Louvered fin heat exchangers are frequently used in heat transfer applications where air is one of the working fluids. By punching delta winglet vortex generators in the louvered fins, the size of the tube wakes can be reduced. The objective of this paper is to study the influence of the louver and delta winglet geometry on the thermal and hydraulic performance of such a compound heat exchanger. To this end, three-dimensional numerical simulations were performed. The effect of the louver and delta winglet geometry on the heat exchanger's performance results in conflicting design requirements. A small fin pitch and large louver angle cause a strong flow deflection and thus a large contribution of the louvers. But in this case the generation of longitudinal vortices is suppressed and thus the effect of the delta winglets is small. Also the delta winglet geometry itself has an important influence. A well-considered geometry and location of the delta winglets is essential for an improved performance. Delta winglets completely located in the tube wakes should be avoided, because in this case they do not cause any enhancement effect. (C) 2012 Elsevier Ltd. All rights reserved.