This paper presents a cross-flow prate type heat exchanger which has been studied and manufactured in the laboratory conditions because of its effective use in waste heat recovery systems. This new heat exchanger is tested with an applicable experimental set up, considering temperatures, velocity of the air and the pressure losses occurring in the system. These variables have measured and the efficiency of the system has determined. The irreversibility of the heal exchanger has been taken into consideration while the design of the heat exchanger is being performed so that the minimum entropy generation number has analysed with respect to second law of thermodynamics in the cross-flow heat exchanger. The minimum entropy generation number depends on parameters called optimum flow path length, dimensionless mass velocity and dimensionless heat transfer area. Variations of the entropy generation number with these parameters are analysed and introduced with their graphics and comments. (C) 1997 Elsevier Science Ltd.