PROGRESS IN TURBINE SHIP PROPULSION. 71 



ter with 29 inches vacuum than with 28 inches, providing it be designed to expand to 

 such low pressures, the necessity of providing sufficient well-disposed surface to pro- 

 duce such a vacuum is not only obvious, but it is furthermore necessary to have a 

 complete understanding of what constitutes good condenser performance with th^ 

 apparatus available. 



An important detail is the drop in steam pressure through the condenser. For 

 good practice this should not exceed i/io inch mercury. The air pump should be of 

 a type capable of completely evacuating the non-condensible vapor from the con- 

 denser so that the condenser shell shall contain nothing but steam, and therefore 

 be of the same temperature throughout except for the small pressure drop within 

 the shell itself. To produce this condition requires not only excellent air remov- 

 ing apparatus, but constant vigilance in the elimination of air leaks. It is the prac- 

 tice of certain large central stations to provide an air bell, which may on occasion be 

 connected to the discharge of air pumps, by means of which the air leakage may be 

 measured. With 40,000 horse-power units, 3^ cubic feet per minute of free air is 

 considered good practice, and should this reach to seven it is the signal for a search 

 to discover the source of the leak. Such a practice would be impracticable on ship- 

 board and only serves to point out how central station engineers regard the ill ef- 

 fects of air leaks. 



With such a condenser having practically uniform steam temperature through- 

 out the shell, there is the greatest temperature difference between circulating water 

 and steam in the bottom pass (assuming steam enters at the top and water at the 

 bottom) ; while, on the other hand, in the top pass there is but a small temperature 

 difference. Hence much more steam is condensed at the bottom than at the top, 

 contrary to what is usually experienced with condensers that do not have the non- 

 condensible vapors completely evacuated. This calls for especial care in having 

 paths for the passage of steam through the upper zones of the condenser to insure 

 the minimum pressure drop. 



With the complete evacuation of the non-condensible vapors, the vacuum should 

 correspond to a temperature not to exceed one or two degrees higher than the 

 temperature of the condensed steam leaving the condenser, which is a point that 

 should be kept constantly under observation. 



OILING SYSTEMS. 



While the oiling system can bear no relation to the economics of turbine pro- 

 pelled ships, it is of vital concern to the reliability of operation, and too much care 

 cannot be given to the layout of the system to the end of obtaining the extreme de- 

 gree of simplicity and reliability. 



It has been generally the custom in Westinghouse installations to provide oil 

 pumps directly driven by gearing connected to the main gears. The double-pinion 

 double-reduction gears, of which a large number have been built, have two oil pumps, 

 one driven from each intermediate gear shaft. The design contemplated for the 



