The copper tubes or hairpins are inserted into the fin pack and tube sheets

The tube sheets, fins and hairpins are then assembled and placed in a tube expander, where a steel ball slightly larger than the inside diameter of the copper tubes is pressed through all the tubes simultaneously.  This expands the tubes into the fins, forming a tight mechanical bond which is essential for good heat transfer.

After the coils are expanded, the top and bottom plates of the coil case are attached.

The circuiting is completed by brazing return bends into the open ends of the hairpins.  Then the headers are mounted and brazed on, completing the coil assembly.