Method for bending tubes

A method of bending tube to form a tubular heat exchanger for a furnace, comprising the steps of:

• providing a stainless steel tube having an outer diameter of approximately 1.75 inches and a wall thickness of approximately 0.035 inches;
• providing a split bend die having a controlled-wrinkle compression groove with a plurality of elongated indentations that are orthogonal to the groove, said bend die having a centerline radius less than 3.5 inches;
• providing a pressure die and a clamp die;
• seating the tube tangentially in said groove of said bend die;
• inserting a plastic plug mandrel into said tube;
• clamping said tube to said bend die with said clamp die;
• and
• moving said tube tangentially toward said bend die with said pressure die while rotating said bend die and clamp die approximately 180.degree. to form an approximately 180.degree. bend having controlled-wrinkles on the inside surface of said tube at said bend.

A method comprising the steps of:

• providing a steel tube having an outer diameter of 2.5 inches or less and a wall thickness of 0.05 inches or less;
• providing a bend die having a controlled-wrinkle tube groove comprising elongated indentations, said bend die having a centerline radius of 3.5 inches or less;
• providing a pressure die and a clamp die;
• seating said tube tangentially in said tube groove of said bend die;
• clamping said tube to said bend die with said clamp die;
• moving said tube tangentially toward said bend die with said pressure die while rotating said bend die and said clamp die to form a bend having controlled-wrinkles on the inside surface of said tube at said bend, said tube being bent without a ball mandrel inside of said tube;
• directing hot combustion gases through said tube wherein turbulence of said combustion gases is increased by flowing over said controlled wrinkles on said inside surface of said tube at said bend;
• and
• providing a flow of air across the outside of said tube to heat said air.