Principles of naval engineering - Historic Naval Ships Association

Chapter 12- PROPULSION STEAM TURBINESBLADES-ROTOR147.95XFigure 12-7.—Section of impulse turbine rotor(with blades).used, the steam passes through one row afteranother, and each row uses part of the energyof the steam.IMPULSE STAGE.- In an impulse turbine, astage is defined as one set of nozzles and thesucceeding row or rows of moving and fixedblades. Since the only place a pressure dropoccurs in an impulse turbine is in the nozzles,another way of defining an impulse stage is tosay that it includes the nozzles and blading inwhich only one pressure drop takes place. Asimple impulse stage is often called a Rateaustage . Turbines consisting of a single Rateaustage (fig. 12-11) are not used as propulsionturbines but are frequently used to drive smallauxiliary units.REACTION STAGE. -In reaction turbines,one row of fixed blades and its succeeding rowof moving blades are taken as constituting onestage. Since the fixed blades in a reactionturbine are comparable to the nozzles in animpulse turbine, this definition of a reactionstage may seem very similar to the definitionof an impulse stage. However, there is this139.23Figure 12-8.— Hero's steam turbine.important difference: a reaction stage includestwo pressure drops, whereas an impulse stageincludes only one.VELOCITY-COMPOUNDED IMPULSE TUR-BINE.— One way of increasing the efficiency ofan impulse turbine is by velocity-compounding—that is, by adding one or more rows of movingblades to the rotor. 3 Figure 12-12 showsan impulse turbine that has two rows of movingblades on the rotor. This type of turbine iscalled velocity-compounded because the residualvelocity of the steam leaving the first row ofmoving blades is utilized in the second row ofmoving blades. If a third row is added, thevelocity of the steam leaving the second rowis utilized in the third row. The fixed blades,Velocity-compounding can also be achieved when onlyone row of moving blades is used, provided the steamis directed in such a way that it passes through theblades more than once. This point is discussed inmore detail in chapter 16 of this text, in connectionwith helical-flow auxiliary turbines.325