780 



SCIENCE. 



[N. S. YoL. IV. No. 10». 



heat expended in superheating the enter- 

 ing steam effects a comparatively large re- 

 duction in the expenditure of steam in the 

 engine; each thermal unit thus employed 

 saving several thermal units otherwise 

 wasted. The process is one, mainly, at 

 least, of prevention rather than of cure of 

 that fault ; and prevention is, as usual, here 

 found to be vastly more effective than at- 

 tempted cure. 



(3) Superheating is superior to any other 

 known means of reduction of internal waste. 

 Jacketing ordinarily suppresses but a frac- 

 tion of that waste, and the multiple-cylinder 

 engine has also its limitations ; while super- 

 heating may not only extinguish it, but may 

 also check wastes due the resistance to flow 

 of the denser, wet steam through steam and 

 exhaust ports, and may sensibly improve 

 the vacuum attainable in the condenser, 

 with corresponding reduction of back pres- 

 sure, of the quantity of condensing water 

 demanded, and of the load on the air pump. 



Superheating even a few degrees improves 

 considerably the performance of the engine, 

 and, in the average case, superheating one 

 hundred degrees Fahrenheit will entirely 

 extinguish that waste. 



(4) The hitherto unconquered obstruc- 

 tions to the use of superheated steam in 

 the engine have been those resulting from 

 destruction of packing and decomposition 

 of lubricants, with consequent friction and 

 ' cutting ' of the rubbing surfaces. The 

 introduction of metallic packings and of 

 high-test lubricants has now enormously 

 reduced the difficulties of application of 

 superheating. No trouble need now be 

 found at the engine with sufficient super- 

 heating, under usual conditions of opera- 

 tion, to annihilate cylinder condensation. 

 It seems not at all improbable that even this 

 limit may be ere long safely, and perhaps 

 even largely, overpassed, with resulting im- 

 provement of thermodynamic efficiency. 



(5) The obstruction at the boiler has 



been, and still remains, difficulty of con- 

 struction of a superheater, or of a super- 

 heating system, which will be at once effec- 

 tive, safe and durable. 



The comparatively low temperatures at 

 which modern boilers discharge their gase» 

 into the uptake, while reducing these diffi- 

 culties largely, introduces the complemen- 

 tary one of increased necessary area of 

 superheating surface, and consequent vol- 

 ume, weight and cost of the superheaters. 

 The real difficulty is to-day found at this 

 point, and the production of a superheater 

 which will safely withstand the effects of 

 high temperature of flue gases, will effec- 

 tively transfer heat from gas to steam, and 

 will have a satisfactorily long life, still 

 challenges the engineer as one of his most 

 serious, yet attractive and important 

 problems. 



(6) The more wasteful the engine, the 

 larger the promise of gain by superheating, 

 and small engines will profit by it more than 

 large, slow engines more than fast, and 

 simple engines more than the multiple- 

 cylinder systems, which latter require such 

 auxiliaries less as their cascade action is the 

 greater and its steps more numerous. 



(7) The larger the waste to be checked in- 

 the engine, the farther should the super- 

 heating be carried. That degree which 

 would serve every purpose in the simple, 

 slow, small mill engine would be entirely 

 too high for safe use, and quite inexpedient,, 

 in the high-speed compound of large size, 

 while that which would be ample for the 

 latter would be entirely insufficient for the 

 former. 



(8) The extent of superheating should 

 be adjustable — not only to the particular 

 size and type of engine in view, but also in 

 the same engine — to the extent to which 

 expansion is carried. 



A perfectly satisfactory system of super- 

 heating should be adjustable in this respect 

 with the load on the engine, and still free 



