All About Engines

Steam Turbines 173 The larger turbines are often constructed in tandem—that is to say, there is a high-pressure drum and a low-pressure drum on the same shaft, in separate casings, and the steam passes by a large pipe from one to the other. As there is a bearing between the two the portions of the shaft are shorter and stiffer than if the whole power were obtained from a single drum. The high-pressure casing is then usually made of cast steel and the blades of copper. Where only low pressures are to be employed the blading is often in two sets, as in Fig. 104, Plate 14. The steam enters at the middle and flows through the two sets of blading in opposite directions ; but the blades in each half are, of course, set so that the turning effect on the shaft is always in the same direction. With this arrangement there is no end thrust. Another plan followed by many makers is to use both impulse and reaction blading in the same turbine, as in Fig. 105 on Plate 15. The steam first acts upon one or more impulse wheels, and then flows through reaction blading. The Most Powerful Steam Turbine in the World The largest steam turbine in the world was con- structed by C. A. Parsons and Co. in 1913 for the Commonwealth Edison Co., of Chicago, and there- fore has the additional interest that it was made by a British firm for a great American company. Constructed to give over 35,000 horse-power, it is guaranteed to require only 8’1 lb. of steam per horse- power per hour, or only 40 per cent, of that required