32 BOILEU TESTS. 



With this accumulation of examples, no other conclusion can 

 be drawn than that one of the vital principles underlying the 

 attainment of economy in the generation of steam, is a low tem- 

 perature of the escaping gases. What the temperature should 

 be to secure the best results is to some extent uncertain. In 

 the examples of horizontal tubular boilers cited, the best aver- 

 age results where anthracite coal is used are secured with an 

 average temperature of 343 degrees, and where Cumberland 

 coal is used with an average of 383 degrees. It will not be 

 far out of the way if we consider 375 degrees as a proper limit 

 for anthracite coal, and 415 degrees for Cumberland coal. 

 These are named for the general case . Individual boilers may , 

 in rare instances, give excellent economy where the waste tem- 

 perature exceeds these figures, and there are two or three 

 examples furnished in the paper where this is true. There are 

 so many instances referred to where a boiler secures a low 

 grade of economy with more than 375 degrees in the flue when 

 anthracite coal is used, and more than 415 degrees when Cum- 

 berland coal is used, not only among boilers of the hori- 

 zontal tubular type, but among those of all other types, that it 

 seems reasonable to lay down these temperatures for a limit. 



The relation between the heating surface and grate surface is 

 important, and the question arises as to what that relation 

 should be to obtain the highest efficiency. Keeping to the 

 common horizontal boiler, let us select from the anthracite coal 

 tests the boilers in which the ratio is below 40 to 1 and above 

 30 to 1, and compare with those in which the ratio is more 

 than 40 to 1, taking, however, only those cases where the tem- 

 perature of the gases is low and the rate of combustion is above 

 9 pounds per square foot of grate per hour. The tests used are 

 those made on Boilers Nos. 9, 10 and 15 for the small ratios, and 

 Nos. 12, 25, and 36 for the large ratios, and the averages of 

 the two sets are as follows : 



