What is characteristic of an externally fired furnace?

An externally fired furnace is distinctive due to its design, which involves being situated outside the boiler structure, leading to it being surrounded by brickwork rather than being enclosed by water. This setup allows for improved efficiency in heat transfer to the fluid that is being heated, as the combustion occurs in a separate chamber where the heat is absorbed by the water in the boiler.

This design contrasts sharply with other systems where the furnace might be completely enclosed by water, as seen in traditionally water-cooled boilers. Externally fired furnaces are versatile and can accommodate various fuels, including gases and liquids, not just solids. Also, while they can operate at varying temperatures, the characteristic feature is related to their construction and the medium surrounding them rather than specifically about operating temperatures.

Thus, the description of being "not surrounded by water but surrounded by brickwork" accurately captures the essence of an externally fired furnace and differentiates it from other types of furnaces and their configurations.