Chapter 12-PROPULSION STEAM TURBINES 



The rate of doing work on the blade may 

 also be derived from consideration of the 

 thermal energy and kinetic energy entering 

 the system and leaving the system. Although 

 the actual derivations are not given here, it 

 may be of interest to note the relationships 

 expressed in the following equations: 



2 2 



W(Vi - V2) 

 Wk = W (hi - h2) + 50, 000 



and 



Btu per second 



^^ri, - W(Ri - R^ + RJ W(V? - v|) 

 b 50, 000 



Btu per second 



The pressure and velocity changes that oc- 

 cur in the nozzle and in the blades of an impulse 

 turbine are shown in figure 12-6. As may be 

 seen, the pressure is the same at the entrance 

 and at the exit of the blade; the only pressure 

 drop occurs in the nozzle. Figure 12-7 shows 

 a section of an impulse turbine rotor, with the 

 blades in place. 



Theory of Reaction Turbines 



Reaction turbines, as their name implies, 

 are moved by reactive force rather than by a 

 direct push or impulse. Although we com- 

 monly think of reactive force as having been 



NOZZLE 



BLADE;:^ «- V5 



"discovered" by Newton, it is interesting to 

 note that the first reaction turbine— and, in- 

 deed, perhaps the first steam engine of any 

 kind ever made— was developed by the Greek 

 mathematician Hero about 2000 years ago. 

 This turbine, shown in figure 12-8, consisted 

 of a hollow sphere which carried four bent 

 nozzles. The sphere was free to rotate on the 

 tubes that carried steam from the boiler, be- 

 low, to the sphere. As the steam flowed out 



147.94 

 Figure 12-5.— Velocity diagram for impulse 

 blading. 



through the nozzles, the sphere rotated rapidly 

 in a direction opposite to the direction of steam 

 flow. 



Reaction turbines used in modern times 

 utilize the reactive force of the steam in quite 

 a different way. In a modern reaction turbine, 

 there are no nozzles as such. Instead, the blades 

 that project radially from the periphery of the 

 rotor are formed and mounted in such a way 

 that the spaces between the blades have, in cross 

 section, the shape of nozzles. 2 Since these 



Figure 12-4.— Curved impulse blade. 



147.93 



The distinction between actual nozzles and the blading 

 which serves the purpose of nozzles in reaction tur- 

 bines is mechanical rather than functional. The pre- 

 vious discussion of steam flow through nozzles ap- 

 plies equally well to steam flow through the 

 nozzle-shaped spaces between the blades of reaction 

 turbines. 



323 



