Multiple-Pressure HRSGs

Before going into this option, one should clearly understand when multiple — pressure options are justified. From the discussion on HRSG simulation, it can be seen that the exit gas temperature in an HRSG depends on the steam pressure and temperature. The higher the steam pressure, the higher the exit gas temperature. Hence when high pressure steam is generated, it will not be possible to cool the exhaust gases to an economically justifiable level with a single-pressure HRSG. Hence multiple-pressure steam generation is warranted. Also, one can maximize energy recovery by doing several things such as rearranging heat transfer surfaces, splitting up economizers, superheaters, and evaporators so that the gas temperature profiles match the steam and water temperatures and no large imbalance exists between the gas and steam temperatures. This can be done by using a program such as the HRSGS program (see Q8.37). In small HRSGs, multiple-pressure steam generation may not often be viable due to the complexity of the HRSG design and cost.

Multiple-Pressure HRSGs

FIgure 1.18a Secondary surfaces to improve HRSG efficiency. 1, turbine; 2, deaerator; 3, HRSG; 4, mixing tank; 5, pump; 6, deaerator coil; 7, condenser; 8, heat exchange; 9, condensate heater.

Multiple-Pressure HRSGs

Figure 1.18b Continued

Multiple-Pressure HRSGs

Posted in Industrial Boilers and Heat Recovery Steam