Classification of heat exchangers according to flow direction
According to the direction of the flow heat exchangers can be classified as follows:
1) Counterflow;
2) Crossflow;
3) Parallel flow.

In counteflow type of heat exchangers, the two fluids flow parallel to each other, but in opposite directions. This is one of the most efficient configurations for single-pass arrangements.
In crossflow arrangement, the two fluids flow normal to each other. Examples of this arrangement are tube-fin heat exchanger and plate-fin heat exchanger.
In parallel flow arrangement, the fluids flow parallel to each other in the same direction. This arrangement is less efficient than counterflow.