56 BOILEE TESTS 



to the passages in the walls presented an area of 32 square 

 inches, or about four-tenths of one square inch for one square 

 foot of grate. This proportion is only about one-fourth of 

 that provided in the case first noticed (Boiler No. 15), where 

 the admission of air to Cumberland coal gave about 6 per cent, 

 advantage . 



The conclusion drawn from these examples is that a consid- 

 erable advantage attends the admission of air above the fuel 

 when bituminous coal is employed, the amount of gain depend- 

 ing somewhat upon the method employed. There is no advan- 

 tage in the system when mixtures of anthracite screenings and 

 bituminous coal are used, if carried out according to either the 

 first or fourth methods ; and, finally, little or no benefit is 

 derived when anthracite coal is burned. 



FLUE HEATERS. 



Another question connected with the general subject of the 

 boiler economy, is that of the economy produced by the 

 employment of a feed-water heater in the flue, and the tests 

 show that this may be of considerable importance. That the 

 use of a flue heater, in connection with a boiler having too little 

 heating surface, and sending a high degree of waste heat into 

 the chimney, is productive of economy, no one can have good 

 reason to doubt ; and this is clearly shown by the results of 

 tests No. 121 and No. 122, which are cases in point, where a 

 temperature of 618 degrees existed, and the heater added 29 

 per cent, to the evaporation per pound of coal. The special 

 interest in the question is not in cases like this, where the 

 waste heat is due to defects in the design and operation of 

 the boiler, but rather in cases of reasonably good practice, 

 where the temperature of the flue is not what may be regarded 

 as excessive. There are a number of such cases given, and 

 the general results produced are repeated in Table No, 9. 



