In the Heat Recovery Steam Generator (HRSG), as in any boiler, water converts to superheated steam in three different sections. The economizer, the evaporator, and the Superheater.
Heat transfer is dependent on the mass flow of the hot gases, mass flow of the water or steam, the temperature difference, and the surface area. The critical point in the design of an HRSG is to have the gas temperatures leaving the evaporative section as close to the saturated temperature as possible. The difference known as the pinch point is in the order of 5 °C.
The gas temperature entering the HRSG is in the range of 580 °C and gradually reduces to 130 °C or less as it flows over the various heat transfer sections.
For Rankine cycles to be more efficient the steam pressure has to be high. Currently many of the steam turbines in Combined Cycle plants operate at 130 bar – the High Pressure (HP) circuit. At pressures of 130 bar the saturation temperature will be 330 °C. The gas exiting the evaporative section will be in the order of 335 °C. This means absorption of the balance heat of the flue gas has to be in the economizer. This is not possible.
To absorb more heat two additional evaporators are used. One operating at lower pressure 40 bar – Intermediate Pressure (IP) circuit with gas exiting at 255 °C . The second one at 7 bar – Low Pressure (LP) circuit with gas exiting at 170 °C is used. This together with economizers and superheaters in these pressure levels enable absorbing more heat.
This reduces the Flue gas temperature to the range of 130 °C.
Exhaust steam from the HP turbines return to the reheater coils in the HRSG. The IP steam mixes with the reheater an flows to the IP Turbine. The LP steam mixes with IP turbine exhaust steam and flows to the LP turbine.
The IP and LP steam integrates into the steam cycles with the HP, IP and LP steam turbines designed suitably to get the optimum performance. Three different feed water pumps feed the three separate pressure circuits in the HRSG.
By heating the condensate from the condensers in condensate heaters placed in the HRSG, cycle efficiency and heat absorption further increase.
The attached figure shows the flow diagram and the temperature gradient in a Triple pressure HRSG.