What is a steam boiler?

A steam boiler is a closed vessel that is partially filled with water. This design is critical because it allows water to be heated, creating steam that can be used for various applications, such as heating or powering engines. The crucial function of a steam boiler is to convert water into steam under pressure, and this process requires a closed environment to efficiently generate steam from the heated water.

The other options do not accurately define a steam boiler. For instance, a vessel that is always empty would not serve any purpose in steam generation. An open tank for cooling water does not contain steam or high-pressure water, and it serves a completely different function. A device used only for heating does not capture the full scope of a steam boiler's versatility, which can also include producing steam for engines or other mechanical processes, not solely for heating. Hence, the correct answer emphasizes the essential characteristics of a steam boiler, particularly its partial water fill and closed nature.