What is the definition of an unfired boiler?

An unfired boiler is defined as a pressure vessel that generates steam through an external heat source rather than through combustion within the vessel itself. The correct choice indicates that it generates steam but specifically excludes process steam generators, which are typically designed for industrial processes rather than general steam generation.

This definition highlights the purpose of unfired boilers, which can be utilized in various applications such as heating or as part of a steam conditioning system, without the combustion process typically associated with fired boilers.

In contrast, the other options do not accurately represent the nature of unfired boilers. For instance, options that suggest the inclusion of heat exchangers or the generation of steam as part of the primary function miss the specific exclusion of certain types of steam generators. Understanding this distinction is crucial for identifying and categorizing different types of boiler systems correctly.