Why are boilers designed with multiple combustion gas passes?

Boilers are designed with multiple combustion gas passes to allow more time for heat transfer between the combustion gases and the water or steam inside the boiler. Each pass enables a greater surface area contact between the gases and the heating surfaces, enhancing the transfer of heat. This extended interaction means that the heat can be absorbed more efficiently, improving the overall efficiency of the boiler. By maximizing heat transfer, the boiler can generate steam or hot water more effectively while using less fuel.

In contrast, the other reasons do not fully explain the primary design purpose of multiple passes. While thermal stress can be a consideration with the design, the main goal of having multiple passes is directly related to efficiency through prolonged heat exchange. The idea of soot falling out in the last pass refers to maintenance and cleaning strategies rather than the fundamental design principle. Lastly, while shorter boiler tubes can be a design benefit in certain contexts, the number of passes primarily relates to enhancing heat transfer effectiveness rather than merely to reduce tube length.