Principles of naval engineering - Historic Naval Ships Association

Chapter 10- PROPULSION BOILERSFigure 10-6 shows a Boiler Tube RenewalSheet for a double-furnace boiler and illustratesthe method used to identify tubes.The water wall tube row is not identified bya letter in figure 10-6. The tubes in this row areoften identified by the letter W and a followingletter which indicates the type of tube or itsposition in the row. Still another letter (an R oran L) may be added to indicate that the tube isstudded or finned on the right-hand side or onthe left-hand side.The tubes which screenthe superheater fromdirect radiant heat are identified in figure 10-6as the LA, LB, and LC rows. Within each row,the individual tubes are numbered: LA-1, LA-2,LA-3, and so forth.Figure 10-6 identifies the superheater supportand drum support tubes as the LD row.Note that these are NOT superheater tubes.The superheater tubes in this boiler, as inmostboilers, are installed horizontally. The superheatersupport tubes and the drum support tubesare installed vertically; they are identified asLD-1, LD-2, LD-3, and so forth.The first row of division wall tubes is identifiedin figure 10-6 as the LE row. The secondrow of division wall tubes may be identified asthe LF row or as the D row. Identification oftubes in this row is usually made by using therow identification (LF, D, or whatever rowidentification is used for the particular boiler)followed by a letter to indicate the type of tubeor its position in the row; still another letter(an R or an L) may be added to indicate that thetube is studded or finned on the right-hand sideor on the left-hand side.Tubes in the main generating bank are identifiedby lettering the rows and numbering theindividual tubes, as shown in figure 6-6. Thetwo rows nearest the saturated-side furnace areslightly larger than the rest of the generatingtubes; they serve as water screen tubes. Thesetwo rows are often called the RA and the RBrows. Individual tubes in these rows are identifiedby number in the same way that the rest ofthe generating tubes are identified.Note that the superheater tubes are alsoidentified in figure 10-6. Identification of superheatertubes is discussed in the section thatdeals with superheaters.The discussion of boiler tube identificationgiven here is based on one particular type ofboiler— that is, a double-furnace boiler. Thesame general principles of tube identificationapply to most other drum-type boilers now innaval use, but the details of tube identificationare necessarily different in different types ofboilers.SuperheatersMost propulsion boilers now in naval servicehave convection-type superheaters, with waterscreen tubes installed between the superheaterand the furnace to absorb the intense radiantheat and thus protect the superheater.Most convection-type superheaters haveU-shaped tupes which are installed horizonallyin the boiler and two headers which are installedmore or less vertically at the rear ofthe boiler. One end of each U-shaped tube entersone superheater header, and the other endenters the other header. The superheater headersare divided internally by one or more divisionplates which act as baffles to direct theflow of steam. In some cases the superheaterheaders are divided externally as well as internally.Figure 10-7 illustrates some convectiontypesuperheater arrangements that are usedon double-furnace boilers. Part A is apian viewof the superheater tubes, showing how the tubesenter the headers. Part B shows a superheaterin which each header is divided into two sections,and illustrates the flow of steam throughthe superheater. Part C illustrates the flow ofsteam through a superheater in which one headerhas one internal division and the other headerhas two internal divisions. As may be seen fromfigure 10-7, the steam makes several passesthrough the furnaces. The number of passes isdetermined by the number of header divisionsand by the relative locations of the steam inletand the steam outlet.The superheater tubes are installed so thattheir U-shaped ends project forward toward thefront of the boiler. In a double-furnace boiler,the superheater tubes project forward into aspace between the water screen tubes and sometubes called furnace division wall tubes . Thesuperheater tubes and the surrounding waterscreen and division wall tubes are thus togetherthe dividing line between the superheater-sidefurnace and the saturated-side furnace. In asingle-furnace boiler, the superheater tubesproject forward into a space in the main bankof generating tubes. The tubes between thesuperheater tubes and the furnace serve aswater screen tubes.Some recent boilers have walk- in or cavitytypesuperheaters. In this type of superheater,237