How does a firetube boiler differ from a watertube boiler?

A firetube boiler is characterized by having combustion gases pass through tubes that are surrounded by water. This design allows heat from the gases to transfer efficiently to the water, generating steam or hot water for heating and power generation. The movement of the combustion gases through the tubes creates a direct heating effect on the water outside the tubes.

In contrast, a watertube boiler operates differently as it has water flowing through tubes, which are heated by surrounding combustion gases. The focus here is on the fact that firetube boilers utilize the gases to heat the water indirectly, rather than having the water flow through the fire tubes themselves.

The other options refer to characteristics that do not define the primary distinctions between firetube and watertube boilers. Electric resistance heating is irrelevant to these types of boilers, as they primarily operate on combustion processes. Similarly, the construction material being aluminum does not apply to either type as they are typically made from materials designed to withstand high pressures and temperatures, such as steel or iron. Understanding these differences is crucial in grasping the operational principles of different boiler types in stationary engineering.