178 SUPERHEATED STEAM IN MARINE PRACTICE. 



reasonable to expect superheater equipment to have a life practically as great as 

 the boiler itself. If this is obtainable, many of the objections to superheating 

 should be removed. In many cases, even if the life of the equipment were less 

 than eight years but where fuel economies of lo per cent and upward are ob- 

 tained, there would still be considerable on the credit side of the account. 



21. It would seem from the very rapid adoption of fire-tube superheaters that 

 the designers and builders of this apparatus have definitely determined the funda- 

 mentally correct basis for proportioning and fitting this apparatus. The cut-and-try 

 method could not give the uniformly successful results which this design has 

 given. It would seem that this type of apparatus, which has had world-wide success 

 in locomotives, must be credited with flexibility and a basic law that makes it ef- 

 ficient, through a wide range of loads. It seems quite probable to the writer that 

 other designs are also capable of being proportioned on a scientifically correct 

 basis, but that they have not had a sufficient amount of study put upon them, and 

 that the lack of their general adoption is thus accounted for. 



SUPERHEATER DESIGN AND CONSTRUCTION. 



22. Superheaters may be broadly classified on the basis of their location as 

 follows : 



1st. Separately fired superheaters, i.e., those subjected to gases coming from a 

 source other than the furnaces of the main boilers. 



2d. Waste-gas superheaters, i.e., those subjected to gases on their way from the 

 boiler to the stack. 



3d. Live-gas superheaters, i.e., those subjected to gases which have not left the 

 main-boiler evaporating surface. 



2;^. The superheaters in each of the three classes above mentioned may be, and 

 have been, constructed on one of the two structural methods: (a) Tubular; (b) 

 Cellular. 



24. By tubular is meant a construction which requires the steam to pass 

 through tubes for a greater part, or all, of its path during which heat is added. 



25. By cellular is meant a construction which requires the steam to pass 

 through a chamber, usually of irregular shape, and to receive heat from gas 

 flowing through tubes which pass through the steam chamber. 



26. In other words, with tubular construction steam is carried inside the tube, 

 which is subject to bursting pressure, and with cellular construction gas is carried 

 inside the tube, which is subject to collapsing pressure. 



27. To make these structural dififerences more clearly understood, it may be 

 of interest here to devote some space to illustrating and describing a number of 

 superheater designs which have been produced and to point out some of the char- 

 acteristics of these designs, as well as their advantages and limitations. Among 

 the early designers of superheaters, particularly for marine application, tributes 

 should be paid to Partridge (1840?), Lamb (1858), Beardmore (1859), Boden and 



