Page 243 - Steam Turbines--Design, Applications, and Rerating by Heinz-Bloch, Murari-Singh

222 Chapter Twelve

TABLE 12.1 Basic Comparison Between Impulse and Reaction Turbines
Feature Impulse turbine Reaction turbine
Efficiency Better with small volume Better with medium- and high-
coefficients, poorer with volume coefficients
medium- and high-volume
coefficients
Rotor Disk construction Drum construction
Blading (Fig. 12.2) Few stages, wide in axial More stages, narrow in axial
direction direction
Fixed blades mounted in Fixed blades mounted in casing
diaphragms or blade carrier
Moving blades on disks of rotor Moving blades on drum
Maintenance Longer time elapse between Somewhat shorter elapsed time
major overhauls between major overhauls
SOURCE: Asea Brown-Boveri, Baden, Switzerland.

any advantage in using shrouds for mechanical drives as well must be
judged from the economic circumstances of each case. Cost and vari-
able speed experience are possible issues. To find the overall efficiency
of a reaction turbine, one must also allow for losses due to the thrust
balance piston. Viewed as a whole, however, the fact remains that the
impulse machine has its advantages when capacities are low, while for
medium and high ratings the merits of the reaction type of construc-
tion are of interest to specifying engineer and purchaser.

Figure 12.3 Comparing the effi-
ciency η of impulse and reaction
blading; a = impulse blading; b =
reaction blading with tip seal-
ing; c = reaction blading with
shrouding; δ= volume coefficient
= V/r u; V = flow volume; r =
˙
˙
2
radius (center of blade passage);
u = rotational velocity (at r).
(Asea Brown-Boveri, Baden,
Switzerland)