A heat exchanger having a pair of manifolds and flat tubes for heat exchange between a first fluidum flowing inside the tubes and a second fluidum flowing outside of the tubes. The manifolds receive ends of the tubes and have an inlet and an outlet for introducing the first fluidum into the flat tubes and for discharging therefrom. Each manifold has at least two adjacent parallel channels with a partition wall therebetween. Each channel is further defined by at least a second wall, with at least part of the second wall having a curved surface. The partition wall defines two parallel substantially flat surfaces facing the channels. Each channel has a perimeter in cross-section defined by a continuous line having curved and straight portions.