What defines an internally fired boiler?

An internally fired boiler is characterized by having the furnace surrounded by water. This design ensures that the heat generated from the combustion of fuel occurs within a space that is directly in contact with the water, promoting efficient heat transfer. The water surrounding the furnace helps absorb the heat produced, leading to effective steam generation or hot water production. This configuration can also enhance safety, as the water acts as a barrier to heat and can help contain any potential issues within the furnace.

In contrast, other characteristics mentioned in the other options do not define an internally fired boiler. For instance, while efficiency can vary between different designs, the efficiency of an internally fired boiler can be optimized and does not inherently fall below that of externally fired units. The use of refractory and firebrick relates more to the materials and construction of the boiler rather than its classification as internally or externally fired. Additionally, while some boilers may require a setting for installation, it is not a defining feature of being an internally fired boiler.