What is the function of the metal projections on the economizer tubes?

The correct answer highlights that the metal projections on economizer tubes serve to extend the heat transfer surface area. This is significant because an increased surface area allows for more efficient heat exchange between the flue gas and the water inside the tubes. This design improves the overall efficiency of the heat recovery process, as more heat can be transferred to the water, thus enhancing the performance of the economizer in recovering waste heat from the exhaust gases of a boiler system.

Understanding this function is crucial for anyone involved in the operation or maintenance of boiler systems, as maximizing heat transfer efficiency directly impacts energy consumption and operational costs.