22 BOILER TESTS. 



1. COMPARISON OF BOILERS WHICH PRODUCE SATURATED 

 STEAM WITH THOSE PRODUCING SUPERHEATED STEAM. 



The boilers may be divided into two general classes, accord- 

 ing as they furnish saturated steam or superheated steam. 

 The first class are, as a rule, of the horizontal tubular type, 

 and the second class of the vertical type. In most of the 

 boilers which superheat, that is, vertical boilers, the superheat 

 is obtained by sacrificing the water-heating surface, and pro- 

 viding a large area of steam-heating surface. In the horizontal 

 tubular boiler, the proportion of heating surface to grate sur- 

 face is generally 33 or more to 1, and this is almost wholly 

 water surface. In many of the vertical boilers, the proportion 

 of water surface to grate is only 20 to 1 , that of steam surface 

 to grate about 10 to 1, making a total of 30 to 1, and this total 

 surface is less than that given for the horizontal boiler. Con- 

 sidering that the economic performance of the boiler is affected 

 by the proportion which exists between the heating surface and 

 grate surface, as is shown in a later part of the paper, these 

 boilers are handicapped at the outset by a deficiency of surface, 

 even though the steam-heating surface is considered equally 

 as effective as the water-heating surface. Let us make a com- 

 parison between the two types of boilers, however, taking the 

 proportions which exist. Although the evaporation per pound 

 of coal is less in the superheating boiler than in the boiler 

 which does not superheat, it does not follow that the actual 

 efficiency of one is different from that of the other. Super- 

 heated steam is of greater value than saturated steam, according 

 as it is more or less superheated. The reason for this is, first, 

 that superheated steam contains the greater amount of heat ; 

 and, second, that heat expended in superheating steam secures 

 a greater return, in cases at least where it is used for power, 

 than the original heat expended in evaporating water. If, for 

 example, steam is superheated 100 degrees, the added heat 

 which it contains is 48 thermal units per pound, or 4.8 per 

 cent, of the heat required to convert a pound of water, supplied 

 at 212 degrees, into steam of 80 pounds pressure. In other 



