SAT HEATED AND SUPERHEATED STEAM. 23 



words, if the boiler evaporates with one pound of coal 10 

 pounds of water supplied at 212 degrees into steam of 80 

 pounds pressure, and superheats that steam 100 degreees, the 

 excess of heat due to its superheat corresponds to an additional 

 evaporation of 0.48 pounds of water. When superheated 

 steam is used for motive power, the equivalent value of the 

 added heat is more than the quantity named. Many tests have 

 been made which show that this is true. Comparative tests by 

 the author show that for every ten degrees of superheating, the 

 amount of steam consumed by an engine is reduced from 1 to 

 1.5 per cent. If, to be on the safe side, we take a saving 

 of 1 per cent, of steam for 10 degrees of superheating as the 

 proper allowance, and assume that common steam contains 1 

 per cent, of moisture, which, in round numbers corresponds to 

 20 degrees of superheat, then the equivalent evaporation cor- 

 responding to the effect of 100 degrees of superheating, in the 

 example cited, is 1.2 Ibs. of water per pound of coal. 



In order that the superheating boilers may have the benelit 

 of every advantage to which they may be entitled, let us make 

 the comparison on the supposition that the steam is used for 

 motive power, and that, as just noted, 10 degrees of super- 

 heating is equivalent in value to 1 per cent, increase in evap- 

 oration. The tests on the two types of boilers which can be 

 fairly compared are those made with anthracite egg, or broken, 

 coal, the principal results of which are given in Tables No. 1 

 and No. 2. 



The number of tests referred to in the vertical boiler table is 

 less than those of the horizontal table, but they may be taken 

 as representative examples, since the comparative results ob- 

 tained in several cases where a different kind of fuel was used, 

 are, in general terms, the same. 



The average proportion of heating surface to grate surface 

 for the horizontal tubular boilers is 44.7 to 1, while the total 

 ratio in the vertical tubular boilers is 34.7 to 1. Comparing 

 the water-heating surface in the two cases, that in the vertical 

 boilers is about one-half of that in the horizontal boilers. The 

 average percentage of ash in the two cases is almost identically 



