A boiler with the furnace outside the shell and surrounded by brickwork is what type?

• A. Internal furnace
• B. External furnace
• C. Integral furnace
• D. Modular furnace

Answer: (B)

The key idea is where the furnace sits relative to the boiler shell. If the furnace is outside the shell and surrounded by brickwork, combustion happens in a separate brick-lined chamber and the hot gases heat the water through the boiler walls from outside. This is called an externally fired boiler. It differs from an internal furnace, where the furnace is inside the boiler shell and surrounded by water, so heat transfers from inside the vessel. The terms integral or modular refer to construction style rather than the furnace’s location, so they don’t describe this outside-brick arrangement. So, the described setup is an externally fired furnace.