are of the water-tube type. 



Fire-tube boilers^ were once used extensively 

 in marine installations and are still used in the 

 propulsion plants of some older merchant ships. 

 However, fire-tube boilers are not suitable for 

 use as propulsion boilers in modern naval ships 

 because of their excessive weight and size, the 

 excessive length of time required to raise steam, 

 and their inability to meet demands for rapid 

 changes in load. The only fire-tube boilers cur- 

 rently in naval use are some small auxiliary 

 boilers. 2 



Type of Circulation 



Water-tube boilers are further classified 

 according to the cause of water circulation. By 

 this mode of classification, we have natural 

 circulation boilers and controlled circulation 

 boilers. 



In natural circulation boilers, the circulation 

 of water depends on the difference between the 

 density of an ascending mixture of hot water and 

 steam and a descending body of relatively cool 

 and steam-free water. The difference in density 

 occurs because the water expands as it is heated 

 and thus becomes less dense. Another way to 

 describe natural circulation is to say that it is 

 caused by convection currents which result from 

 the uneven heating of the water contained in the 

 boiler. 



Natural circulation may be either free or 

 accelerated. Figure 10-1 illustrates free natural 

 circulation. Note that the generating tubes are 

 installed at a slight angle of inclination which 

 allows the lighter hot water and steam to rise 

 and the cooler and heavier water to descend. 

 When the generating tubes are installed at a 

 greater angle of inclination, the rate of water 

 circulation is definitely increased. Therefore, 

 boilers in which the tubes slope quite steeply 

 from steam drum to water drum are said to 

 have accelerated natural circulation. This type 

 of circulation is illustrated in figure 10-2. 



Most modern naval boilers are designed for 

 accelerated natural circulation. In such boilers, 

 large tubes (3 or more inches in diameter) are 



As, for example, the old "Scotch marine boiler." 

 2 



Auxiliary boilers (some water-tube, some fire-tube) 

 are installed in diesel -driven ships and in many steam- 

 driven combatant ships. They are used to supply steam 

 or hot water for galley, and other "hotel" services 

 and for other auxiliary requirements in port. 



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