Handbook of Turbomachinery Second Edition Revised - Ventech!

Figure 14 Draft tube setting.
same as the inlet of the draft tube. Thus
V 2 e =2g ¼ðpa peÞ=g ðHs hfÞ ð20Þ
Cavitation occurs when the value of pe becomes equal to the vapor pressure
of the fluid at the local temperature. The cavitation parameter s can be
defined as the ratio of exit velocity head and head on the machine. Thus,
s ¼ðV 2 e =2gÞ=H ¼½ðpa pvÞ=g ðHs hfÞŠ=H ð21Þ
where pe has been replaced by the vapor pressure pv. Usually, the head loss
hf is small and is dropped from the equation. Thus a number called the
Thoma cavitation coefficient, named after the German engineer Dietrich
Thoma (1881–1943), can be defined as
s ¼½ðpa pvÞ=g HsŠ=H ð22Þ
In order to avoid cavitation, it is necessary that s > sc, where sc is called the
critical cavitation coefficient. The values of sc are determined experimentally
by plotting s versus efficiency. For large values of s, efficiency is nearly
constant. When sfalls below sc, however, the efficiency rapidly drops off.
This is also accompanied by increased noise and vibration. Typical values of
sc are given in Table 4.
Copyright © 2003 Marcel Dekker, Inc.