In fire tube boilers, what is the main function of the shell structure?

The main function of the shell structure in fire tube boilers is to support the tube sheets. The shell is a crucial component that houses the fire tubes through which hot gases pass. It provides structural integrity and stability to the entire boiler system, ensuring that the tube sheets, which connect the fire tubes to the shell, are securely held in place. Proper support is vital because the tube sheets must withstand the thermal and pressure stresses produced during boiler operation.

While insulation, containing combustion gases, and regulating steam pressure are important aspects of a boiler's overall function, they do not encompass the shell's primary purpose. Insulation typically comes from other materials designed to minimize heat loss, while the containment of combustion gases is a given in the design of any boiler. Regulation of steam pressure is managed through control systems and safety mechanisms rather than the shell itself. Thus, the shell's key role is its support function in maintaining the structural integrity of the boiler system.