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Development of Compound Steam Turbines for Industrial Applications P M V Subbarao Professor Mechanical Engineering Department Fluid Dynamic Solutions to Techno-economical Viability……

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Classification of Steam Turbines

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From Books of Sir Charles Parson In 1884 or four years previously, I dealt with the turbine problem in a different way. It seemed to me that moderate surface velocities and speeds of rotation were essential if the turbine motor was to receive general acceptance as a prime mover. I therefore decided to split up the fall in pressure of the steam into small fractional expansions over a large number of turbines in series, so that the velocity of the steam nowhere should be great. A moderate speed of turbine suffices for the highest economy.

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This principle of compounding turbines in series is now universally used in all except very small engines, where economy in steam is of secondary importance. The arrangement of small falls in pressure at each turbine also appeared to me to be surer to give a high efficiency. The steam flowed practically in a non-expansive manner through each individual turbine, and consequently in an analogous way to water in hydraulic turbines whose high efficiency at that date had been proved by accurate tests.

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Impulse Vs Parson (50% Reaction) Fixed blade row accelerates the steam Moving blade row changes only the direction of the steam Force comes from rate of change of momentum Fixed blade row accelerates the steam Moving blade row changes both the speed and direction of the steam Force comes from rate of change of momentum V a1 V r1 V r2 V a2 V a1 V r1 V r2 V a2

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The Reaction (~Parson’s) Stage V a0 =V a2 Fixed blades expand steam from V a2 to V a1 using a blades whose angle vary from 1 to 2. U V r1 V a1 V r2 V a2 11 11 22 22 Assuming that the gap between stator and rotor doesn’t alter the flow conditions. ?!?.... Also in a 50% reaction stage, the moving blades are a mirror image of the fixed blades, V a1 V r1 V r2 V a2

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The thermodynamics of 50% Reaction : SSSF V a1 V r1 V r2 V a2

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The Fluid Dynamics of 50% Reaction V a1 V r1 V r2 V a2

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Theory of Parson’s Blading V a1 = V r2 α 1 = β 2 approximately U V r1 V a1 V a2 11 11 22 11 V a1 V r1 V r2 V a2

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Strength of Blading For an ideal Impulse Blade : For an ideal Parson Blade:

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Blade Power Ideal Impulse Stage : Ideal Parson Stage :

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Blade Efficiency Available power in Impulse Stage : Available power in 50% Reaction stage :

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Stage Efficiency Impulse Stage : 50% Reaction stage :

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Maximum Efficiency of Impulse Blade

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Maximum Efficiency of Parson Blade

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Blade Power at Maximum Efficiency COnditions Ideal Impulse Stage : Ideal Parson Stage :

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Moderate Capacity of Parson : Same Blade Velocity So at optimum U/V a1, an impulse stage produces TWICE the power of a 50% reaction stage for same blade speed! This means that an impulse turbine requires only half the number of stages as a 50% reaction turbine for a given application! This fact has a major impact on the construction of the turbine It is also responsible for some of the greatest miss understandings, since people assume that this means that impulse blading is cheaper overall - this is NOT true! Impulse turbines have fewer stages, but they must use a different form of construction which is expensive

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