This paper reports the difference between the irreversibility and available energy loss of heat transfer process in heat exchanger. Based on the irreversibility analyzing in parallel flow and counter-flow heat exchangers, the driving force equipartition principle is derived. Then by the deduction of putting the principle in counter-flow heat exchanger, the principle application feasibility is discussed further. The results show that the changing trend of irreversibility and available energy loss are different obviously. With NTU variation, the available energy loss is declining gradually until stability while the irreversibility is increased first then reduced sharply. It is clear that process with big irreversibility is not always bad, and the available energy loss per unit heat transfer is more suitable for heat exchange criterion. Further, the heat exchangers or heat transfer process can be optimized by changing the temperature distribution. Crown Copyright (C) 2015 Published by Elsevier Ltd. All rights reserved.