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Figure 11-5i Controlled Extraction/Admission Back-Pressure Turbine.

steam flow in one direction. Very large units often employ a tandem-compound design. With this design, steam enters the first section, or high-pressure end, and expands to an intermediate pressure. The steam is then transferred through a crossover pipe to the low-pressure end. Both the high- and low-pressure ends are on the same shaft. Typically, the low-pressure end of the turbine is a single-casing section with double flow, although, in some cases, units may have two low-pressure casings that produce triple flow.

Very large tandem-compound units have a high-pressure element, an intermediate (or reheat) element, and a triple-flow, low-pressure element. Cross-compound units are similar to tandem-compound units, except that the high- and low-pressure ends are not on the same shaft. When used as turbine-generators, each end is usually connected to a separate generator. When used as a mechanical drive, two ends may be connected to the same shaft by reduction gears.

Cycle Enhancements

The reheat cycle is a modification of the simple Rankine cycle, which was developed to take advantage of the increased efficiency available with higher pressures while avoiding excessive moisture in the low-pressure stages of the turbine. The steam, after partial expansion, is withdrawn, superheated at constant pressure in the boiler, and returned to the turbine. This approach is used to improve performance in utility and other large plants with multiple staged turbines.

Reheating also improves the quality (i.e., reduces the moisture content) of exhaust steam. As discussed below, if steam quality falls below about 90% for an extended period of time, blade erosion and reduced efficiency result. By incorporating a reheat cycle, optimal boiler and condenser pressures can be obtained without concerns about low-quality turbine exhaust. Figure 11-6 is a schematic representation of a reheat cycle and Figure 11-7 shows the cycle in a temperature-entropy (T-s) diagram.

Fig. 11-6 Schematic Representation of Steam Turbine Reheat Cycle.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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