Hello @samYF and welcome. The language on this forum is English, so please translate your post (and topic title) next time.
Google translate
Hello everyone, I’ve encountered a rather tricky problem: I need to calculate the surface area of duct fittings, such as elbows, tees, and crosses.
Looking forward to your replies.
Also 
How to get help on the Dynamo forums.
That being said. Look for / into Solid, Geometry, Faces and Surface nodes.