What characterizes an internally fired boiler?

An internally fired boiler is characterized by having its combustion chamber located within the shell of the boiler itself. This design allows for better heat absorption and more efficient heat transfer to the water or steam inside the boiler. By enclosing the grate within the boiler shell, the combustion process occurs directly where the heat generated is needed most, thereby improving thermal efficiency.

The other options do not accurately describe an internally fired boiler. For example, if the combustion chamber were outside the shell, it would classify the boiler as an externally fired boiler, which operates under different principles. Similarly, internally fired boilers can operate using various fuels, not limited to gas alone, so options related to specific fuel types like gas do not apply. While some internally fired boilers may have a high water capacity, this characteristic is not unique to them and can be found in many other boiler types.