<P> Countercurrent exchange is a mechanism occurring in nature and mimicked in industry and engineering, in which there is a crossover of some property, usually heat or some component, between two flowing bodies flowing in opposite directions to each other . The flowing bodies can be liquids, gases, or even solid powders, or any combination of those . For example, in a distillation column, the vapors bubble up through the downward flowing liquid while exchanging both heat and mass . </P> <P> The maximum amount of heat or mass transfer that can be obtained is higher with countercurrent than co-current (parallel) exchange because countercurrent maintains a slowly declining difference or gradient (usually temperature or concentration difference). In cocurrent exchange the initial gradient is higher but falls off quickly, leading to wasted potential . For example, in the diagram at the right, the fluid being heated (exiting top) has a higher exiting temperature than the cooled fluid (exiting bottom) that was used for heating . With cocurrent or parallel exchange the heated and cooled fluids can only approach one another . The result is that countercurrent exchange can achieve a greater amount of heat or mass transfer than parallel under otherwise similar conditions . See: flow arrangement . </P>

In a countercurrent exchange system the fluids in two tubes flow in opposite directions