One or more steel plates rolled into a cylinder welded together with the ends closed by rounded heads is called a:

The correct answer, steam or water drum, refers to a key component found in steam boilers. A steam or water drum is designed to hold water and steam within a cylindrical body that is constructed from steel plates. The welding and rounded ends provide the strength and integrity necessary to withstand the high pressures associated with steam generation.

In the context of boiler operation, the steam or water drum serves as a vessel where water is stored, heated, and from which steam is drawn. This plays a crucial role in maintaining efficient heat transfer and ensuring a reliable supply of steam for various processes.

The other options, although related to steam systems, refer to different types of equipment entirely. A combustion chamber, for instance, is where fuel is burned to generate heat but does not hold water or steam. A packaged boiler is a complete boiler system ready for installation, which includes many components, but the term doesn't specifically describe the cylindrical shape containing water and steam. Lastly, a steam header refers not to a storage vessel but rather to a pipe or distribution system that directs steam to various parts of a plant, which also does not match the description provided in the question. Thus, understanding the specific function and design of the steam or water drum is critical in distinguishing it from other components