GENERAL CONDITIONS OF ECONOMY. 35 



horizontal boiler are in cases where the ratio is larger than 9 

 to 1. From these facts the conclusion is drawn that the high- 

 est efficiency with anthracite coal is obtained when the tube 

 opening is from one-ninth to one-tenth of the grate surface. 



When bituminous coal is burned the requirements appear to 

 be different. The effect of a large tube opening does not seem 

 to make the extra tubes inefficient when bituminous coal is used. 

 The highest result on any boiler of the horizontal tubular class, 

 fired with bituminous coal, is obtained where the tube opening 

 is largest. This is Boiler No. 40, which gives an evaporation 

 of 12.47 pounds, and the ratio of grate surface to tube opening 

 is 5.4 to 1. The next highest result is produced in Boiler No. 

 42, already alluded to , which gives 12,42 pounds, and the ratio 

 is 5.2 to 1. Another high result is produced by Boiler No. 44. 

 This is 12.03 pounds, and the ratio is 4.1 to 1. Table No. 5 

 gives three high results, averaging 12.01 pounds, and here the 

 average ratio is 7.1 to 1. These instances are sufficient to 

 exhibit the need of a larger area of tube opening when bitumin- 

 ous coal is used than when anthracite is used, and this might be 

 expected in view of the gaseous nature of the products of com- 

 bustion. Without going to extremes, the ratio evidently most 

 to be desired when bituminous coal is used is that which gives 

 a tube opening having an area of from one-sixth to one-seventh 

 of the grate surface. 



One set of tests is given which bears on the question as to 

 the effect which size of tubes has upon the economy. These 

 are the tests made on Boilers No. 28 and No. 29, in one of which 

 140 3-inch tubes are used, and in the other 100 3^-inch. 

 The boiler with the smaller tubes gave the best result, but the 

 improved performance was evidently due to the increased heat- 

 ing surface, of which there was an addition of one-sixth, rather 

 than to any difference in the diameter of the tubes. It might 

 be inferred from the fact that bituminous coal requires a larger 

 collective area of tubes for best results than anthracite coal, 

 that it may also require a larger individual area, and therefore 

 larger diameter of tubes. This inference is not borne out by 

 a comparison of the tests on Boilers No. 42 and No. 40, one of 



