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Handbook of Turbomachinery Second Edition Revised - Ventech!

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denoted by hd. Thus the efficiency <strong>of</strong> the draft tube can be defined as the<br />

ratio <strong>of</strong> real diffusion effects versus ideal diffusion effects, that is,<br />

Zd ¼ðhd hfÞ=hd ¼ 1 hf=½V 2 e V2 t Þ=2gŠ ð18Þ<br />

Draft tubes are broadly classified into four categories. These are<br />

shown in Fig. 13. The first type is shaped like the frustum <strong>of</strong> a cone and is<br />

simply called the straight divergent tube. It has an efficiency <strong>of</strong> about 85%<br />

and is useful for low specific speed vertical-shaft Francis turbines. The<br />

Figure 13 Types <strong>of</strong> draft tubes: (a) straight divergent type, (b) hydracone, (c,d)<br />

elbow type.<br />

Copyright © 2003 Marcel Dekker, Inc.

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