The furnace walls in a bent-tube watertube boiler are formed by water filled tubes, called:

In a bent-tube watertube boiler, the walls of the furnace are created using tubes that are filled with water. These tubes, referred to as waterwalls, serve a vital function in containing the boiler's internal environment and facilitating heat transfer from the combustion gases to the water. The presence of water in these tubes allows for efficient absorption of heat, which contributes to steam generation.

Waterwalls are an integral design feature in many watertube boilers and specifically engineered to withstand high temperatures and pressures while promoting thermal efficiency. Their configuration helps to create a boundary that protects the surrounding structure from intense heat, while also maximizing the surface area for heat exchange.

The other terms mentioned typically refer to different components or characteristics of boilers. For example, while furnace walls and fireside walls may imply the structural components of the furnace area, they don't specifically denote the presence of water-filled tubes as the waterwalls do. Waterside walls is not a standard term in this context, as the focus here is on how the walls interact with the combustion process rather than simply being on the water side. Thus, the correct identification of these water-filled tubes as waterwalls underlines their functional importance in the operation of bent-tube watertube boilers.