Tube header for heat exchanger

A heat exchanger with at least one header box and a plurality of tubes extending therefrom, the header box comprising a tube header having:

• a header plate defining a header plane, the header plate having a core side and a header side and row of oblong passages extending through the header plate,
• each passage bordered by a ferrule monolithically formed with the header plate, the ferrule having a surrounding wall with a wall thickness extending perpendicular to the header plane;
• and
• wherein a transitional area between the ferrule and the header plate has a reduced thickness that is smaller than the wall thickness of the ferrule, and the header plate is made of cold-formed sheet metal,
• wherein each passage has a pair of opposing wide sides and a pair of opposing narrow sides, each transitional area is formed at an end of a taper with a steady slope toward the ferrule on the core side over a taper length at least along the wide sides of each passage, the taper gradually reducing the thickness of the header plate toward the transitional area, wherein the reduced thickness reaches a minimum thickness in a portion of the taper within the taper length and close to the ferrule.