Which type of boiler has its combustion chamber located outside the boiler drum?

The type of boiler that has its combustion chamber located outside the boiler drum is known as an externally-fired boiler. In this design, the combustion of fuel takes place in a separate chamber, and the heat produced is then transferred to the water in the boiler drum through heat exchange surfaces. This arrangement allows for more flexibility in fuel type and can lead to improved combustion efficiency compared to internally-fired boilers, where the combustion chamber is situated within the boiler drum itself.

Externally-fired boilers are commonly used in applications where it's beneficial to handle larger combustion volumes or for specific types of fuel. Furthermore, this design can potentially reduce the risk of overheating the boiler metal structure, as the combustion gases are kept separate from the water/steam environment.

In contrast, internally-fired boilers incorporate the combustion chamber into the boiler's main structure, which can limit design options and raise safety considerations related to the high temperatures within the drum. The Scotch Marine Boiler, while popular in marine applications, features a horizontally oriented fire-tube design and is internally fired. Forced draft boilers use a fan to introduce combustion air but do not specifically indicate the location of the combustion chamber relative to the drum. Thus, the defining characteristic of an externally-fired boiler is its separate combustion chamber, making it distinct and clearly