Water wall tubes serve to protect the refractory and do what else?

Water wall tubes play a crucial role in boiler design and function by enhancing the heating surface area. This increase in heating surface allows for better heat exchange between the combustion gases and the water inside the tubes. As the water is heated more effectively, it becomes steam, which enhances the overall efficiency of the boiler system.

The water wall tubes, typically located on the sides of the furnace, absorb a significant amount of heat from the flue gases, thereby improving the boiler's thermal performance. This increase in heating surface contributes to higher overall efficiency and output of steam, which is essential for various applications in power generation and heating systems.

In this context, the other options do not align with the primary function of water wall tubes in a steam boiler system. They either focus on aspects that are not directly influenced by the presence of water wall tubes or are related to other components and operational strategies within the boiler system.