Page 132 - Steam Turbines Design, Applications, and Rerating

Turbine Blade Design Overview 113

Figure 6.6 Axial entry root blad-
ing in control stage of a reaction
turbine. (Asea Brown-Boveri,
Baden, Switzerland)

6.3 Types of Airfoils and
Blading Capabilities
Blade airfoils fall into three categories: constant area airfoils, tapered
airfoils, and tapered twisted airfoils. The constant area airfoil is an
impulse blade and it is generally used in short blades at the high pres-
sure end of a turbine. The tapered airfoil is used on longer blades when
a reduction in centrifugal stress is needed. The tapered twisted airfoil
is basically a reaction blade. It is used when both a reduction in cen-
trifugal stress and a change in blade angles, from hub to tip for ther-
modynamic efficiency, are required.
Drawn blading is used on both single-stage and multistage turbines
when stresses permit. General guidelines for drawn blade limitations
are 6000 r/min in speed, 3 to 3.5 in (75 to 89 mm) in blade height and
about 1000 hp (746 kW) in stage loading. Milled blades are typically
used when drawn blades are unacceptable because of high-stress
action. Capabilities of existing blading include and often exceed 17.25-
in (438 mm) length, tip speeds of 1386 ft/s (422.5 m/s) and horsepowers
per blade of over 400 (300 kW).