BOILEBS No. 28 and No. 29. 



135 



Area of grate surface, . . . . 42 sq. ft. 42 sq. ft. 



Area through tubes, 5.8 sq. ft. 5.8 sq. ft. 



Ratio of heating surface to grate surface, 42.8 to 1 30. 8 to 1 



Ratio of grate surface to tube area, . 7.3 to 1 7.3 to 1 



Width of air spaces and metal bars in grates, each, . . 3-8 in. 



Distance of grate to shell, each, 27 in. 



Distance of flat bridge to shell, each, ..... 9 in. 



Eesults of Tests, Boilers No. 28 and No. 29. 



The tests on Boilers No. 28 and No. 29 had for an object the 

 determination of the effect which the size of tubes has upon 

 the economy. This object cannot fairly be said to have been 

 attained, because the boiler which has the small tubes has such 

 a large number of them that the heating surface is increased a 

 considerable amount above that of the other boiler. The test 

 is therefore a determination of the effect of increased heating 

 surface as well as of reduced diameter of tubes. Boiler No. 

 28, with the small tubes, gave the better result, the increased 

 evaporation being 4.5 per cent, based on coal, and 3.2 -er 

 cent, based on combustible. 



It is to be noted that the temperature of the escaping gases, 

 here given, is highest in case of the boiler with the larger area 

 of heating surface. It is doubtful whether these figures indi- 



