Which type is NOT an externally fired boiler?

The vertical fire tube boiler is not considered an externally fired boiler because it operates with the combustion process occurring within the boiler itself. In an externally fired boiler, the heat source comes from outside the boiler. Instead, the vertical fire tube design utilizes tubes where hot gases pass through, providing heat directly to the water or steam within the boiler.

The other types listed, such as cast iron set on brick, Heine, and horizontal return tube set on brick, involve configurations that allow for external combustion setup or incorporate elements where heat is generated outside the water container, characterizing them as externally fired boilers. Understanding the definitions and operational differences between externally and internally fired boilers is crucial in stationary engineering, particularly for identifying their specific applications and efficiencies.