By nature, HRSGs are inefficient, particularly the unfired units, because of the large gas mass flow associated with the low exit gas temperature from the gas turbine. The large mass flow forces one to use a boiler with a large cross section, though the steam generation may not be compatible with the size of the HRSG. The low ratio of steam to gas flow (15-18%) also results in a small heat sink at the economizer leading to higher stack gas temperature. Hence single-pressure units are inefficient. In addition,

1 Gas/steam temperature profiles are dictated by the steam pressure and steam temperature, unlike in a steam generator, where one can easily attain about 300°F stack gas temperature in a single-pressure unit even with high steam pressures on the order of 2000-2500 psi. In a single­pressure HRSG, the exit gas temperature is a function of the steam pressure and temperature. With 600psig steam superheated to 700°F, it is difficult to get the economizer exit gas temperature below 380°F in an unfired HRSG.

2 The higher the steam pressure, the lower the exit gas temperature (single-pressure unit). This point is explained under HRSG simulation: see Q8.36.

3 The higher the steam temperature, the lower the steam generation and the higher the exit gas temperature. This is due to the smaller amount of steam generated with higher steam temperature and hence a smaller heat sink at the economizer.

4 Partial load operation of a gas turbine also results in poor HRSG performance, as shown above.

So how can we improve the HRSG performance? There are several options.