What distinguishes a fire-tube boiler from a water-tube boiler?

The distinguishing characteristic of a fire-tube boiler is that it has hot gases passing through tubes that are surrounded by water. This design allows for the heat from the combustion gases to be transferred to the water surrounding the tubes, which in turn produces steam or hot water. The arrangement facilitates effective heat transfer, making fire-tube boilers particularly efficient for certain applications, especially in smaller operations or where lower pressure steam is required.

In contrast, water-tube boilers operate by having water within the tubes and the hot gases surrounding the tubes. This allows water-tube boilers to handle higher pressures and larger capacities than fire-tube designs. Therefore, understanding the flow of gases relative to water is fundamental in differentiating between these two types of boilers.

The physical size aspect of a boiler, referred to in one of the choices, is not a reliable differentiating factor since both fire-tube and water-tube boilers come in various sizes tailored to specific applications. Additionally, water-tube boilers are known for their ability to operate at high pressures, which also sets them apart from fire-tube boilers. Thus, the focus on the correct description of hot gas flow in relation to water is what makes the first choice the correct answer.