SUPERHEATED STEAM IN MARINE PRACTICE. 

 By Henry B. Oatley, Esq., Member. 



[Read at the twenty-third general meeting of the Society of Naval Architects and Marine Engineers, held in 



New York, November 18 and 19, 1915.] 



INTRODUCTION. 



1. The purpose of this paper is to invite attention, in a general way, to the 

 position now occupied by superheated steam in marine practice, and especially to 

 the prominent place it now has in the minds of foreign shipowners and builders. 



2. Superheated steam, per se, will not be dwelt upon, as technical publications 

 and society transactions during the last ten years have devoted considerable spact 

 to this phase of the subject. Brief mention later in the paper will be made of gen 

 eral conditions of superheating in lines other than marine practice. 



3. Superheating is largely a question of degree or amount. For the purposes 

 of this article the following divisions will be made: — 



1st. Low superheat, zero to 50° F. 



2d. Moderate superheat, 50° to 125° F. 



3d. High superheat, 125° F. upwards. 



4. Lmv Degree Superheat, perhaps being the easiest to obtain, has, in many in- 

 stances, been tried and found of some advantage. A common, and in some cases 

 unintentional, development of low superheat has been by throttling or wire-draw- 

 ing. A boiler pressure of 10 pounds or more above steam chest pressure permits, 

 by wire-drawing through the throttle, a reduction in moisture, and, if the moisture 

 is in small amounts, slightly superheated steam may actually be obtained; how- 

 ever, if some distance exists between the throttle and the steam chest, radiation 

 losses will cause a loss of a part, or all, of the superheat. 



5. Low degree superheaters are generally exposed to relatively low tempera- 

 ture gases. In such cases a change in the gas temperature becomes a point more 

 to be considered than where the gases are at high temperatures. The difference 

 between the gas and the steam temperature is, of course, small and a reduction in gas 

 temperature which might be occasioned — for example, opening the furnace door — 

 would easily bring the gas temperature below the steam temperature. If such a 

 condition existed, the superheater would become a condenser, /. e., would give up 

 heat to the gases. This should, of course, be guarded against. It should also be 

 considered that, below certain temperatures, gases deposit soot in large quantities. 

 This may be quite easily appreciated by those who have operated feed water heat- 

 ers or "economizers," many being provided with tube-scraping arrangements, 

 either manually or automatically operated, which are intended to remove accumu- 

 lated soot, thus enabling the apparatus to be operated at its designed efficiency. 



