Page 241 - Steam Turbines Design, Applications, and Rerating

220 Chapter Twelve

(a) (b)

Figure 12.1 Operating principles of steam turbines: (a) impulse tur-
bine; (b) reaction turbine. (Asea Brown-Boveri, Baden, Switzerland)

12.2 Impulse and Reaction
Turbines Compared
The essential differences between the two types can be considered in
two categories, thermodynamic and constructional. Differences of the
latter kind are largely determined by the former. The thermodynamic
differences have been described earlier and, except for the question of
efficiency, are not repeated here. For turbine reliability, it is the con-
structional differences that are most important, and so these will be
examined more closely. The main differences between the two designs
can be seen in Table 12.1.

12.3 Efficiency
A variety of factors, of course, have to be accounted for when calculat-
ing efficiency. To compare impulse and reaction designs, therefore, it is
helpful to take a dimensionless characteristic, the most suitable being
the volume coefficient δ. This is defined in Fig. 12.3, which shows the
efficiencies of the two types.
If we first compare impulse blading (curve a) to reaction blading
with tip sealing, i.e., with no shroud, (curve b), we arrive at the rela-
tionships stated in Table 12.1. It can also be seen from Fig. 12.3 that
with the reaction turbine a substantial improvement in efficiency is
possible, especially at low volume coefficients, by using shrouds (curve
c). On economic grounds this technique has become standard practice
for power station turbines above a certain capacity. Whether there is