Steam Turbine Power Cycles

Steam turbines operate on the Rankine Cycle. The simple ideal Rankine cycle, which is described in detail later in the chapter, can be reduced to four processes:

1. Boiler feedwater (condensate) is pressurized and injected into a boiler.

2. Water is heated and evaporated in the boiler. The resulting steam may be superheated to increase its enthalpy and reduce moisture.

3. Steam is expanded in the turbine to a lower pressure. A small portion of steam thermal energy is converted to kinetic energy that is used to drive a rotating load.

4. Steam is condensed by a cooling medium in the condenser. In a back-pressure turbine, exhaust steam is delivered to a remote heating load, where condensation occurs.

A steam turbine is considered part of a cogeneration system when an application involves the sequential use of a single source of energy for both the generation of power and useful thermal energy output. As with other prime mover systems, these applications are broadly classified as either topping or bottoming cycles.

A topping cycle uses a back-pressure or extraction turbine as a pressure-reducing valve. As high-pressure steam is expanded to a lower pressure, shaft power is generated by the turbine at a minimal cost, as there is only a slight drop in the enthalpy content of the steam (usually less than 10%). This application is ideal for facilities that require both high-and low-pressure steam simultaneously.

A bottoming cycle uses excess steam, discharged from a high-pressure process, to generate shaft power. Bottoming cycles are also used for applications which discharge high-temperature exhaust gas, convert it to steam in a heat recovery steam generator (HRSG), and pass it through a steam turbine.

Traditional combined cycles, which integrate a topping cycle and a bottoming cycle into one sequential process, use recovered heat (generally from a combustion engine's exhaust) to produce additional shaft power in a steam turbine. When a condensing turbine is used, it functions as a bottoming cycle. In a cogeneration type combined cycle, a back-pressure or extraction turbine may be used with low-pressure steam passed on to a thermal load. In this application, the steam turbine may be classified as a second topping cycle or an intermediate cycle.

Shaft power can be extracted from virtually any pressure drop. Some multi-stage turbines are designed to operate with great efficiency at very low steam pressures. In general, however, condensing turbines are most cost-effective at steam pressures above 100 psig (7.9 bar) and back-pressure turbines should operate with at least a 4:1 pressure drop ratio (ratio of absolute inlet to exhaust pressure). When rejected steam or heat recovery-generated steam is available at lower pressures, bottoming cycle steam turbine applications may still be used. However, for most industrial size turbines, the size and cost of the turbine outweigh the value of available power output in low-pressure applications.

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