High efficiency in heat transfer makes heat pipe heat exchanger a common device applied in waste heat recovery systems. In order to optimize it, a mathematical model, which considers the effect of both convective heat transfer in the finned tube bank and the internal heat resistance of the heat pipes, was developed. This model contains all the resistances that were not considered in previous studies. Finally, an equation to determine the optimum position of separating plate in heat pipe heat exchanger was obtained. It was resulted that the optimum position is a function of geometrical arrangement of heat pipes in heat exchanger like transverse spacing, initial temperature and mass flow rates of streams, as much as internal and construction features of heat pipes. When the ratio of internal to evaporator overall resistance increases, the descending slope of the optimum position ratio, X, becomes steeper, and when this ratio goes upper than 2, X reaches the constant value of 0.5. If the ratio of evaporator to condenser overall resistance becomes 1, X reaches the constant value of 0.5 again. The optimum position ratio is 0.65 when the evaporator to condenser volumetric flow rate is 0.13 and moves toward the evaporator section as volumetric flow rate ratio increases.
