What type of boiler has a cylindrical shell connected to tubes between the upper and lower tube sheets?

The correct response is based on the design characteristics of a vertical firetube boiler. This type of boiler features a cylindrical shell that typically houses the fire tubes, which run between upper and lower tube sheets. The main function of these fire tubes is to allow hot gases from combustion to travel through them, transferring heat to the water surrounding the tubes and generating steam.

In contrast, the other types of boilers mentioned have distinct structural differences. A modular boiler typically consists of multiple smaller boiler units that can be combined to meet varying steam production needs, rather than being a single cylindrical structure. A firebox boiler has a design where the combustion chamber is often box-like and separate from the water tank, making it quite different from a standard firetube design. The locomotive boiler, while also having a cylindrical shape, is specially designed with its own unique components for locomotive engines, combining both fire tubes and water tubes but is primarily recognized for its application in trains.

These structural and operational distinctions clarify why the vertical firetube boiler is the best answer to the question regarding the design described.