The identification of fire-tube boilers and water-tube boilers as the two main types of boilers is grounded in fundamental engineering principles governing their design and operation. Fire-tube boilers operate by having hot gases from combustion pass through tubes that are surrounded by water, effectively transferring heat to the water and producing steam. This design is often simpler and less expensive, making fire-tube boilers a popular choice for smaller applications. In contrast, water-tube boilers circulate water inside tubes that are heated externally by the combustion gases. This arrangement allows for greater efficiency and the ability to reach higher pressures and steam outputs. Water-tube boilers are typically used in larger industrial applications due to their robust construction and capability of handling high-capacity steam generation. While electric, gas, single-pass, multi-pass, condensing, and non-condensing boilers represent important categories within the broader structure of boiler technology, they do not encapsulate the fundamental classification based on how the heat transfer occurs and the physical configuration of the boiler. This makes fire-tube and water-tube boilers the primary types in the context of boiler engineering.