STEAM BOILERS 321 



line; as the water does not completely cover the tubes, 

 the upper part forms a superheater. 



When the exhaust steam is released into the stack, the 

 tubes have a tendency to leak. To avoid this, some 

 manufacturers sink the tube sheet below the water level 

 (Fig. 213). This form reduces the superheating surface, 

 and moreover, since the conical smoke chamber is sub- 

 jected to internal pressure, it is likely to be weak. Fig. 

 214 is a special type of vertical boiler in which are water 

 tubes laid up in courses. The boiler shell can be removed 

 from the caisson of tubes so that all parts are accessible 

 for cleaning and repairing. 



441. Externally fired boilers (Fig. 215) are generally 

 of the cylindrical tubular type and can be used for sta- 

 tionary work only. These are probably the most simple 

 as well as most easily handled and kept in repair of all, 

 but they are very bulky, requiring a great amount of floor 

 space. The furnace for such boilers is a part of the set- 

 ting and is made under the front end. The flames sur- 

 round the lower part of the shell and pass to the rear, 

 where they enter the tubes and return to the front, thence 

 up the stack. 



When setting externally fired boilers, care should be 

 taken that one end or the other, generally the rear, be 

 free to move forward or backward, since the variation of 

 temperature will cause the boiler to contract and expand 

 enough to crack the masonry upon which it rests. 



442. Internally fired boilers. This class comprises 

 several types, the locomotive type (Fig. 216), the return- 

 flue type (Fig. 217), and the Lancashire. The first two 

 of these types are the most used for traction or portable 

 work, while the latter is adapted only to stationary use. 



443. Locomotive type. The locomotive fire-tube type 

 was probably the first of the modern boilers to come into 



