BOILERS AND GENERATION OF STEAM 303 



Both the boiler and fireplace are enclosed in fire-brick. Boilers 

 may be divided roughly into two general classes: water-tube, 

 and fire-tube boilers. The distinction between the two is 

 that in water-tube boilers water flows through the tubes and 



FIG. 158. Return Tubular Boiler. 



the fire is on the outside of the tubes, while the conditions are 

 reversed in the case of fire-tube boilers. 



The most widely used of all boilers in America and England 

 is the return tubular boiler (Fig. 158). This is a closed vessel 

 made of steel or iron, simple in construction, and easy to clean 

 and repair. The first horizontal tubular boilers were ordinary 

 iron storage tanks, 30 or 40 ft. long and 48 to 56 in. in diameter. 

 This type of boiler frequently exploded at the girth seam over 

 the fires, and 50 or 60 Ibs. was considered high pressure. 



