176 SUPERHEATED STEAM IN MARINE PRACTICE. 



crating with what we have classed as "high superheat." In reality the adoption 

 of superheated steam is world-wide, and is permeating nearly all branches of steam 

 engineering. Marine engineers in the United States have recognized the advan- 

 tages of superheated steam for both merchant and naval service, but the growth 

 does not seem to have been as pronounced here as in European countries. There 

 has probably been some good reason for the slower advance, and it would be of 

 interest to determine the reason, or reasons, for this situation. 



AIM IN SUPERHEATER DESIGN. 



12. In the development of a superheating apparatus there are a number of 

 points which should be ever present in the mind of the designer. It is believed 

 that the following represent fundamental requirements: 



1st. Efficiency. 

 2d. Simplicity. 

 3d. Accessibility. 

 4th. Durability. 



13. Efficiency in design is basic. Upon this success or failure of any apparatus 

 must be eventually determined, and without it a low-priced device is of no value. 

 In a great many instances a high efficiency has been offset by one or more of the 

 three points which will be discussed below. It is interesting in this connection to 

 note : — 



"For efficient superheating, the tubes should be small, or, if large, should be 

 fitted either with cores to form a thin annulus of steam or with deflecting plates to 

 rotate the steam and throw the heavier and colder particles against the hot tube."* 



14. Another fundamental widely accepted by designers of superheaters is for 

 a high velocity of steam past the superheating surface. Cathcart refers to this as 

 follows : — 



"Rapid relative movement of the heat-delivering and heat-receiving surfaces 

 increases the rate of heat-transfer, because this movement keeps the temperature 

 difference at its maximum." 



15. Simplicity is perhaps an easy problem. All superheaters are, in a sense, 

 simple as compared with apparatus that has moving parts. Simple construction is 

 directly of value, in that it generally prevents excessive cost. This latter may ap- 

 pear either as first cost or installation cost. In either case it represents less money 

 to be invested, and thought and study are required to make such investment a 

 minimum. 



16. Accessibility demands all the ingenuity that the designer possesses. An 

 efficient and simple apparatus would be discarded were it to be so inaccessible as 

 to prevent inspection, renewal and repairs. 



•Cathcart, Journal of the American Society of Naval Engineers, August, 1915. 



