Internal furnaces are:

Internal furnaces are typically designed to be surrounded by water; this allows for efficient heat transfer and helps to manage the high temperatures produced within the furnace during operation. The water surrounding the furnace absorbs heat from the combustion process, which contributes to the overall efficiency of the system and helps maintain the structural integrity of the equipment. By having water as a buffer, the risk of overheating and damage to the furnace structure is reduced.

The other options don't accurately represent the primary function or characteristics of internal furnaces. While some of the options may describe related concepts in boiler design or operation, they do not specifically capture the defining feature of internal furnaces, which is their relationship with water for heat management and safety.