The HRSG simulation concept is helpful in predicting the performance of an HRSG at various modes of operation. The HRSG need not be designed to perform this study. Figure 1.17a Shows a multiple-pressure HRSG used in a combined cycle plant with nine modules. Module 1 superheater is fed by module

3, which consists of a superheater, evaporator, and economizer. Module 2 is a reheater. Module 7 evaporator feeds module 4 superheater. Module 5 economizer

Figure 1.17b Temperature profiles and performance of the HRSG.

Feeds module 2, and module 9 evaporator feeds module 6 superheater. Module 8 economizer feeds both modules 5 and 7.

The HRSGS program can be used to arrive at the design case performance as shown in Fig 1.17b. The US value (product of overall heat transfer coefficient and surface area) for each surface is also shown. One may also use this information to predict the HRSG performance at other off-design cases and study, for example, the effect of steam pressure or the feedwater temperature on the HRSG performance.