ENGINEERING DESIGN

Modern steam power plants, such as coal or nuclear fueled, operate on the Rankine cycle. Gas turbine plants using fuel such as gas or light petroleum (e.g., kerosene) use the Brayton cycle. Various enhancements to improve thermal efficiency are employed in modern power plants. In each case, however, practical restrictions prevent further gains. For example, lowering the final exit temperature of the flue gas increases boiler efficiency; however, the flue gas temperature cannot be lowered too much or else the water vapor in the flue gas will condense (SO2 actually raises dew point temperature significantly).

The Carnot cycle efficiency delineates achieving heat addition at the maximum temperature possible and rejecting heat at the lowest possible temperature. Thus, it is desirable to increase the maximum steam pressure which increases the specific work of the cycle (if same temperature is maintained), but this change always increases moisture content of steam at turbine exhaust which can damage turbine blades. In order to achieve this superheat and/or reheat may be employed. However, the maximum steam temperature is limited by materials considerations. Superheat allows heat to be added at a higher temperature. More superheat increases the specific work of the cycle and decreases the moisture content at the turbine exhaust. Reheat involves reheating the steam before passing it through the latter stages of the turbine; this results in a reduction in the moisture present in the last stages of the turbine which reduces erosion of turbine blades (prolongs blade lifetime). Because of safety concerns (thermal limits) very little, if any, superheat is used in nuclear plants, and nuclear plants use moisture separation versus the true reheat used in a boiler.

Another thermal efficiency enhancement technique is regenerative feedwater heating. Steam is bled from the turbine at various points in the expansion process, and the energy is then transferred to the feedwater. This feedwater heating reduces the irreversibility due to the sensible heat addition to otherwise subcooled liquid, thereby allowing heat to be added at a higher temperature; that is, the thermal efficiency of the basic cycle is improved by adding heat using internal cycle energy particularly latent heat energy which otherwise would be rejected in the condenser. Several stages of feedwater heaters are used based on optimizing the cost of feedwater heaters versus the gain in thermal efficiency. Feedwater heating can increase efficiency by up to 20%; however, the cycle power output is lowered because total steam flow across all stages of the turbine is reduced.

Condensation at low pressure may also be used to increase thermal efficiency since the lower saturation pressure means lower temperature. In this case the condenser is operated at sub-atmospheric pressure (1 in Hg to 2 psia). The reduced minimum pressure increases the specific work of the cycle, but also increases moisture content at turbine exit. The very low pressure at the turbine exit results in low density steam that produces high velocity flow which can aid turbine blade erosion in the event moisture is present.

For a rough idea of the numbers involved, the table below compares the steam cycle of coal and nuclear plants:

Characteristic

Coal Plant

Nuclear Plant

Thermal efficiency

40%

33%

Steam pressure and temperature 3500 psi and 1000°F 1000 psi and 550°F
Feedwater temperature

500°F

440°F

Heat dissipation techniques include

The former method is the least expensive but can result in thermal pollution. To avoid thermal pollution the latter two methods have been employed. Cooling ponds, and its specialized version---the spray pond, are man-made lakes. Most cooling towers are of the wet (vs. dry) variety which employs direct water-to-air contact that can cool more from the evaporation but which means water loss. Another classification of cooling towers is the air draft mechanism: natural vs. mechanical draft. The natural draft towers are large, tall structures whereas mechanical draft towers are shorter and employ either forced or induced draft fans.


Last updated: November 7, 1997

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