What is the difference between counterflow and parallel flow heat exchangers?

In counterflow, fluids flow in opposite directions - the cold fluid exits near where hot fluid enters, achieving the highest temperature approach and maximum heat transfer efficiency. In parallel flow, both fluids enter at the same end and flow in the same direction - the outlet temperatures approach each other but the cold fluid can never exceed the hot fluid outlet temperature. Counterflow is preferred because it achieves higher LMTD for the same terminal temperatures and can achieve closer temperature approaches.