Internal furnaces are?

Internal furnaces are surrounded by water as they are typically designed for specific heating applications within a boiler system. The furnace, which serves as the combustion chamber, is located inside a reservoir of water that is heated to generate steam. This design facilitates effective heat transfer from the combustion of fuel to the water, ensuring efficient steam production.

The surrounding water plays a critical role in maintaining safe operating conditions, as it absorbs heat during combustion and helps regulate the temperature within the furnace. This arrangement also contributes to the boiler's overall efficiency by maximizing heat utilization.

In contrast to the other options, the function of internal furnaces specifically revolves around their interaction with water for steam generation, rather than related support structures or processes such as superheating or strength enhancement.