The L fin design is for design temperatures less than 150 centigrade . This fin offers a certain degree of atmospheric protection of the tube wall against corrosion. The broad contact surface between the tube and the finning enhances heat transfer. This fin design is the most economical. 

This fin is created by forming an aluminum strip into an L-shape which is then tension wound around the primary tube. Each L section is positioned against each other to ensure complete tube coverage. Also this manufacturing technology allows the use of relatively thin wall tubes. 
The material: Any metal
Fin material: Aluminum
L fin design temp: less than 150 centigrade.