Chapter 12- PROPULSION STEAM TURBINES 



■BLADES 



-ROTOR 



147.95X 

 Figure 12-7.— Section of impulse turbine rotor 

 (with blades). 



used, the steam passes through one row after 

 another, and each row uses part of the energy 

 of the steam. 



IMPULSE STAGE.- In an impulse turbine, a 

 stage is defined as one set of nozzles and the 

 succeeding row or rows of moving and fixed 

 blades. Since the only place a pressure drop 

 occurs in an impulse turbine is in the nozzles, 

 another way of defining an impulse stage is to 

 say that it includes the nozzles and blading in 

 which only one pressure drop takes place. A 

 simple impulse stage is often called a Rateau 

 stage . Turbines consisting of a single Rateau 

 stage (fig. 12-11) are not used as propulsion 

 turbines but are frequently used to drive small 

 auxiliary units. 



REACTION STAGE. -In reaction turbines, 

 one row of fixed blades and its succeeding row 

 of moving blades are taken as constituting one 

 stage. Since the fixed blades in a reaction 

 turbine are comparable to the nozzles in an 

 impulse turbine, this definition of a reaction 

 stage may seem very similar to the definition 

 of an impulse stage. However, there is this 



139.23 

 Figure 12-8.— Hero's steam turbine. 



important difference: a reaction stage includes 

 two pressure drops, whereas an impulse stage 

 includes only one. 



VELOCITY-COMPOUNDED IMPULSE TUR- 

 BINE.— One way of increasing the efficiency of 

 an impulse turbine is by velocity-compounding— 

 that is, by adding one or more rows of moving 

 blades to the rotor. 3 Figure 12-12 shows 

 an impulse turbine that has two rows of moving 

 blades on the rotor. This type of turbine is 

 called velocity-compounded because the residual 

 velocity of the steam leaving the first row of 

 moving blades is utilized in the second row of 

 moving blades. If a third row is added, the 

 velocity of the steam leaving the second row 

 is utilized in the third row. The fixed blades, 



Velocity-compounding can also be achieved when only 

 one row of moving blades is used, provided the steam 

 is directed in such a way that it passes through the 

 blades more than once. This point is discussed in 

 more detail in chapter 16 of this text, in connection 

 with helical-flow auxiliary turbines. 



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