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Handbook of Turbomachinery Second Edition Revised - Ventech!

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the water uniformly around the circumference, it is necessary to reduce the<br />

area available for flow. Thus the casing takes the shape <strong>of</strong> a spiral, which<br />

gives the maximum area at the entrance and almost zero at the tip, hence the<br />

name spiral casing. Water from the spiral casing is guided onto the runner<br />

by a set <strong>of</strong> guide vanes, which are sometimes called wicket gates, placed<br />

circumferentially. These can be set to any angle desired and their shape is<br />

designed so that the water passes over them with minimum friction losses.<br />

The water then passes through the runner and then discharges into a draft<br />

tube.<br />

As already mentioned, the flow in the runner <strong>of</strong> modern Francis<br />

turbines is not purely radial, but a combination <strong>of</strong> radial and axial flows.<br />

The width <strong>of</strong> the runner depends on the specific speed. The higher the<br />

specific speed, the higher the width <strong>of</strong> the runner, since higher specific speeds<br />

necessarily mean larger flows and correspondingly larger flow areas. The<br />

runners can typically be classified as slow, medium or normal, and fast<br />

runners, with their specific speed ðNsÞ ranges being 60–120, 120–180, and<br />

180–300, respectively. These are pictorially shown in Fig. 9.<br />

Figure 9 Variation <strong>of</strong> Francis turbine runners.<br />

Copyright © 2003 Marcel Dekker, Inc.

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