Gas Turbine Handbook : Principles and Practices

Gas Turbine Systems Theory 69
to the disc using a “ fir-tree” design. This “fir-tree” configuration allows
room for expansion (due to thermal growth) while still retaining
the blades against centrifugal force. While almost all blades employ
the fir-tree root design, the blades themselves may be solid or hollow
(to provide for blade cooling), and with or without tip shrouds. Some
blades, primarily in the lower pressure stages where the blade length
can be very long, also include a damping wire through sets of blades.
Each of these designs serves a specific purpose but they can be generalized
as follows:
Hollow-air cooled— blades are used in the high pressure stages where
gas temperatures exceed the limits of the specific blade material.
Figure 4-15 shows the effects of cooling the airfoils. Solid
blades are used where gas temperatures are below the critical
limits of the blade material.
Tip Shrouds—form a band around the perimeter of the turbine
blades, which serve to seal against tip leakage, and dampen
blade vibration.
Figure 4-15. Turbine temperature profile.