What type of boiler has the furnace surrounded by water?

The correct choice describes an internally fired boiler, which features a furnace that is surrounded by water. This design allows for efficient heat transfer and helps maintain the structural integrity of the boiler, as the water absorbs the heat generated from the combustion process, reducing the risk of hot spots that could potentially lead to failures.

In an internally fired boiler, the furnace is located within the boiler shell, ensuring that the heat produced during combustion is effectively absorbed by the water, facilitating steam generation. This type of boiler is common in various industrial applications due to its efficiency and safety.

Other types of boilers like externally fired boilers have furnaces that are not surrounded by water; instead, the furnace is external to the boiler shell. Smoke tube boilers, on the other hand, feature a design where the combustion gases flow through tubes that are immersed in water but do not have a furnace surrounded by water. Fire tube boilers consist of tubes filled with hot gases, with water surrounding those tubes, and are distinct from the internal firing design concept. Hence, understanding the internal configuration of the furnace in relation to the water in the boiler is crucial in identifying the type of boiler being described.