Which type of boiler has heat and gases of combustion passing through tubes surrounded by water?

The correct choice refers to a firetube boiler, which operates by having the hot gases produced from combustion pass through tubes that are surrounded by water. In this design, the combustion gases transfer their heat to the water outside the tubes, allowing the water to heat up and eventually produce steam or hot water.

This method is efficient for generating steam, as it maximizes the surface area of the tubes exposed to the water, ensuring optimal heat transfer. Firetube boilers are often favored in applications where steam is needed quickly and in smaller quantities.

Other types of boilers, such as watertube boilers, function differently; in those, water flows through tubes that are surrounded by hot gases. This results in different operational characteristics, making each type of boiler suitable for specific applications and requirements. Cast iron and electric boilers have their own designs and mechanisms, further distinguishing them from firetube boilers.