Modern Engineering Thermodynamics

13.7 Rankine Cycle with Regeneration 469
WHAT IS WRONG WITH USING WET STEAM?
High-efficiency modern steam turbines are designed to expand the steam into the wet vapor region in the low-pressure
stage of the turbine. When the steam quality is less than 1.0, some of the vapor (1 – x) condenses into tiny liquid water
droplets. Some of these droplets are deposited on the turbine walls, promoting corrosion, and some are entrained in the
steam flow, eventually coalescing into large droplets. When large water droplets strike the front of high-speed moving
blades, they can cause impact erosion, as shown by Figure 13.18.
FIGURE 13.18
Front edge of turbine blades. (Source: Photo courtesy of
Sanders, W. P. Turbo-Technic Services, Inc., Aurora, Canada.)
FIGURE 13.19
Trailing edge of turbine blades. (Source: Photo courtesy of
Sanders, W. P. Turbo-Technic Services, Inc., Aurora, Canada.)
This type of impact damage occurs where blades have a tip velocity around 800 ft/s and a steam quality of 97% (3.0%
moisture) or less. Water droplets can also damage blade trailing edges, as shown in Figure 13.19. This is more of a gouging
operation and produces blade thinning and stress concentration at positions where cracks and blade failure can occur.
Superheating the turbine inlet steam minimizes the exhaust steam moisture that produces these detrimental effects.
13.7 RANKINE CYCLE WITH REGENERATION
A regeneration process in a system is a feedback process whereby energy is transferred internally from one part of
a system to a different part of the system to improve the system’s overall energy conversion efficiency. This internal
transport of energy within a system is called regeneration, and the associated equipment is called a regenerator.
In the case of heat engines, regeneration usually involves utilizing otherwise waste exhaust thermal energy to
preheat fluid in another part of the same system. More specifically, in the case of the Rankine cycle heat engine,
a certain percentage of the vapor passing through the prime mover is removed and used to preheat the boiler
feedwater to a temperature between the condenser outlet temperature and the boiler outlet temperature. This significantly
reduces the thermal irreversibility that occurs when relatively cold condenser outlet water is pumped