Heatflow

The HRSG is a heat exchanger where liquid water absorbs the thermal energy supplied by the Gas Turbine exhaust. The main form of heat transfer at play here is conduction. The hot exhaust gases enter from the left (1) and diffuse to fill the space available.

From the right (2), feedwater travels along pressurised pipes and gradually absorbs the heat from the fumes. The transfer/exchange is completed in a few stages so as to maximise the level of energy recovered. The thermal energy is given up (by the exhaust fumes) gradually so as you move across the HRSG the temperature is lower the further right that you measure it.

The overall result of the exchange is that the liquid H2O which enters on the bottom right changes state and exits as superheated vapour. It is then sent off via a long overhead pipeline to drive the Steam Turbine.

After working through the turbine the H2O is cooled to form condensate which is sent back as feedwater to the HRSG.

Overall the water system is closed and therefore there is an indefinite supply of vapour to the Steam Turbine and an indefinite supply of feedwater to the HRSG. All this is achieved with a finite fixed amount of water moving that is continuously changing state within the system.

The exhaust fumes enter the HRSG from the turbine flue connection on the left (1) and diffuse to fill the space available thus heating the entire channel.

The heat source (within the Combustion Chamber of the Gas Turbine) is an exothermic reaction of methane and oxygen, resulting in the left-hand side of the HRSG (closer to the heat source) being far hotter than the right-hand side.

As the thermal energy is passed to the water, the profile of the exhausts drops gradually from 620°C to 79°C at which point the exhaust leaves the HRSG via the tall chimney stack (2). The residual heat in the exhaust fumes is then lost to the atmosphere, even though the majority of the thermal energy has been recovered previously and sent to the Steam Turbine.

HRSG Summary of Thermal Heat Exchange