Principles of naval engineering - Historic Naval Ships Association

Chapter 10- PROPULSION BOILERSArrangement of Steam andWater SpacesFLAME AND GASES147.83Figure 10-1.— Natural circulation (free type).installed between the steam drum and the waterdrums. These large tubes, called downcomers ,are located outside the furnace and away fromthe heat of combustion, thereby serving as pathwaysfor the downward flow of relatively coolwater. When a sufficient number of downcomersare installed, all small tubes can be generatingtubes, carrying steam and water upward, and alldownward flow can be carried by the downcomers.The size and number of downcomersinstalled varies from one type of boiler to another,but some are installed on all modern navalboilers.Controlled circulation boilers are, as theirname implies, quite different in design from theboilers that utilize natural circulation. Controlledcirculation boilers depend upon pumps,rather than upon natural differences in density,for the circulation of water within the boiler.Because controlled circulation boilers are notlimited by the requirement that hot water andsteam must be allowed to flow upward whilecooler water flows downward, a great variety ofarrangements may be found in controlled circulationboilers.Controlled circulation boilers have been usedin a few naval ships during the past few years.In general, however, they are still consideredmore or less experimental for naval use.Natural circulation boilers are classified asdrum-type boilers or as header-type boilers,depending upon the arrangement of the steamand water spaces. Drum-type boilers have oneor more water drums (and usually one or morewater headers as well). Header-type boilershave no water drum; instead, the tubes enter agreat many water headers.What is a header, and what is the differencebetween aheader andadrum? The term HEADERis commonly used in engineerlngto describe anytube, chamber, drum, and similar piece to whicha series of tubes or pipes are connected in such away as to permit the flow of fluid from one tube(or group of tubes) to another. Essentially, aheader is a type of manifold. As far as boilersare concerned, the only distinction between adrum and a header is the distinction of size.Drums are larger than headers, but both servebasically the same purpose.Drum-type boilers are further classifiedaccording to the overall configuration of theboiler, with particular regard to the shapeformed by the steam and water spaces. Forexample, double-furnace boilers are often called"M-type boilers" because the arrangement oftubes is roughly M-shaped. Single-furnaceboilers are often called "D-type boilers" becausethe tubes form (rouglily) the letter D.3Number of FurnacesAll boilers that are now commonly used inthe propulsion plants of naval ships may beclassified as being either single-furnace boilersor double-furnace boilers. The D-type boiler isa single-furnace boiler; the M-type boiler is adouble-furnace (or divided-furnace) boiler.Furnace PressureRecent developments in naval boilers make itconvenient to classify boilers on the basis of theAn interesting variation in this terminology occurredwhen the single-furnace or D-type boiler becamestandard for steam-driven destroyer escorts and thussubsequently became known as a "DE-type boiler."The term "DE-type boiler" is still used rather freely;its use should be discouraged, however, as this generaltype of boiler is now installed on many shipsother than destroyer escorts.233