What type of boiler has water surrounding the tubes?

The boiler that has water surrounding the tubes is classified as a fire tube boiler. In a fire tube boiler, the combustion gases flow through tubes that are surrounded by water. This design allows the hot gases to transfer heat to the water, which is heated and converted into steam. The tubes themselves are surrounded by water, and the heat transfer occurs from the gases inside the tubes to the water external to them, making this a fire tube configuration.

In contrast, water tube boilers have water-filled tubes that are heated by combustion gases that pass outside the tubes, where the tubes are located within a larger water container. Cast iron sectional boilers consist of cast iron sections that enclose water and are usually not classified under the tube types. Steam is a state of water and does not refer to a boiler's construction type. Understanding these distinctions helps clarify why a fire tube boiler is characterized by its design of having water surrounding combustion gas-filled tubes.