What is the key feature of an externally fired boiler?

An externally fired boiler is characterized by its design where the combustion chamber is separate from the boiler shell itself. This means that the furnace, where the fuel is combusted, and the grates used for feeding the fuel are located outside of the boiler's primary structure. This design allows for better heat transfer and increased efficiency in producing steam or hot water, as the heat from the combustion can be effectively transferred to the water or steam within the boiler shell without the constraints of being housed together.

This distinction is critical in understanding the operation and efficiency of different types of boilers, as it impacts how heat is generated and transferred within a system. The separation of the combustion area from the boiler allows for more flexibility in design and can lead to improved designs that are specific to the needs of a particular application.