It then can be seen that the shell fluid velocity has a component in the direction perpendicular to the tubes and a component in the direction parallel to the axis of the tubes. The velocity component in the direction perpendicular to the tubes is usually the most important for heat transfer. The fluid velocity in the direction perpendicular to the tubes depends on the separation between the baffles. The closer the
baffles are to each other, the smaller is the flow area, and the higher is the fluid velocity. This has a direct influence on the heat transfer coefficient, which increases with velocity. Of course, this increment in the velocity represents an increase in the frictional pressure drop.