Steam Turbine is a type of turbo machine. Turbo machine are those devices in which energy is transferred either to or from, a continuously flowing fluid by the dynamic action of one or more moving blade rows. In steam turbine energy is transferred from fluid to blade rows and is decreasing along the flow directions. It is power producing thermodynamics device. Steam turbine converts heat energy of steam (at high pressure and temperature) into mechanical energy. The so utilised can be used in various filed of industry such as electricity generation, transport, in driving of pumps, fan and compressor etc. the basic cycle on which steam turbine works is Rankine Cycle. The reciprocating steam engine was still inefficient, cumbersome, had a very low power to weight ratio, and was a high maintenance piece of machinery. The development of the steam turbine was a vast improvement in all of these respects. A turbine consist of one set of stationary blades or nozzles and an adjacent set of moving blades or buckets. These stationary and rotating elements act together to allow the steam flow to do work on the rotor. The ork is transmitted to the load through the shaft or shafts.
Figure 1.1 Block diagram of a Turbine
Steam turbines date back to 120 B.C. when the first steam turbine was developed by Hero of Alexandria. Subsequently number of steam turbines came up but the practically successful steam turbine appeared at the end of nineteenth century when Gustaf De Laval designed a high speed turbine built on the principle of reaction turbine in 1883. Before this in 1629 G. Branca developed the first impulse turbine. Branca’s impulse turbine and Hero’s reaction turbine are shown in Fig. 1.1.
Figure 1.2 Hero and Branca’s turbine.
In nineteenth century some more steam turbines were developed by Sir Charles A. Parsons and C.G. Curtis which gave a filip to the development to the modern steam turbine. Over the period of time the modern steam turbines evolved with capacity from few kilowatts to 350,000 kW and in speed from 1000 rpm to 40,000 rpm. Steam turbines offer the advantages over other prime movers in terms of simplicity, reliability and low maintenance costs. Reciprocating steam engines use pressure energy of steam while steam turbines use dynamic action of the steam. Steam turbines require less space as compared to diesel engine or steam engine and also the absence of reciprocating parts & reciprocating motion in steam turbine results in lesser vibrations and lighter foundation. In steam turbine the expanding steam does not come into contact with lubricant and so exhaust steam leaves uncontaminated.
The basic principle on which steam turbine works is Newton’s Second law of motion. The motive power of a high velocity jet impinging on a curved blade. The steam from boiler is expanded in a nozzle where due to fall in pressure of steam, thermal energy of steam is converted into kinetic energy of steam, resulting in the emission of a high velocity jet of steam which impinges on the moving vanes or blades, mounted on a shaft; here it undergoes a change in direction of motion which give rise to a change in momentum and therefore, a force. An ideal steam turbine is considered to be an isentropic process, or constant entropy process, in which the entropy of the steam entering the turbine is equal to the entropy of the steam leaving the turbine. Steam turbines are mostly 'axial flow' types; the steam flows over the blades in a direction Parallel to the axis of the wheel. 'Radial flow' types are rarely used.It should be noted that the blade obtains no motive force from the static pressure of the steam or from any impact of the jet, because the blade is designed such that the steam jet will glide on and off the blade without and tendency to strike it. Figure 1.3 Working of steam turbine
References: * Emmet, W.L.R., The Curtis Steam Turbine, Proceedings of the American Philosophical Society, Vol. 42, No. 172 (Jan. - Apr., 1903), page no-68-84.
* El-Wakil M. M., Powerplant Technology, Tata Mcgraw Hill Educaition Private Limited, New Delhi, 2010.
* Venkanna, B.K., Fundamentals of Turbomachinery, PHI Learning Private Limited, New Delhi, 2011.
* Yadav, R., Applied Thermodynamics, Central Publishing House, Allahabad, 2006.
* Yahya S. M., Turbines, Compressors and Fans (Fourth Edition), Tata Mcgraw Hill Educaition Private Limited, New Delhi, 2011.
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