Excerpt from Shipbuilding and Shipping Record, Vol. 10: August 23, 1917
The great disadvantage of the reaction turbine is to be found at the high-pressure end. When steam at high pressure is em ployed, the volume per pound of steam is very small, and this results in blades of short length being required, so short in fact that the clearance area between the cylindrical surface of the drum or the cylinder and the respective blade tips is quite an appreciable percentage of the total peripheral area through which the steam passes. In order to reduce this clearance to a minimum, it is customary to thin the blade tips down to a knife edge, these being just in contact with the drum or cylinder when the turbine is erected, and grinding away to adapt themselves to running condi tions. But as a result of the deﬂection of the shaft due to'the weight of the rotor and to centrifugal force when running, a small clearance always shows itself when running, through which the steam passes without doing any work, with the result that there is always a loss of efficiency at the high-pressure end.
The numerous stages required for the reaction turbine necessitate the employment of a comparatively long drum, and with super heated steam, the employment of which is becoming increasingly common due to the undoubted economy resulting from its use, the expansion or distortion due to the high temperature may affect this long drum differently to the cylinder, with the result that the rubbing of the blades, even if it does not have disastrous effects, inevitably tends to increase the clearances with corresponding loss of efficiency.
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