What material is commonly used in modern furnaces to form the furnace walls compared to early watertube boilers?

In modern furnaces, the walls are typically formed from materials that can withstand high temperatures and provide durability. Watertubes were commonly used in early watertube boilers as they allowed for effective heat transfer and high pressure capabilities. In contrast, modern furnaces often use steel casing for the walls, which is strong and capable of handling the stresses imposed by heating cycles. Steel provides a sturdy structure capable of supporting the weight of the heat exchanger and other components while also allowing for efficient heat conduction.

The watertube option, while relevant in the context of boilers, does not apply to the construction of modern furnace walls, which prioritize the use of steel for increased efficiency and reliability. Steel is specifically designed to manage the conditions within a furnace, making it the more appropriate choice for the structural material in comparison to the original watertube designs.