In firetube boilers, the pressure vessel is commonly referred to as a:

In firetube boilers, the pressure vessel is commonly referred to as the "shell." This term specifically describes the outer casing or body of the boiler that contains the water and steam. The shell is designed to withstand high pressure and temperature generated during the operation of the boiler.

The shell is integral to the boiler's structure, providing the necessary support and containment for the water and steam. It is typically cylindrical in shape, which allows it to effectively handle the internal pressure and distribute the thermal stresses that occur during operation.

While the other terms may relate to similar concepts in different contexts, they are not appropriate for describing a firetube boiler's pressure vessel. "Vessel," "tank," and "cylinder" do not capture the specific function and design characteristics that "shell" does in this scenario, making it the most accurate option.