Finned tubes with collars

Finned tubes with collars, The collar that I have labeled in the picture is made of close packed fins on the tube.  The collar is made out of fins.  If you look below you can see what I mean.  The fins are far apart then when you get to the collar the fins get very close together to form the collar.  Please see the pictures below:

The other characteristics of the tubes we need are as follows:

-Fin material is copper

-Tube material is stainless steel or 90/10 copper nickel

-The entire tube is coated with lead free tin solder to connect the fins to the tube

-Tube sizes we use are 1/2 inch up to 1 inch.  5/8 inch, 3/4 inch, and 7/8 inch are the most common tube sizes

-Tubes are 18 or 20 bwg wall thickness.

-The height of the fins are 1/8 inch or higher.