34 BOILER TESTS. 



gases in the same boiler is invariably higher when bituminous 

 coal is used than when anthracite coal is used, and this points 

 to the same characteristic. In practice, the deposit of soot on 

 the surfaces when bituminous coal is used interferes with the 

 full efficiency of the surface, and an increased area is demanded 

 as an offset to the loss which this deposit occasions. It 

 would seem, then, that if a ratio of 36 to 1 is sufficient for 

 anthracite coal, from 45 to 50 should be provided when bitu- 

 minous coal is burned, especially in cases like those referred 

 to, where the rate of combustion is above 10 or 12 pounds per 

 square foot of grate per hour. 



The size of shell in horizontal tubular boilers appears to have 

 little effect on the economy. The best of all the results with 

 anthracite coal, which is 11.53 pounds of water from and at 

 212 degrees per pound of combustible, was obtained in a case 

 where the diameter of the shell was 48 inches, and this result 

 is all that can be expected from any boiler, whether the shell is 

 large or small. 



The number of tubes controls the ratio between the area of 

 grate surface and area of tube opening. Boilers No. 42 and No. 

 45 have a very large number of tubes, and consequently a small 

 ratio of grate to tube opening. The ratio is 5.2 to 1. They 

 also have the very large area of heating surface represented by 

 ratios of 65 and 60 to 1. Notwithstanding the ample provision 

 of surface and other favorable conditions, the evaporation with 

 anthracite coal is no higher than boilers give which have surface 



o o 



of much less extent, though of such character that the tube 

 opening bears a smaller proportion to the grate surface. The 

 conclusion which is well warranted by this fact is that a certain 

 minimum amount of tube opening is required for efficient work. 

 This conclusion is borne out by the result of the tests with 

 anthracite coal on Boiler No. 12, where the products of com- 

 bustion make two circuits through the shell and the ratio of 

 grate surface to tube opening is 11.6 to 1. The ratio of heating 

 surface to grate here is 42 to 1, and the average evaporation per 

 pound of combustible from and at 212 degrees is 11.16 pounds. 

 The best results obtained with anthracite coal in the common 



