Which of these is NOT a type of fire tube boiler?

The choice that is identified as not being a type of fire tube boiler is indeed accurately selected. A fire tube boiler operates by passing hot gases through tubes that are surrounded by water, and the various types of fire tube boilers are characterized by their specific designs and configurations.

The Scotch Marine boiler, vertical boiler, and HRT (Horizontal Return Tubular) boiler are all well-known types of fire tube boilers. The Scotch Marine boiler features a cylindrical design with internal flues, which maximizes heat exchange and efficiency. Vertical boilers, as the name suggests, are oriented vertically, which can be advantageous in terms of space efficiency. The HRT boiler is designed with horizontal tubes that return the hot gases back through the water for improved heating.

In contrast, the Cross Drum boiler is classified as a water tube boiler. In water tube boilers, water circulates within tubes that are heated externally by gases, which is fundamentally different from the operation of fire tube boilers. Understanding these distinctions is essential, as it highlights the differences between boiler types based on their design and operational characteristics.