Posted By: Shraddha Raut

June 18, 2016


Steam turbine is one of the great inventions of Parsons, producing power by utilizing the velocity of a jet of steam, instead of using the pressure of the steam to drive a piston as in the ordinary reciprocating engine. It was evident that a jet of steam could be made to turn a wheel by acting on blades set around its circumference, or alternatively it could be used to develop power by its own reaction when escaping tangentially from an orifice in a rotating wheel or arm. Both devices had already been suggested by innumerable inventors, but was not practically implemented due to the excessive velocity of the steam. Even steam at a comparatively low pressure escaping into the atmosphere may easily be travelling at more than 2500 feet per second, or over I700 miles an hour, while twice this velocity may be attained by high-pressure steam flowing into a good vacuum. To make use of such velocities effectively in a simple turbine, the blades or other moving elements would have to travel at about half the speed of the steam, for otherwise an undue proportion of the energy of the jet would be uselessly carried away in the steam leaving the wheel. The blade speeds required for efficiency would therefore be so high that they would be prohibited by reason of centrifugal force alone, apart from other considerations. He could therefore only secure a proper relationship between steam speed and blade speed by reducing the former to a manageable amount. Now the speed of a jet of steam will obviously depend upon the difference of pressure that causes the flow. It occurred to Parsons that he could attain his end by the device of causing the whole expansion of the steam to take place by a series of steps, each partial drop of pressure being only sufficient to generate a velocity that could be efficiently utilized by blades running at a moderate speed. To put this idea into effect he constructed a turbine consisting of a cylindrical rotor enclosed in a casing. The steam flowed along the annulus between the two, parallel to the axis of the machine, and in so doing it had to pass through rings of blades fixed alternately in the casing and rotor. The passages between the blades of each ring formed virtually a set of nozzles in which a partial expansion of the steam could take place. In passing through each ring of fixed blades the steam acquired a certain velocity due to this expansion, and the jets so formed gave up their energy in driving the succeeding row of moving blades. The passages between the latter blades also acted as nozzles, permitting a further partial expansion, so that the moving blades were impelled partly by the 'action' of the steam entering them and partly by the 'reaction' of the steam leaving them. The principle of subdividing the whole expansion of the steam into a number of stages, so that only comparatively moderate velocities have to be dealt with, still forms the basis of all efficient turbine design. The secondary principle of utilizing the 'reaction' of the steam expanding in moving blades has remained typical of the Parsons turbine. It is not, however, a crucial characteristic of an efficient turbine, and certain inventors subsequent to Parsons, notably C. G. Curtis in the United States and Professor A. Rateau in France, preferred for constructional reasons to confine the expansion of the steam to fixed nozzles. Machines of the latter type, in which the steam drives the blading of each stage by virtue of its velocity, only, are known as 'impulse' turbines. Although they have attained an honorable position in the industry, it is generally recognized that the 'reaction' principle, chosen by Parsons for his original turbine, is conducive to the highest efficiency, so that large machines which are nominally of the impulse type are now often designed to work with a certain amount of reaction in their blading. In addition to laying down the broad lines necessary to success in the development of the new kind of prime mover, Parsons had many practical problems to solve before his ideas could be embodied in an actual machine. Not only had a suitable form of blading to be invented and appropriate manufacturing methods devised, but the design generally had to conform to conditions quite outside the range of ordinary engineering practice. For example, to obtain the desired blade velocity in the small turbine, first a rotational speed of 20000 revolutions per minute had to be adopted. This was over fifty times as fast as the fastest reciprocating engine of the day, and it involved the invention of a new kind of bearing which would permit a long rotor, inevitably out of mathematically perfect balance, running at such a speed without vibration. Means had also to be provided for the continuous lubrication of these bearings, and a totally new method of controlling the speed of the machine had to be devised. Again, it was realized that the flow of the steam would result in an end-thrust on the blading, and to prevent this being transmitted to the bearings, where it might have caused trouble, Parsons neutralized it by the ingenious expedient of admitting the steam midway along the rotor and causing it to flow equally towards each end. His subsequent invention of 'dummy pistons' rendered the double flow principle unnecessary for machines of moderate output, but without it the large and efficient high-speed machines of to-day could hardly be built. A study of Parsons' first turbine patent, taken out in 1884, will show how clearly he appreciated the difficulties in his path and how thoroughly he had considered the means of overcoming them. The success of Parsons' first little turbine marked the beginning of the most revolutionary change in the history of steam engineering. By developing power from the velocity of steam rather than from its static pressure, the turbine was exempt from the mechanical limitations of the reciprocating engine. Its invention has enabled the power that could be produced by a given weight and size of machinery to be multiplied a hundredfold and it has provided that purely rotational motion at high speed so desirable for the driving of electrical generators and many other classes of machinery. In addition to these advantages, it has brought about a remarkable economy in the use of steam.

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