PRINCIPLES OF NAVAL ENGINEERING 



bottom of the boiler. Water drums are usually 

 round in cross section; headers may be round, 

 oval, or square. Headers are provided with 

 access openings of the type shown in figure 

 10-5. Water drums are usually made with man- 

 holes similar to the manholes in steam drums. 



HANDHOLE PLATE 



GASKET 



HEADER 



Figure 



NUT 



38.21 

 10-5.— Header handhole and handhole 

 plate. 



Generating and Circulating Tubes 



Most of the tubes in a boiler are generating 

 or circulating tubes. There are four main kinds 

 of generating and circulating tubes: (1) gener- 

 ating tubes in the main generating tube bank; (2) 

 water wall tubes, (3) water screen tubes, and 

 (4) downcomers. The tubes are made of steel 

 similar to the steel used for the drums and 

 headers. Most tubes in the main generating 

 bank are about 1 inch or 1 1/4 inches in outside 

 diameter. Water wall tubes, water screen tubes, 

 and the two or three rows of generating tubes next 

 to the furnace are generally a little larger. 

 Downcomers are larger still, being on the aver- 

 age about 3 to 11 inches in outside diameter. 



Since the steam drum is at the top of the 

 boiler and the water drums and headers are at 

 the bottom, it is obvious that the generating and 

 circulating tubes must be installed more or less 

 vertically. Each tube enters the steam drum and 

 the water drum (or water header) at right angles 

 to the drum surfaces. This means that all tubes 

 in any one row are curved in exactly the same 

 way, but the curvature of different rows is not 

 the same. Tubes are installed normal to the 

 drum surfaces in order to allow the maximum 

 number of tube holes to be drilled in the tube 

 sheets with a minimum weakening of the drums. 

 However, nonnormal installation is permitted if 



certain advantages can be achieved in design 

 characteristics. 



What purpose do all these generating and 

 circulating tubes serve? The generating tubes 

 are the ones in which most of the saturated 

 steam is generated. The water wall tubes serve 

 primarily to protect the furnace refractories, 

 thus allowing higher heat release rates than 

 would be possible without this protection. How- 

 ever, the water wall tubes are also generating 

 tubes at high firing rates. Water screen tubes 

 protect the superheater from direct radiant heat. 

 Water screen tubes, like water wall tubes, are 

 generating tubes at high firing rates. Down- 

 comers are installed between the inner and outer 

 casings of the boiler to carry the downward flow 

 of relatively cool water and thus maintain the 

 boiler circulation. Downcomers are not designed 

 to be generating tubes under any conditions. 



In addition to the four main types of gen- 

 erating and circulating tubes just mentioned, 

 there are a few large superheater support tubes 

 which, in addition to providing partial support for 

 the steam drum and for the superheater, serve 

 as downcomers at low firing rates and as gen- 

 erating tubes at high firing rates. 



Since a modern boiler is likely to contain 

 between 1000 and 2000 tubes, some system of 

 tube identification is essential. Generating and 

 circulating tubes are identified by LETTERING 

 the rows of tubes and NUMBERING the indi- 

 vidual tubes in each row. A tube row runs from 

 the front of the boiler to the rear of the boiler. 

 The row of tubes next to the furnace is row A, 

 the next is row B, the next is row C, and so 

 forth. If there are more than 26 rows in a tube 

 bank, the rows after Z are lettered AA, BB, CC, 

 DD, EE, and so forth. Each tube in each row is 

 then designated by a number, beginning with 1 

 at the front of the boiler and numbering back 

 toward the rear. 



The letter which identifies a tube row is 

 often preceded by an R or an L, particularly 

 in the case of water screen tubes, superheater 

 support tubes, and furnace division wall tubes. 

 When an R or an L is used AFTER the regular 

 letter and number identification of atube, it may 

 indicate either that the tube is bent for a right- 

 hand or left-hand boiler or that the tube is 

 studded or finned on the right-hand side or on 

 the left-hand side. 



236 



