In the HRT boiler, the hot combustion gases pass:

In the HRT (Horizontal Return Tube) boiler, the design facilitates efficient heat transfer from the hot combustion gases to the water in the boiler. In this type of boiler, the hot gases flow along the underside of the steam drum, which is crucial for maximizing the surface area that the gases come into contact with. This path allows the heat from the gases to be transferred effectively to the water, increasing the overall efficiency of the heating process.

After this initial passage, the hot gases then return through the firetubes back to the boiler stack. This design ensures that the combustion gases are adequately utilized for heating before they are expelled through the stack, minimizing heat losses and improving energy efficiency. The return path through the firetubes also helps to maintain a higher steam production rate, as the water is continuously heated by the gases flowing through the system.

In summary, the flow of hot combustion gases along the underside of the drum and their return through the firetubes serves an essential function in the boiler's operation, ensuring effective heat transfer and efficiency.