CHAPTER 12 



PROPULSION STEAM TURBINES 



In beginning the study of steam turbines, 

 the first point to be noted Is that we have now 

 reached the part of the thermodynamic cycle 

 In which the actual conversion of thermal 

 energy to mechanical energy takes place. 



We know by simple observation of pres- 

 sures and temperatures that the steam leaving 

 a turbine has far less thermal energy than It 

 had when It entered the turbine. By observa - 

 tlon, again, we know that work Is performed as 

 the steam passes through the turbine, the work 

 being evidenced by the turning of a shaft and 

 the movement of the ship through the water. 

 Since we know that energy can be transformed 

 but can be neither created nor destroyed, the 

 decrease of thermal energy and the appearance 

 of work cannot be regarded as separate events. 

 Rather, we must Infer that thermal energy ha,s 

 been transformed Into work— that Is, mechanical 

 energy In transition. 



Disregarding such Irreversible energy 

 losses as those caused by friction and by heat 

 flow to objects outside the system, It can be 

 shown that two energy transformations are In- 

 volved. First, there Is the thermodynamic 

 process by which thermal energy Is trans- 

 formed Into mechanical kinetic energy as the 

 steam flows through one or more nozzles. 

 Second, there Is the mechanical process by 

 which mechanical kinetic energy Is transformed 

 Into work as the steam Impinges upon pro- 

 jecting blades of the turbine, thereby turning 

 the turbine rotor. 



In order to understand the process by which 

 thermal energy is converted Into mechanical 

 kinetic energy, we must have some understand- 

 ing of the process that takes place as steam 

 flows through a nozzle. The second energy 

 transformation, from kinetic energy to work, 

 is best understood by considering some basic 

 principles of turbine design. 



STEAM FLOW THROUGH NOZZLES 



The basic purpose of a nozzle Is to con- 

 vert the thermal energy of the steam Into 

 mechanical kinetic energy. Essentially, this Is 

 accomplished by shaping the nozzle In such a 

 way as to cause an Increase In the velocity of 

 the steam as It expands from a high pressure 

 area to a low pressure area. The nozzle also 

 serves to direct the steam so that it will flow 

 In the right direction to Impinge upon the tur- 

 bine blades. 



Within certain limitations, the velocity of 

 steam flow through any restricted channel such 

 as a nozzle depends upon the difference be- 

 tween the pressure at the Inlet of the nozzle 

 and the pressure at the region around the out- 

 let of the nozzle. Let us begin by assuming 

 equal pressure at Inlet and outlet. No flow 

 exists In this static condition. Now, if we 

 maintain the pressure at the inlet side but 

 gradually reduce the pressure at the outlet 

 area, the steam will begin to flow and Its 

 velocity will Increase as the outlet pressure 

 Is reduced. However, If we continue to re- 

 duce the outlet pressure, we will reach a 

 point at which the velocity of steam is equal 

 to the velocity of sound in steam . At this 

 point, a further reduction in pressure at the 

 outlet region will not produce any further in- 

 crease in velocity at the entrance to the noz- 

 zle, nor will it produce any further Increase 

 In the rate of steam flow. 



The ratio of outlet pressure to Inlet pres- 

 sure at which the acoustic velocity (also called 

 the critical flow ) Is reached 'is known as the 

 acoustic pressure ratio or the critical pres- 

 sure ratio. This ratio is about 0.55 for super- 

 heated steam. In other words, the velocity of 

 flow through nozzles is a function of the pres- 

 sure differential across the nozzle, and steam 

 velocity will increase as the outlet pressure 



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