The evaporator has undulating ribs with parallel transverse fins on their flat portions. The ratio of the fin height (H) to the pitch (P) of the flat portions is between 0.75:1 and 0.90:1. The pitch can be 1.8 to 2.2 mm, while the fins (23) can be 2.3 to 2.7 mm wide and at an angle of 28 to 38 deg.
