What is an example of a single pass boiler?

A vertical firetube boiler is an example of a single pass boiler because it allows the flue gases produced from combustion to pass through the tubes only once before being vented to the atmosphere. This type of boiler has a relatively simple design, with the combustion gases flowing vertically through the firetube, transferring heat to the water surrounding the tube.

This single flow path makes it efficient for certain applications, especially where space is limited or where straightforward installation and maintenance are priorities. The design allows for effective heat transfer while minimizing the complexity of the system.

In contrast, other types of boilers mentioned in the choices might have multiple passes of the flue gases or different configurations that do not fit the definition of a single pass system. For example, a horizontal firetube boiler may typically operate in a multi-pass configuration, increasing heat transfer efficiency but diverging from the single-pass classification. Similarly, horizontal and vertical water-tube boilers also generally utilize multiple passes or more complex layouts, which differentiates them from the simplicity of a vertical firetube setup.