Which type of furnace is surrounded by brickwork rather than by water?

The type of furnace that is surrounded by brickwork rather than water is known as an externally fired furnace. In this design, the combustion process occurs outside the area where the heat transfer takes place within the boiler or process system. This means that the furnace itself is enclosed with insulating brickwork, which can withstand high temperatures and protect surrounding structures.

Externally fired furnaces are often chosen for their capability to ensure better heat transfer efficiency and to manage higher operating temperatures without the risk of damaging water tubing that might be present in internally fired designs. They are widely used in applications where direct contact with water could lead to boiling or steam generation, which is not the case when the combustion chamber is separate from the heat transfer area.

In contrast, internally fired furnaces have a different configuration where combustion happens within or in close proximity to the heat transfer medium, typically water. Other furnace types mentioned, like the cyclone and staged combustion furnaces, have their unique designs and operational principles but are characterized differently regarding their heat transfer and combustion processes.