Which of the following is NOT a main type of boiler?

In the context of boiler types, the correct classification focuses on the primary construction and operational principles that define them. Fire-tube boilers, water-tube boilers, and electric boilers each represent distinct categories based on how they create and transfer heat.

Fire-tube boilers operate by passing hot gases through tubes surrounded by water, which allows for efficient heat transfer. Water-tube boilers, on the other hand, circulate water in tubes that are heated by combustion gases outside the tubes, making them suitable for high-pressure applications. Electric boilers utilize electric heating elements to generate steam or hot water, offering an alternative to traditional combustion methods.

The term "steam boiler" often refers to any boiler that produces steam, but it does not specify a particular construction type; therefore, it is broader and more of a general term rather than a distinct type of boiler. This makes it the outlier in the list, as it does not fit the specific classification criteria like the other options do. Thus, "steam boiler" is not categorized as a main type of boiler in the same sense as the other three.