Environmental Engineering Reference
In-Depth Information
low-pressure turbine. The superheater and reheater sections of the boiler are usually situated past
a bend in the boiler, called the neck . In order to optimize thermal efficiency, the combustion air
is preheated to a temperature of 250-350 Cinthe air preheater section of the boiler. Near the
burners, heat is transferred from the combustion gases to the boiler tubes by radiation. Away from
the burners, heat is transferred mainly by convection. Coal and oil flames are highly luminous in
the visible portion of the spectrum because of the radiation from unburnt carbon and ash particles.
Natural gas flames are less visible because of the absence of particles in the flame. However,
most of the radiative transfer of heat from all flames occurs in the nonvisible infrared portion
of the spectrum. The theoretical (Carnot) thermodynamic efficiency of a heat engine that works
between a temperature differential of 838 K (565 C) and 298 K (25 C)—that is, between the
temperature of the superheater and the condensation temperature of water in the condenser—is
η = (
64%. However, as mentioned in the introduction, typical efficiencies of
steam power plants are in the 33-40% range. Because heat is added to the water and steam at
all temperatures between these limits, the Rankine cycle efficiency is necessarily lower than the
Carnot value. Furthermore, parasitic efficiency degradation occurs because of heat losses through
the walls of the boiler, ducts, turbine blades and housing, and frictional heat losses.
T L )/
Steam Turbine
Steam turbines were first employed in power plants early in the twentieth century. They can handle
a much larger steam flow, much larger pressure and temperature ratios, and a much larger rotational
speed than can reciprocating piston engines. Today virtually all steam power plants in the world
employ steam turbines. The steam turbine arguably is the most complex piece of machinery in the
power plant, and perhaps in all of industry. There are only a score of manufacturers in the world
that can produce steam (and gas) turbines.
The antecedent of the steam turbine is the water wheel. Just as water pushes the blades of
a water wheel, steam pushes the blades of a steam turbine. Considering the high pressure and
temperature of the steam, that the turbine must be leak proof, the enormous centrifugal stresses
on the shaft, and the fact that steam condenses to water while expanding in the turbine, thereby
creating a two phase fluid flow, one gets an idea of the technological problems facing the designer
and builder of steam turbines.
The development of the modern steam turbine can be attributed to Gustav deLaval (1845-1913)
of Sweden and Charles Parsons (1854-1931) of England. deLaval concentrated on the development
of an impulse turbine, which uses a converging-diverging nozzle to accelerate the flow speed of
steam to supersonic velocities. That nozzle still bears his name. Parsons developed a multistage
reaction turbine. The first commercial units were used for ship propulsion in the last decade of the
1800s. The first steam turbine for electricity generation was a 12 MW unit installed at the Fisk
power plant in Chicago in 1909. A 208-MW unit was installed in a New York power plant in 1929. Impulse Turbine
In an impulse turbine a jet of steam impinges on the blades of a turbine. The blades are symmetrical
and have equal entrance and exit angles, usually 20 . Steam coming from the superheater, initially
at 565 C and over 20 MPa, when expanded through a deLaval nozzle will have a linear velocity
of about 1650 m s 1 . To utilize the full kinetic energy of the steam, the blade velocity should be
about 820 m s 1 . Such a speed would generate unsustainable centrifugal stresses in the rotor. To
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