What is the design feature of integral furnaces?

Integral furnaces are characterized by their design that incorporates water tubes lining the furnace walls. This design is essential for enhancing heat transfer efficiency and controlling the temperature within the furnace. The water tubes absorb heat from the combustion process, allowing for efficient energy transfer to the water, which is then converted into steam. This feature is particularly valuable in applications where steam generation is required, such as in power plants or industrial processes.

By utilizing water tubes, integral furnaces can effectively manage high temperatures and pressures while improving safety and performance. The water tubes not only act as structural components of the furnace but also serve a critical purpose in the overall thermal efficiency of the system. The interaction between the hot gases and the water in the tubes helps to ensure that the temperature is kept at optimal levels for effective combustion and steam production.

The other choices do not accurately describe the defining characteristic of integral furnaces. While there may be furnaces that employ open flame combustion, use electric heating, or are designed solely for solid fuels, these features are not synonymous with the unique water tube design that sets integral furnaces apart.