Which type of boiler allows combustion gases to pass through tubes within the boiler?

The correct choice is a fire tube boiler. In this type of boiler, the combustion gases produced by burning fuel travel through tubes that are surrounded by water. As the hot gases pass through these tubes, they transfer heat to the water in the surrounding shell, causing the water to heat up and eventually convert to steam.

This design allows for efficient heat transfer, making fire tube boilers a popular choice in various applications, particularly where lower pressure steam is required. The configuration of the tubes provides a significant surface area for heat exchange, contributing to the boiler's effectiveness in generating steam rapidly.

Other types of boilers do not use the same mechanism for heat transfer. For example, in a water tube boiler, water tubes carry the water while gases circulate outside the tubes, which is the opposite of the fire tube design. An environmental boiler typically focuses on minimizing emissions and may not reference a specific mechanism like the other options. The term "steam tube boiler" is not commonly used in the industry and does not accurately describe a boiler type.