174 SUPERHEATED STEAM IN MARINE PRACTICE. 



6. Moderate Superheat has, during the last twenty years, been more widely 

 used. Superheaters developing between 50° and 125° of superheat have been con- 

 structed in a multitude of ways, and have been operated with a greater or less 

 degree of satisfaction, but, in marine service at least, have not obtained any uni- 

 form degree of application. This is particularly true of fire-tube boilers, as com- 

 pared with water-tube construction. 



7. High Degree Superheat, i. e., above 125° F., has, during the past ten years, 

 been meeting with pronounced favor in merchant service, as well as in naval con- 

 struction. This statement is believed to apply much more to England and conti- 

 nental countries than to the United States. 



8. Gray, in a paper presented before the Institution of Naval Architects on 

 April 2, 1914, gives some very interesting figures as to steam temperatures. In 

 the tests which he describes it was found that on a triple-expansion engine having 

 180° of superheat in the high-pressure chest, 100° of superheat was carried 

 through to the intermediate chest. This practically all disappeared before reach- 

 ing the low-pressure chest. In another ship having quadruple engines a superheat 

 above 210° F. was carried to the high-pressure chest. This was decreased to about 

 125° of superheat in the first intermediate, and to just a trace, perhaps 10° F. of 

 superheat, in the second intermediate, the low-pressure steam being very nearly 

 dry. This point is considered of interest as showing that for triple and quadruple 

 engines high superheat should be used if it is desired to obtain the large economies 

 which are given in the tests reported within the past few years. 



SUPERHEATING DEVELOPMENT. 



9. To say that superheating is almost as old as the use of steam, and that it 

 was from very early times known to be extremely efficient, is probably stating 

 a platitude. Technical literature, in many branches of engineering, has referred 

 at length to Watt, Hirn and others, who many years ago worked assiduously to 

 find satisfactory methods by which they could obtain and utilize this desirable 

 quality in steam. In 1859 the late Mr. John Penn* advocated strongly the use of 

 superheaters and pointed out the advantages to be attained. In an article appear- 

 ing in the Mechanics Magazine^ some reference is made to difficulties experienced 

 at that early date. It was not due to lack of knowledge, but rather to lack of suitable 

 materials and also methods for manufacturing steam containers, in these early 

 days, which prevented the widespread adoption of superheated steam. Watt had 

 no steel mills and high-grade steel foundries from which to obtain the necessary 

 plates, tubes, castings, etc., with which to build satisfactory superheating appa- 

 ratus. These limitations prevailed, to a great extent, till within fifteen or twenty 

 years of the present time and, naturally, retarded the growth of the superheater 



♦Institution of Mechanical Engineers, 1859. 

 tLondon, April 26, 1861. 



