STEAM TURBINES 229 



the appearance of white smoke. When these signs are observed 

 the oil supply should immediately be increased to the greatest 

 possible amount. If this does not reduce the temperature of 

 the bearing or prevent its further heating, the turbine should 

 be shut down. To continue will result in burning out the 

 bearing, and it is better to stop before this happens. The high 

 speed of the shaft renders it impossible to nurse a hot turbine 

 bearing along as is done frequently in the funning of recipro- 

 cating engines. 



Use of Superheated Steam. As there are no internal rub- 

 bing surfaces in the steam turbine, superheated steam may be 

 employed without causing any of the lubrication troubles 

 attending its use in reciprocating engines. Because of the 

 greater amount of heat contained in a pound of superheated 

 steam, the economy of a turbine working with superheated 

 steam is greater than that of one working with saturated 

 steam; also, the efficiency is increased because the superheated 

 steam causes less frictional resistance to the motion of the 

 blades. To show the value of superheated steam in turbine 

 work, it may be stated that 50 F. of superheat reduces the 

 steam consumption about 6%; 100 F. of superheat reduces 

 it about 10%; and 150 F. of superheat reduces it about 13f%. 

 The use of high superheat, however, produces expansion of 

 the rotor and the casing and may cause the blades to interfere; 

 as a result, the usual degree of superheat in steam-turbine 

 practice is 100 F., and seldom exceeds 150 F. 



Exhaust-Steam Turbine. The steam turbine shows better 

 economy than the steam engine when working with low-pressure 

 steam in connection with a high vacuum; but when working 

 with high-pressure steam and a vacuum of about 26 in., the 

 engine is the more economical. As a consequence, a combi- 

 nation of the steam engine and the steam turbine has been 

 adopted. The engine uses the high-pressure steam from the 

 boilers and expands it to about atmospheric pressure. This 

 exhaust steam then passes into the turbine, which exhausts 

 into a condenser carrying a high degree of vacuum, and the 

 expansion is carried to the extreme practicable limit. The 

 turbine thus used in connection with an engine is termed an 

 exhaust-steam turbine. 



