PRINCIPLES OF NAVAL ENGINEERING 



blades are mounted on the revolving rotor, 

 they are called moving blades . 



Fixed or stationary blades of the same shape 

 as the moving blades are fastened to the casing 

 in which the rotor revolves; these fixed blades 

 are installed between successive rows of the 

 moving blades. The fixed blades guide the 

 steam into the moving blade system and, since 

 they are also shaped and mounted in such a 

 way as to provide nozzle- shaped spaces be- 

 tween the blades, the fixed blades also act as 

 nozzles. The general arrangement of the fixed 

 and moving blades, together with the pressure 

 and absolute velocity relationships in a reaction 

 turbine, are shown in figure 12-9. Figure 12-10 

 shows a section of a reaction turbine rotor with 

 one row of moving blades and one row of fixed 

 blades. 



A reaction turbine is moved by three main 

 forces: (1) the reactive force produced on the 

 moving blades as the steam increases in veloc- 

 ity as it expands through the nozzle- shaped 

 spaces between the blades; (2) the reactive 

 force produced on the moving blades when the 

 steam changes direction; and (3) the push or 

 "impulse" of the steam impinging upon the 

 blades. Thus, as previously noted, a reaction 

 turbine is moved primarily by reactive force 

 but also to some extent by direct impulse. 



From what we have already learned about 

 the function of nozzles, it will be apparent that 



thermal energy is converted into mechanical 

 kinetic energy in the blading of a reaction 

 turbine. The second required energy transfor- 

 mation—that is, from kinetic energy to work— 

 also occurs in the blading. A velocity diagram 

 such as was used to analyze the work done on 

 impulse blading may be similarly used to analyze 

 the work done on reaction blading; however, the 

 angles and velocities are different in the two 

 types of blading. 



Since the velocity of the steam is increased 

 in the expansion through the moving blades, the 

 initial velocity of the entering steam (V^) must 

 be lower in a reaction turbine than it would be 

 in an impulse turbine with the same blade speed 

 (V^); or, alternatively, the reaction turbine must 

 run at a higher speed than a comparable im- 

 pulse turbine in order to operate at approxi- 

 mately the same efficiency. 



TURBINE CLASSIFICATION 



As we have seen, turbines are divided into 

 two general groups or classes— impulse turbines 

 and reaction turbines— according to the way in 

 which the steam causes the rotor to move. 

 Turbines may be further classified according 

 to (1) the manner of staging and compounding, 

 and (2) the mode of steam flow through the 

 turbine. 



Staging and Compounding 



PRESSURE 



ABSOLUTE 

 VELOCITY 



NOZZLE-^ 



BLADES 



38.76X 

 Figure 12-6. — Nozzle position and pressure- 

 velocity relationships in an impulse turbine. 



Thus far in this chapter, we have more or 

 less assumed that an impulse turbine had one 

 set of nozzles and one row of blading on the 

 rotor, and that a reaction turbine had one row 

 of fixed blades and one row of moving blades. 

 In reality, however, propulsion steam turbines 

 are not this simple. Instead, they use several 

 rows of blading, arranged in various ways. 



It has been shown that the amount of ther- 

 mal energy which can be utilized in a turbine 

 depends upon the relationship between the veloc- 

 ity of the entering steam (Vi) and the blade 

 speed (Vb). It might seem reasonable, there- 

 fore, to think that the work output of the tur- 

 bine could only be increased by increasing 

 Vi and Vb in the proper ratio. However, 

 mechanical considerations and problems con- 

 cerning strength of materials impose certain 

 limits on blade speed. In modern naval ships, 

 the amount of available energy per pound of 

 steam is so great that there is no practicable 

 way of utilizing the major portion of it in one 

 row of blades. When several rows of blades are 



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