Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGFIXEDBLADESENTERINGSTEAMPRESSUREVELOCITY38.77. 2XFigure 12-9.— Arrangement of fixed and movingblades and pressure-velocity relationships ina reaction turbine.which are fastened to the casing rather thanto the rotor, serve to direct the steam fromone row of moving blades to another.As may be seen in figure 12-12, the velocitycompoundedimpulse turbine has only one pressuredrop and therefore, by definition, onlyone stage. This type of velocity- compoundedimpulse stage is usually called a Curtis stage.PRESSURE-COMPOUNDED IMPULSE TUR-BINE,— Another way to increase the efficiencyof an impulse turbine is to arrange two or moresimple impulse stages in one casing. The casingis internally divided by nozzle diaphragms. Thesteam leaving the first stage is expanded againthrough the first nozzle diaphragm, to the secondstage; from the second nozzle diaphragm,to the third stage; and so on. This type of turbineis known as a pressure- compounded turbinebecause a pressure drop occurs in eachstage. Figure 12-13 shows a pressure-compoundedimpulse turbine with four stages. Apressure- compounded impulse turbine is frequentlycalled a Rateau turbine, since it is essentiallya series of simple impulse (Rateau)stages arranged in sequence in one casing.PRESSURE-VELOCITY-COMPOUNDED IM-PULSE TURBINE. -An impulse turbine whichconsists of one velocity-compounded (Curtis)stage followed by a series of pressure-ROTORMOVING BLADES38.771XFigure 12-10.— Section of reaction turbine rotor,showing fixed and moving blades.compounded (Rateau) stages is generally referredto as a pressure-velocity-compoundedimpulse turbine. Turbines of this type arecommonly used in the propulsion plants ofnaval ships.PRESSURE - COMPOUNDED REACTIONTURBINE. -Because the ideal blade speed in areaction turbine is so high in relation to thevelocity of the entering steam (V^), all reactionturbines arepressure-compounded— thatis, theyare so arranged that the pressure drop frominlet to exhaust is divided into many steps bymeans of alternate rows of fixed and movingblades. The pressure drop in each set offixed and moving blades (i.e., in each stage) istherefore small, thus causing a lowered steamvelocity in all stages and consequently a loweredideal blade velocity for the turbine as a whole.COMBINATION IMPULSE AND REACTIONTURBINE.—A combination impulse and reactionturbine employs a velocity-compounded impulse326