Recently developed graphite plates with large in-plane thermal conductivity are considered as promising alternative to conventional metallic plate heat exchangers (PHE). A new analytical model is developed to study the impact, and the potentials, of the emerging orthotropic graphite-based plates in PHE under various convective regimes. Closed-form relationships are obtained for temperature and heat flux distributions, and applied to perform a comprehensive parametric study on the orthotropic conductivity effects. Our results show that increasing the in-plane thermal conductivity leads to significant changes in heat flow pattern and reduction in temperature variation along the plate. In spite of the remarkable effects of in-plane thermal conductivity on the heat flow pattern, through-plane thermal conductivity plays the key role in controlling the total heat transfer between the hot and cold fluid streams through the plate. Moreover, a new critical through-plane conductivity is proposed to calculate the maximum value of thermal conductivity that provides the highest heat transfer rate through orthotropic slabs. The critical value also includes convective heat transfer resistance of the fluid side and the plate thickness effects. To verify the present model, an independent numerical study is conducted using COMSOL Multiphysics. The analytical results are compared with the obtained numerical data as well as an existing data set in the literature and show a great agreement with less than 5% relative difference. (C) 2014 Elsevier Ltd. All rights reserved.