598 A century's riioouEss of the steam enotne. 



quality as to rediuo appreoial)ly. it not entirely, this heat exchange. 

 Eitlier fluid or cylinder wall Weino- nonoondueting, heat exchange is 

 impossible. 



Steam drying and superheating has come to bo recognized as an 

 essential process in the economical operation of the steam engine. 

 Separators at or nc^ir the engine cylinder are now made very efficient 

 in the removal of all i)articles of water fiom the steam entering the 

 engine, and thussuperlu'ating is very eti'ectively facilitated; hut su])er- 

 heating itself is a problem in construction and in operation which is 

 not even yet completely solved. Nevertheless, all engines exhibiting 

 maxinuun economy to-day etuploy steam effectively dried and moie 

 or less superheated. These processes are not only practi{!ed in the 

 passage of the steam into the engine, but they are also often employed 

 between cylinders where the engine is of the multiple-cylinder type. 

 Here separation is always pi-acticable and easily made effective; l)ut 

 superheating, even where it is provided for, is seldom accomplished 

 in * reheaters." One heat unit employed in superheating the steam 

 preliminarily to its introduction into the cylinder, whether high- 

 })ressure. intermediate, or low, is worth several tMuployed in evapo- 

 rating additional steam; yet such are the practical ditficulties that even 

 the best of modiM-n engines are rarely supplied with steam superh(Mited 

 more than 50 ' F., and effei-tive superheating between cylinders is \-eiy 

 .seldom accomplished. \Vhere it is successfully introduced the effect 

 is probably always to very considei'ably iiu])rove the action of the 

 machiMc and reduce its I'xjienditui'c of steam and of fuel. The highest 

 modern records ai'e iield by engines in which the ideal thermodynamic 

 conditions are most closely appioximated in this respect. The usual 

 variation of etficien<-v with xariation of engine speed is not, in this 

 ease, so ob.-jcr\ al)le. and is far less important. 



The fundauiental deductions from experii^nce. as well as fi'om scien- 

 tific examination of the case, and the principles controlling the con- 

 struction and operation of the steam engine in which high eificiency is 

 sought, are the following: 



(1) Make the steam pressure adopted as high as, under existing 

 conditions, is safe. 



{'2) Adopt the lowest practicable ))ack pressure. 



(3) Expand through the widest range of temperature and pressure 

 found commercially satisfacto^3^ 



(4) Adopt as high (Migine speed as is safe. 



(5) Employ dry and. if practicable, moderately superheated steam 

 in all cylinders. 



(«)) So design the machine that friction and external wastes of heat 

 shall be reduced to the lowest practica))le amounts. 



(7) In the application of any expedient for promoting efficiency, 

 seek that limit at which further gain is compensated by the additional 



