6

At ISO rated conditions

Correction factor for elevation

Correction factor for compressor inlet temperature

Correction factor for excess inlet losses

Correction factor for excess exhaust losses

Correction factor for steam or water injection

Correction factor for part load operation

Fig. 8-42 Large Capacity Two-Pressure HRSG Applied in 220 MW Combined-Cycle Cogeneration Plant. Source: Deltak

Heat Recovery at Multiple Steam Pressures

Heat recovery with single-pressure HRSGs becomes limited at higher pressures due to higher saturation temperatures. In some cases, multiple-pressure HRSGs may be used to serve process loads or gas turbine steam injection requirements, improving overall efficiency. One to four separate pressure sections may be used, each with superheater, boiler, and economizer sections. Multiple-pressure operation is also effective for increasing combined-cycle system efficiency where admission steam turbines include high-pressure and low-pressure inlets.

Figure 8-42 shows a large-capacity, two-pressure HRSG applied in a 220 MW gas-turbine combined-cycle cogeneration plant. High-pressure steam is generated at 1,500 psig/1,000°F (104.5 bar/538°C) and low-pressure steam is generated at 223 psig (16.4 bar). Extraction steam is exported to a nearby fertilizer plant and supplementary firing is used to increase steam turbine generating capacity when gas turbine output falls off during high ambient temperature conditions.

Figure 8-43 shows a triple-pressure HRSG applied in a 41 MW gas turbine cogeneration plant that serves a

Fig. 8-42 Large Capacity Two-Pressure HRSG Applied in 220 MW Combined-Cycle Cogeneration Plant. Source: Deltak university hospital campus. High-pressure extraction steam is used for campus heat distribution and air conditioning cold generation. Intermediate-pressure steam, generated at 85 psig (6.9 bar), is used for heating and cooling the inlet air to the gas turbine, and low-pressure steam is generated at 25 psig (2.7 bar) for selected heating loads. Supplementary firing allows steam output to be varied as required.

Figure 8-44 shows the temperature profile of an unfired HRSG featuring three operating pressure levels. Illustrated is the distribution of heat exchanger sections and the associated temperature differences between exhaust gas and water and steam temperatures. This shows the manner in which heat-absorbing sections operating at certain temperature are located in the gas stream to minimize the amount of heat transfer surface required. There are ten discrete heat exchange sections distributed based on the gas temperature available and the fluid temperature requirement. Note that the high-pressure economizer is divided into three separate sections to provide appropriate temperature zones for the intermediate-pressure superheater, evaporator, and economizer and that the intermediate pressure-generating bank and economizer sections are intermeshed with sections of the high-pressure economizer to optimize performance.

Fig. 8-43 Triple-Pressure HRSG Applied in 41 MW Gas Turbine Cogeneration Plant Serving a University/Hospital Campus. Source: Deltak
Fig. 8-44 Temperature Profile of Triple-Pressure Unfired HRSG. Source: Combustion Engineering, Inc. Reprinted with Permission from Combustion Fossil Power, 4th Ed., 1991.
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