Why are boilers designed with more combustion gas passes?

Boilers are designed with multiple combustion gas passes primarily to allow more time for heat transfer. As combustion gases pass through the boiler, they transfer heat to the water or steam within the tubes. By increasing the number of passes, the gases remain in contact with the heat transfer surfaces for a longer duration. This extended contact enhances the efficiency of heat transfer, allowing the boiler to extract maximum thermal energy from the combustion process.

In addition to improving heat transfer, having more passes also contributes to better overall efficiency and can help in maintaining a stable temperature within the boiler. This is particularly important in applications where consistent steam quality or specific heating requirements are necessary. The design choice reflects a balance between achieving optimal performance and managing the physical constraints of the boiler system.