Page:The Steam Turbine.djvu/36

 the fixed and moving buckets at each velocity compounded stage: the best velocity ratio in a four row multiple impulse is only one-seventh and the efficiency about 44 per cent., and therefore much lower than that of reaction blading, which as we have stated is under favourable conditions 75 to 85 per cent.

The advantages, however, to be derived from the use of some multiple impulse elements at the commencement of the turbine are that because there is very little loss in them from leakage, therefore in spite of their low intrinsic efficiency, one or more multiple impulse wheels can in certain cases usefully replace reaction blading. The explanation is that in turbines of the compound reaction type of moderate power and slow speed of revolution the blades are often very short at the commencement, and consequently there is in such cases excessive loss by leakage through the clearance space, which brings the efficiency below that of impulse blading. In most cases one multiple impulse wheel is preferred, followed by reaction blading. Such