Heat exchanger

A heat exchanger comprising:

• a pair of cylindrical headers vertically arranged at an interval in a radial direction;
• and a plurality of flat tubes arranged at intervals in an axial direction of the headers, both ends of each of the tubes being connected to the respective headers, the heat exchanger further comprising a plurality of tube groups arranged in an air flow direction, the tube groups comprising the plurality of tubes including the headers at both ends, the heat exchanger exchanging heat of air flowing through a core section formed by the tube groups and heat of a heating medium flowing through a plurality of heating medium flow holes forming flow channels in each of the tubes, wherein
• three headers and three tube groups are arranged in the air flow direction to form the core section,
• when a width dimension of the tubes is defined as Tw, a flow channel cross-sectional area per tube is defined as S, and the number of heating medium flow holes per tube is defined as N,
• the width dimension Tw of the tubes is equal to or greater than 5 mm and equal to or smaller than 11 mm,
• N is 3≤N≤5 when 5 mm≤Tw≤6 mm, 4≤N≤5 when 6 mm<Tw≤7 mm, 5≤N≤6 when 7 mm<Tw≤8 mm, 5≤N≤7 when 8 mm<Tw≤9 mm, 6≤N≤8 when 9 mm<Tw≤10 mm, and 7≤N≤9 when 10 mm<Tw≤11 mm, and
• the width dimension Tw of the tubes and the flow channel cross-sectional area S satisfy a relationship of −0.002470×Tw5+0.09542×Tw4−1.436×Tw3+10.50×Tw2−37.08×Tw+51.31≤S≤0.005616×Tw5−0.2314×Tw4+3.746×Tw3−29.70×Tw2+115.4×Tw−173.9.