This is possible when both fluids have infinite contact time, and separating wall has zero thermal resistance. In this situation the temperature difference between hot and cold fluid at any position will be very small and constant along the length of the heat exchanger.

Finite thermal resistance of the separating wall and flow rates of both fluids make the heat exchange irreversible. Finite flow rates of both fluids will have finite residence time depending on flow rates and as a result all the heat is not transferred from hot to cold fluid. Similarly finite thermal resistance of the wall will also limit the transfer of heat to the cold fluid. In such a situation for an adiabatic process . will be non-zero, but will have constant value when .

The practical result of the irreversibility is that the heat exchange is not complete and there is always some heat which is left with the POC on leaving the heat exchanger.

Difference in heat capacities of fluid will influence the heat exchange process. For example if cold fluid can be heated nearly to the entering temperature of hot fluid provided .