Which type of boiler is categorized as externally fired?

The type of boiler categorized as externally fired is indeed the HRT (Horizontal Return Tubular) boiler set in brickwork. This classification stems from the design of the HRT boiler, where the furnace is located outside of the boiler shell, allowing for greater efficiency and a more controlled combustion process. The external firing arrangement contributes to improved heat transfer without exposing the pressure vessel directly to the fire, thereby reducing stress on the materials and extending the lifespan of the boiler.

In contrast, the Scotch marine, locomotive, and vertical fire tube boilers are characterized by internal firing, where the combustion takes place within the boiler structure itself. This results in a different method of heat exchange and influences the design, efficiency, and applications of these boilers, making them internally fired types rather than externally fired. This distinction is essential in understanding the operational and maintenance considerations of varying boiler types in engineering practice.