The common existence of heat transfer phenomena in energy utilization systems highlights the importance of appropriate design and optimization methods of heat transfer systems to an unprecedented level. In this contribution, we defined an alternative thermal resistance for heat exchangers based on the inlet temperature difference of hot and cold fluids, which was influenced by such two aspects as the finite heat capacity rates of hot and cold fluids and the entransy dissipation-based thermal resistance of heat exchangers. Meanwhile, the corresponding energy flow models are proposed to analyze the heat transfer performance of both individual heat exchangers and three basic heat exchanger networks, i.e. series, parallel and multi-loop by the thermo-electrical analogy method, which offer the solution for constructing the energy flow models of any heat transfer system. For validation, we constructed the energy flow model of a double-loop thermal management system. On this basis, the heat transfer characteristic was described on the system level and the overall system constraints were obtained by the Kirchhoffs Law without introducing any intermediate variables. With these system constraints, the optimization equations for the system were derived theoretically by applying the Lagrange multiplier method. Simultaneously solving these equations gave the optimal thermal conductances of each heat exchanger and the best allocations of heat capacity rate directly, which benefited energy conservation. That is, the energy flow model is reliable and convenient for the analysis and optimization of heat transfer systems. (C) 2016 Elsevier Ltd. All rights reserved.