ANS:C - increase by 2¹⁶

When converting a single tube pass heat exchanger to a two-pass configuration, the pressure drop per unit length in the tube side typically decreases. This decrease occurs because in a two-pass configuration, the flow velocity in each pass is lower compared to a single pass, resulting in reduced frictional losses. The exact decrease in pressure drop per unit length depends on various factors such as the geometry of the heat exchanger, flow characteristics, and fluid properties. However, it is generally observed that the pressure drop decreases when moving from a single pass to a two-pass configuration. Therefore, the correct option is: decrease by 22