What types of boilers have the furnace separate from the vessel?

Boilers with a separate furnace from the vessel are typically known as "external furnace" or "fire-tube" boilers. In these systems, the combustion gases pass through tubes that are located within the water-filled vessel, allowing for efficient heat transfer. This design contrasts with "water-tube" boilers, where water flows through tubes located in the furnace. Fire-tube boilers are often used in smaller applications, while water-tube boilers are preferred for larger, high-capacity systems.