Environmental Engineering Reference
In-Depth Information
Fluid friction occurs throughout the turbine, in the steam nozzles, along the blades, and along
the rotor disks that carry the blades. In addition, the rotor and blade rotation impart a centrifugal
action on the steam, causing it to be dragged along the blades. When the blades are not properly
designed, flow separation may ensue, further increasing turbine losses.
Heat transfer losses are caused by conduction, convection, and radiation. Conduction is the
result of heat transfer between metal parts of the turbine. Convection is the heat transfer between
steam and the metal parts. Radiation is the heat given off by the turbine casings to the surroundings.
Heat transfer losses are highest in the high-temperature, high-pressure sections of the turbines.
In addition, there are frictional losses in bearings, governor mechanisms, and reduction gearing.
Also, turbines must supply power for accessories, such as oil pumps. The combined efficiency losses
and syphoning of auxiliary power amount to 10-20%; that is, turbines convert only 80-90% of the
available steam enthalpy into mechanical energy that drives the generator.
Gas Turbine
In a gas turbine plant where oil, natural gas, or synthesis gas may be used as a fuel, the hot combustion
gases are directly used to drive a gas turbine, rather than transferring heat to steam and driving a
steam turbine. This requires a different turbine, appropriate for the much higher temperature of the
combustion gases and their different thermodynamic properties compared to steam. Gas turbines
are easily brought on line and have flexible load match. But their cycle efficiencies are lower than
those of steam plants, and the fuel is more expensive. Therefore, gas turbines are mostly used
for peak load production and for auxiliary power, such as during major plant outages. However,
recently many natural gas-fueled gas turbine plants have been installed in the United States and
other countries; but these usually employ the combined cycle mode, which has a higher efficiency
than the single cycle mode. Gas turbines operate on the principle of the Brayton cycle, which was
described in Chapter 3. Compressed air enters a combustion chamber, where liquid or gaseous fuel
is injected. The combustion of the fuel increases the temperature of the combustion gas, producing
a net work output of the turbine-compressor system. The temperature of the combustion gases is on
the order of 1100-1200 C, which is the maximum tolerable by present-day steel alloys used for gas
turbine blades. Even at these temperatures, thermal stresses and corrosion problems are manifested,
so that turbine blade cooling from the inside or outside of the blades by air or water is necessary.
Gas turbines are of the reaction type, where blades form a converging nozzle in which the
combustion gases expand, thus converting enthalpy to kinetic energy. As in steam turbines, staged
turbines are employed, consisting of several rows of moving and fixed blades.
The working fluid in gas turbines, composed of nitrogen, excess oxygen, water vapor, and
carbon dioxide, is not recycled into the compressor and combustion chamber but is, instead, vented
into the atmosphere. In some systems, a part of the energy still residing in the exhaust gas is recovered
in heat exchangers to heat up the air entering the combustion chamber in order to enhance the overall
thermal efficiency of the Brayton cycle, but eventually the exhaust gas is vented. This is in contrast
to steam turbines where the working fluid, steam, is recycled into the boiler as condensed water. 6
6 Gas turbines have many applications other than for electricity generation. They are used for pipeline pumping
of natural gas, ship propulsion, and foremost for airplane propulsion in turbojet aircraft. Here, air is compressed
in the compressor, and jet fuel (kerosene) is added to the combustion chamber. The combustion gases drive
the turbine that supplies power to the compressor and auxiliary systems (e.g., electricity generation), and the
turbine exhaust gases pass a deLaval nozzle to provide forward thrust to the airplane.
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