In fire tube boilers, which of the following supports the flat surfaces above the tubes?

In fire tube boilers, diagonal stays are crucial for maintaining the structural integrity of the boiler. These stays are specifically designed to provide support to the flat surfaces above the tubes. The pressure and temperature inside the boiler can create stresses that may cause deformation or movement if not appropriately supported. Diagonal stays help distribute these stresses and maintain the alignment of the boiler components, preventing potential failure.

The design of fire tube boilers incorporates these stays to reinforce the upper portions of the boiler where the flat surfaces can be susceptible to pressure forces. This is particularly important for ensuring that the boiler operates safely and effectively, as any compromise in the structure can lead to dangerous situations.

The other options, while they may seem relevant to supports in other contexts or design aspects, do not fulfill this specific role. Vertical supports tend to provide general stability, cross members often serve structural purposes in other systems, and brackets may be used in various mounting applications but do not provide the specific diagonal bracing required in this scenario.