SUPERHEATED STEAM IN MARINE PRACTICE. 177 



17. Durability, which is, of course, another way of saying "long life," is an- 

 other basic requirement. Failure to provide this has caused the abandonment of 

 a great number of superheater designs. The nature of the work the superheater 

 has to do requires its parts to be subjected to varying temperature conditions, both 

 from within and without. Action upon the material of the superheater, both from 

 the products of combustion as well as from the steam, and, in some designs, from 

 intermittent applications of water, has proven destructive. When we look back 

 upon the various designs which have been brought out, it is evident that these con- 

 ditions have not been fully appreciated by those who built such apparatus. 



18. Superheaters are subjected to relatively high gas temperatures, and any 

 change in the heat-transferring ability of the superheating surface will be towards 

 a reduction in its efficiency. This in turn will mean an increase in the 

 temperature of the metal and consequent weakening. Tubes which are subjected 

 to internal pressure are better able to resist deformation than those subjected to 

 external pressure. In general, it may be said that all of the widely adopted and 

 successfully used superheaters have had one point in common, viz., that super- 

 heater pipes are subjected to internal pressure. This has appeared to be a start- 

 ing point in the minds of all designers, probably from the standpoint of safety. 

 Watkinson, in speaking of this condition, says: — 



"Attempts have frequently been made to make part of the boiler-heating sur- 

 face serve for superheating the steam. In boilers of the Serpollet type this is done, 

 and for motor car purposes, where minimum weight is one of the most important 

 factors, this arrangement may be satisfactory. But, wherever this arrangement 

 is used, some of the tubes or some parts of the tubes are alternately wet and red 

 hot."* 



19. The modern tendency towards high degrees of superheat has, of course, 

 meant subjecting superheating apparatus to higher gas temperatures. On first 

 thought this would appear to aggravate destruction of the apparatus, as there are 

 limiting temperatures above which oxidization of all usable materials would take 

 place. Experience, however, has shown the builders how far they may go without 

 harmful results, and progressive designers have taken full advantage of this 

 knowledge. It is, of course, evident that, were many restrictions removed, dif- 

 ferent materials could be used and some advantages obtained thereby. Practical 

 considerations have required compactness in the apparatus, which undoubtedly adds 

 some cost by making it advisable to use high-priced material. 



20. From information obtained it appears that a well-designed superheater, 

 even one that is developing high degrees of superheat, has at least a life as long 

 as that of the boiler tubes. Many instances have come to the attention of the writer 

 where the life of some of these superheaters is above eight years, and in some 

 cases twelve years, without repairs of any extent. It would seem not at all un- 



* Institution of Naval Architects. 



