BOILER No. 12. 



99 



Boiler No. 12 is a horizontal tubular boiler, differing from 

 that of the ordinary type in the arrangement of tubes. The 

 general features of the boiler and the manner in which it is set 

 are shown in the following cuts. The tubes are divided into 

 two sections ; one section serving to carry the products of 

 combustion forward, as in the ordinary boiler, and the other 

 serving to carry them backward to the chimney. The latter 

 are placed centrally with reference to the former, and extend 

 backward from the main tube sheet a distance of three feet, 

 the extended portion being enclosed in a supplementary shell 

 34 inches in diameter, which forms an extension to the rear 

 end of the boiler. 



This arrangement of tubes reduces the area for draught in a 

 given size of shell below what it would be in the boiler of the 

 ordinary type, inasmuch as only about one-half of the tubes 

 are effective in either direction. The ratio of the grate surface 

 to the smallest tube area in the boiler under notice is 11.6 to 1. 



The setting of the boiler is so arranged as to introduce air 

 to the products of combustion as they leave the furnace. The 

 air is supplied 

 through pas- 

 sages which ex- 

 tend hack and 

 forth through the 

 side walls, being 

 finally discharged 

 through perfo- 

 rated plates 

 located in the 

 sides of the fur- 

 nace and in the 

 top of the bridge 

 wall. 



BOILER No. 12, CROSS SECTION THROUGH FURNACE. 



