THE ADVANTAGES OF THE USE OF 

 SUPERHEATED STEAM 



THAT there are advantages to be secured through the use of superheated 

 steam is probably most conclusively shown by the fact that superheaters 

 are installed, almost without exception, in the largest and most economical 

 power plants throughout the world. Regardless of any such evidence, however, 

 there is a deep-rooted conviction in the minds of certain engineers that the use 

 of superheated steam will involve operating difficulties which, taken in connection 

 with the added first cost, will more than offset any fuel saving. There are, of 

 course, conditions under which the installation of superheaters would be in no 

 way advisable. While such instances are perhaps rare, nevertheless, when a 

 superheater installation is contemplated, it must be considered in all of its phases. 

 The actual saving possible by the use of superheated steam must be balanced 

 against such factors as the initial cost and upkeep of the superheater, the 

 efficiency of the design of superheater to be installed, the nature of the service 

 of the plant in question, the design of the prime movers, a consideration of pipe 

 and fittings, and the like. 



No general statement of the saving possible may be made since this may 

 vary widely with a number of the factors above enumerated. 



The logical method of approaching the subject would appear to be, first, 

 a consideration of the saving in fuel possible through the use of superheated 

 steam regardless of other factors ; and, second, a consideration of the bearing of 

 such factors on the advisability of a superheater installation. 



In considering the saving possible by the use of superheated steam, it is too 

 often assumed that there is only a saving in the prime movers, a saving which is 

 at least partially offset by an increase in the fuel consumption of the boilers 

 generating steam. This misconception is due to the fact that the fuel con- 

 sumption of the boiler is only considered in connection with a definite weight of 

 steam. It is true that where such a definite weight is to be superheated, an 

 added amount of fuel must be burned. With a properly designed superheater, 

 where the combined efficiency of the boiler and superheater will be at least as 

 high as of a boiler alone, the approximate increase in coal consumption for 

 producing a given weight of steam will be as follows : 



Superheat Added Fuel Superheat Added Fuel 



Degrees Per Cent Degrees Per Cent 



25 1.59 100 5.69 



50 3.07 150 8.19 



75 4.38 200 10.58 



These figures represent the added fuel necessary for superheating a definite 

 weight of steam to the number of degrees given. A heat unit basis is standard 

 in the consideration of boiler evaporation and from this standpoint, again 

 providing the efficiency of the boiler and superheater is as high as of the boiler 



29 



