Steel Shell, Copper Tubes?

[FriscoJim]

One thing to keep in mind is that the flues act as stays between the fluesheets. Since the boiler is under pressure as well as thermal stress, the fluesheets will be trying to deflect away from each other to some degree. I suspect that the tensile staying force applied to the flues offsets or perhaps negates the compressive force from the thermal expansion. The area above the flues on the front fluesheet has to be calculated and stayed properly as part of the boiler design calcs, just like any flat surface in the boiler, but the area that contains the flues is considered to be stayed by flues for the purposes of the design. Of course, I would think that the flues located closer to the center of the fluesheets would be under more tensile stress than the ones located near the edges, since the deflection of the fluesheet under pressure would be greater in the middle. Just another aspect to think about...