9bbafd4a_Duct_Heaters___Wattco_com__vs__Unfinned_tubular_elements_Finned_tubular_elements
[Unfinned Tubular elements] Made of steel or stainless steel tube [Finned tubular elements] 
Made of a steel tube with a corrugated steel fin wrapped around it and brazed together
Duct Heaters - Wattco.com
Finned tubular elements