Steam Power Plant Cycles

Power is produced in a steam power plant by supplying heat energy to the feedwater, changing it into steam under pressure, and then transforming part of this energy into mechanical energy in a heat engine to do useful work. The feedwater therefore acts merely as a conveyor of energy. The basic elements of a steam power plant are the heat engine, the boiler, and a means of getting water in the boiler. Modern power plants use steam turbines as heat engines; except for very small plants, centrifugal boiler-feed pumps are used.

This basic cycle is improved by connecting a condenser to the steam turbine exhaust and by heating the feedwater with steam extracted from an intermediate stage of the main turbine. This results in an improvement of the cycle efficiency, provides deaeration of the feedwater, and eliminates the introduction of cold water into the boiler and the resulting temperature strains on the latter. The combination of the condensing and feed-water heating cycle (Figure 1) requires a minimum of three pumps: the condensate pump, which transfers the condensate from the condenser hot well into the direct-contact heater; the boiler-feed pump; and a circulating pump, which forces cold water through the condenser tubes to condense the exhaust steam. This cycle is very common and is used in most small steam power plants. A number of auxiliary services not illustrated in Figure 1 are normally used, such as service water pumps, cooling pumps, ash-sluicing pumps, oil-circulating pumps, and the like.

The required improvements in operating economy in the 1970s dictated further refinements in the steam cycle, and these created new demands for power plant centrifugal pumping equipment. This evolution involved a steady increase in operating pressures until 2400 lb/in2 (165 bar*) steam turbines became quite common. Many plants are operating at supercritical steam pressures of 3500 lb/in2 (240 bar). Several central station


FIGURE 1 Simple steam power cycle


FIGURE 1 Simple steam power cycle

FIGURE 2 Natural gas and steam, combined cycle power

plants constructed in the late 1970s are operating between 4000 and 5000 lb/in2 (275 and 345 bar).

Other refinements were directed toward a greater utilization of heat through increased feed-water heating, introducing a need for heater drain pumps-equipment with definite problems of its own. Finally, the introduction of forced or controlled circulation as opposed to natural circulation at 650°F (343°C) in boilers created a demand for pumping equipment of again an entirely special character.

Although direct-contact heaters would have thermodynamic advantages, a separate pump would be required after each such heater. The use of a group of closed heaters permits a single boiler feed pump to discharge through these heaters and into the boiler. The average power plant is based on a compromise system: one direct-contact heater is used for feedwater deaeration, whereas several additional heaters of the closed type are located

FIGURE 3 Nuclear power steam cycle, boiling water reactor

upstream as well as downstream of the direct-contact heater and of the boiler-feed pump (Figure 5). Such a cycle is termed an open cycle. The major variation is the closed cycle, where the deaeration is accomplished in the condenser hot well and all heaters are of the closed type (Figure 6).

Electric power generation technology advanced into the 1970s when the conventional coal and gas fired boilers were replaced with nuclear fission reactors. Nuclear power generation utilizes two concepts for generating steam: boiling water reactors (Figure 3) where the feedwater travels directly to the reactor, and pressurized water reactors (Figure 4) where the feedwater travels through a steam generator.

In the 1980s, the evolution of power generation industry continued with the construction of combined-cycle units. This technology increased the efficiency and improved the heat rate by utilizing the exhaust gases of primary gas turbines to produce valuable steam to drive steam-powered generators (Figure 2). Gas turbine-fired plant construction expanded as emphasis increased on environental issues related to coal- and oil-fired plants.

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

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