What design feature normally surrounds the furnace in certain firetube boiler designs?

In certain firetube boiler designs, the feature that typically surrounds the furnace is known as the waterleg. The waterleg functions as a crucial component because it facilitates the transfer of heat from the hot gases produced by combustion to the water within the boiler. This design enhances the efficiency of heat absorption, ensuring that the maximum amount of energy from the burning fuel is utilized to generate steam or hot water.

The waterleg usually consists of a series of vertically or horizontally arranged water-filled tubes that surround the furnace area. This arrangement allows for effective cooling and helps maintain the proper operating temperature within the furnace, preventing overheating or damage to the boiler structure.

The other options do not perform this specific function. Headers serve as connection points for multiple tubes within the boiler system, while the combustion chamber is primarily where the fuel combustion takes place, and the steam drum serves as a reservoir for steam and water separation in a water-tube boiler. Each component has a distinct role, but the waterleg is essential for surrounding the furnace in certain firetube designs, making it the correct answer.