STEAM TURBINES 225 



Comparison of Turbines and Engines. If the matter of 

 steam consumption alone is considered, the average condensing 

 turbine of less than about 700 H. P. is not so economical as 

 the average compound or triple-expansion condensing engine, 

 although the turbine may be preferred to the engine for other 

 reasons. In larger sizes, however, and particularly in very 

 large units, the economy of the turbine is very noticeable. 

 The turbine possesses the ability to expand the steam to the 

 lowest available condenser pressure without difficulty; bat 

 to do this in a reciprocating engine would require very large 

 valves, and ports and heavy pistons, because of the great 

 volume of steam to be handled at very low pressures. 



Finding Horsepower of Turbines. There is no way of 

 finding the indicated horsepower of a steam turbine, because 

 no form of indicator applicable to the turbine has been invented. 

 Nor is any such instrument likely to be developed, owing to 

 the very great difficulty of determining the energy given up 

 to the blades of a turbine from a jet of steam. The usual 

 way of finding the power of a steam turbine is to use a brake 

 or a dynamometer and thus to determine the brake horse- 

 power, or else to connect an electric generator to the turbine 

 and measure the electrical output at the switchboard. In 

 case the latter method is used, the efficiency of the generator 

 and the turbine together is involved. 



TURBINE TROUBLES 



Clearance of Blades. To obtain free running, it is necessary 

 to allow clearance between the stationary and the moving 

 rows of blades, as well as between the ends of the blades and 

 the casing or the rotor. In impulse turbines, such as the 

 Curtis and the Rateau, the clearance between the rows of blades 

 is important; however, if it is made no greater than is neces- 

 sary for mechanical reasons, the efficiency will not be affected 

 seriously. In the reaction turbine, such as the Parsons, the 

 clearance between the rows is of small consequence as com- 

 pared with the clearance between the ends of the stationary 

 blades and the rotor and between the ends of the moving 



