What connects the tubes of a serpentine watertube boiler to the upper and lower headers?

The correct answer is that the tubes of a serpentine watertube boiler are connected to the upper and lower headers using flanged connections. Flanged connections are designed specifically for high-pressure applications like those found in boilers. They ensure a tight seal and allow for disassembly when maintenance or repairs are necessary.

These connections provide a robust way to handle thermal expansion and contractions that occur due to the extreme temperature fluctuations within the boiler. The flanged design allows for easy alignment of the tubes to the headers and ensures that the stresses induced by the pressure inside the system are effectively managed.

While welded joints are commonly used in boiler construction, specifically in areas that can be permanently sealed, they do not allow for the easy disassembly of components that flanged connections provide. Threaded connections are not typically used in high-pressure systems like boilers, as they can be prone to leaks and do not handle the stresses encountered in such environments very well. Compression fittings are also less suitable for the demands of boiler applications, particularly in high-pressure situations, as they may not provide the same level of integrity as flanged connections.