A new concept of heat transfer enhancement applicable for finned tube air-cooled heat exchangers is proposed. The concept called a 'partial bypass' arises from an effective usage of the ineffective air stream (bypass airflow). The concept is rather simple, focusing on increasing the Q max at the rear part of the heat exchanger. The concept can be implemented either with substantial increase of heat transfer rate without additional pressure drop penalty or with identical heat transfer rate but accompanies a much smaller pressure drop. Both the temperature difference amid inlet air and water and the mass flow rate into the second heat exchanger can be increased simultaneously. This eventually leads to an even higher Q max at the second heat exchanger and as a result a much larger heat transfer rate. The concept is especially helpful in the design of a deep tube bank and can be implemented with sufficient augmentation without additional penalty.