70 PROGRESS IN TURBINE SHIP PROPULSION. 



200 pounds pressure and 200° F. superheat are regarded as a more or less everyday 

 operating condition for large plants. Steam generated at 600 pounds pressure, 

 having exactly the same heat content as that contained in 200 pounds pressure and 

 200° F. superheat, wilj have a superheat of approximately 128° F. This, expanded 

 to 29 inches of vacuum, is theoretically capable of giving 13 per cent more energy 

 than when generated at 200 pounds and expanded to the same vacuum. Doubtless, 

 when operating under the high-pressure conditions, the turbine will be of lower effi- 

 ciency. The high-pressure element will be less efficient on account of the great den- 

 sity and the small volume of the steam, and on the other hand the low-pressure 

 elements will be less efficient because of the great amount of water precipitated by 

 the steam expansion from the high pressure, introducing a brake in the turbine. 

 However, it is reasonable to suppose that the turbine will avail itself of at least 

 50 per cent of this 13 per cent possibility, producing a net saving of 6 or 7 per cent. 

 What may be expected to be derived from higher pressures is exhibited in Fig. 11, 

 Plate 36, and Fig. 12, Plate 37, plotted for various pressures. 



ECONOMIZERS. 



So far as the author is aware, economizers have not been employed in marine 

 installations. It would seem that their capital cost would be well warranted for the 

 case of express steamers operating always at their maximum speed. In the case of 

 250 pounds gauge steam pressure there is a 16 per cent saving in the work of the 

 boiler by a feed temperature of 375° F. as compared with 212° F. To this gain 

 there must be made a deduction of the energy expanded by the induced draft fans 

 and the scrapers for the economizer surfaces. A net gain of 10 per cent should be 

 readily realized. Their installation would seem to present no particular difficulty as 

 is shown in Fig. 13, Plate 40. 



SUPERHEAT. 



With the old direct-connected turbines, superheat has never been recommended, 

 and wisely so considering their immense structure. Even with the much smaller 

 high-speed geared turbines as high superheat as is desirable in land installations 

 may not be practicable at sea on account of the rapid stopping, starting, etc., neces- 

 sary for quick maneuvering, which may cause uneven heating and cooling. In the 

 author's opinion superheats as high as 100° F. should present no operating diffi- 

 culty whatsoever with well-designed high-speed turbines. 100° F. superheat will 

 affect the steam consumption not less than 8 per cent as compared with dry saturated 

 steam, probably saving some 4 or 5 per cent in fuel. 



CONDENSERS. 



Inasmuch as turbines may be designed to expand the steam to vacuum of 29 

 inches, and as the steam consumption with such a turbine will be 7 per cent bet- 



