Modern Engineering Thermodynamics

13.9 Rankine Cycle with Reheat 477
Steam
Nozzle
Moving blades
Moving blades
Steam
Nozzle
Velocity
Moving blades
Stationary
blades
Moving blades
(a) Pressure staging (Rateau)
Velocity
Pressure
Pressure
(b) Velocity staging (Curtis)
FIGURE 13.26
Pressure (Rateau) and velocity (Curtis) staging in the DeLaval impulse turbine.
Thomas Alva Edison’s (1847–1931) development of a practical electric lightbulb in 1879 opened the doors of a
remarkable new technology, electricity.Tomakehislightbulbmarketable,hehadtodevelopandproducea
means of putting electricity directly into the home. He had to conceive and build an entire electrical power
plant and electrical distribution network. This he did, and in 1882, he opened the Pearl Street power station in
New York City, the first such station in the world. By 1890, several electrical power stations were in place in
major cities across the United States, and they were rapidly growing in size and complexity. Initially, reciprocating
steam engines drove the electrical generators, but it became clear rather quickly that this type of prime
mover was not going to be able to meet the needs of this growing industry for very long. Reciprocating steam
engines were too slow, too large, and too unreliable to carry the burden. The steam turbine was cultivated as a
viable replacement prime mover.
By 1900, the Westinghouse Electric Company was manufacturing multistage reaction steam turbines of the
Parson’s type for the electrical power generation industry, and the General Electric Company was developing an
impulse turbine of the DeLaval type with Curtis velocity staging for the same market.
Thus, the search for a suitable prime mover for large-scale electrical generators was the motivation that led to the
successful commercial development of the steam turbine. Though early steam turbines were actually less energy
efficient than their reciprocating counterparts, their potential for improvement was enormous. In addition, they
were about ten times smaller than a reciprocating engine with the same power output. Also, even very large steam
turbines could be made to run efficiently at generator speeds (1800 or 3600 rpm), they were quiet, and they
required little maintenance. It was for these reasons, not the reasons of improved thermal or mechanical efficiency,
that by 1920, the steam turbine had replaced virtually all large-scale reciprocating steam engines. By
1960, virtually all small- and medium-scale reciprocating steam engines had been replaced by electric motors or
internal combustion engines.
13.9 RANKINE CYCLE WITH REHEAT
By 1920, boiler technology had advanced to the point where steam at 650°F, 250 psia was generally available.
In the early 1920s, the regenerative process, initially developed in the late 1890s to improve the thermal efficiency
of reciprocating steam engines, was reintroduced as a means of improving steam turbine power plant
thermal efficiency. Regeneration using steam turbine prime movers required that steam be extracted from
between one or more of the turbine stages and used to preheat the boiler feedwater. During the 1920s, boiler
technology continued to increase rapidly, and by 1930, steam was commonly supplied at 725°F and550psia.
This led to the commercial use of steam reheat, in which steam is extracted from the outlet of a turbine stage,
returned to the boiler to be reheated, then brought back to the inlet of the next turbine stage for further expansion.
After its introduction in the mid 1920s, reheat technology became unpopular during the Depression due
to technical and economic difficulties. Single reheat cycles were again introduced in the 1940s, and double
reheat cycles were introduced in the 1950s. This prevents excessive moisture levels from occurring in the lowpressure
stages and has the effect of slightly increasing the thermal efficiency of the cycle. A simple power plant
utilizing reheat (but no regeneration) is shown schematically in Figure 13.27.