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The HP steam drum and superheater headers are more prone to reduced mechanical life because they are subjected to the highest exhaust gas temperatures. When CCGT power plants are used for load-following operation, characterized by frequent starts and stops or operating at part-load to meet fluctuating electric demand, this cycling can cause thermal stress and eventual damage in some components of the HRSG. As the HRSGs are located directly downstream of the gas turbines, changes in temperature and pressure of the exhaust gases cause thermal and mechanical stress. The HRSGs present operational constraints on the CCGT power plant. In a three-stage steam turbine, HP, IP and LP steam produced by the HRSG is fed into the turbine at different points. With a multiple-pressure HRSG, the steam turbine will typically have multiple steam admission points.
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The high pressure steam in a large CCGT plant can reach 40 – 110 bar. Because the exhaust gases from a gas turbine can reach 600✬, HRSGs for GTs may produce steam at multiple pressure levels to optimize energy recovery thus they often have three sets of heat exchanger modules – one for high pressure (HP) steam, one for intermediate pressure (IP) steam, and one for low pressure (LP) steam. After exiting the steam turbine, the steam is sent to a condenser which routes the condensed water back to the HRSG.ĭesigns and configurations for HRSGs and steam turbines depend on the exhaust gas characteristics, steam requirements, and expected power plant operations.
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The energy delivered to the generator drive shaft is converted into electricity. The superheated steam produced by the HRSG is supply to the steam turbine where it expands through the turbine blades, imparting rotation to the turbine shaft. Preheaters are located at the coolest end of the HRSG gas path and absorb energy to preheat heat exchanger liquids, such as water/glycol mixtures, thus extracting the most economically viable amount of heat from exhaust gases. Saturated steam from the steam drums or once-through system is sent to the superheater to produce dry steam which is required for the steam turbine. In some designs, duct burners are used to add heat to the exhaust gas stream and boost steam production they can be used to produce steam even if there is insufficient exhaust gas flow. An alternative design to steam drums is a once-through HRSG, which replaces the steam drum with thin-walled components that are better suited to handle changes in exhaust gas temperatures and steam pressures during frequent starts and stops. Steam drums also serve storage and water treatment functions. The separated water is recirculated to the evaporator tubes. The steam-water mixture in the tubes enters the steam drum where steam is separated from the hot water using moisture separators and cyclones. As the hot exhaust gases flow past the evaporator tubes, heat is absorbed causing the creation of steam in the tubes. The drum is located adjacent to finned evaporator tubes that circulate heated water.
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The economizer is a heat exchanger that preheats the water to approach the saturation temperature (boiling point), which is supplied to a thick-walled steam drum.
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These modules are referred to as economizers, evaporators, superheaters/reheaters and preheaters. The tubes are arranged in sections, or modules, each serving a different function in the production of dry superheated steam. As the hot exhaust gases flow past the heat exchanger tubes in which hot water circulates, heat is absorbed causing the creation of steam in the tubes. The HRSG can rely on natural circulation or utilize forced circulation using pumps. It is also called a boiler, as it creates steam for the steam turbine by passing the hot exhaust gas flow from a gas turbine or combustion engine through banks of heat exchanger tubes.
#Fspassengers x not identifying steam series#
The HRSG is basically a heat exchanger, or rather a series of heat exchangers.