逆流交换

藉逆流热交换或质量交换，交换的量可以比并流交换（同向流动）更多，因为逆流热交换能维持温差梯度下降得较慢。并流时，初始梯度很高，但是下降得很快，最终有浪费。以右图举例，受加热的流体（向上），离开交换器时，温度比降温后的流体（向下）离开的温度更高。对于平行流动的交换，此为不可能，因为两者的温度只能彼此趋近。所以逆流热交换与并流相比，同样条件下，可以达到更大的交换量。见换热器。

逆流交换的环路和循环常见于自然，尤其生物的系统中。恒温动物在寒冷的环境下，为了减少热能（能量）的散失，血管的排列变得特殊，使动脉的血与静脉的血之间进行逆流热交换。脊椎动物中，此种结构称为迷网，原先是鱼鳃从水吸收氧气的器官名，工业模仿制成各种逆流交换系统。逆流交换是化学工程热力学和制造过程的重要概念，例如从糖用甜菜提炼蔗糖就要考虑。

Countercurrent exchange is a mechanism between two flowing bodies flowing in opposite directions to each other, in which there is a transfer of some property, usually heat or some chemical. 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. It occurs in nature and is mimicked in industry and engineering. It is a kind of exchange using counter flow arrangement.

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 adjacent diagram, 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.

Countercurrent exchange when set up in a circuit or loop can be used for building up concentrations, heat, or other properties of flowing liquids. Specifically when set up in a loop with a buffering liquid between the incoming and outgoing fluid running in a circuit, and with active transport pumps on the outgoing fluid's tubes, the system is called a countercurrent multiplier, enabling a multiplied effect of many small pumps to gradually build up a large concentration in the buffer liquid.

Other countercurrent exchange circuits where the incoming and outgoing fluids touch each other are used for retaining a high concentration of a dissolved substance or for retaining heat, or for allowing the external buildup of the heat or concentration at one point in the system.

Countercurrent exchange circuits or loops are found extensively in nature, specifically in biologic systems. In vertebrates, they are called a rete mirabile, originally the name of an organ in fish gills for absorbing oxygen from the water. It is mimicked in industrial systems. Countercurrent exchange is a key concept in chemical engineering thermodynamics and manufacturing processes, for example in extracting sucrose from sugar beet roots.

Countercurrent multiplication is a similar but different concept where liquid moves in a loop followed by a long length of movement in opposite directions with an intermediate zone. The tube leading to the loop passively building up a gradient of heat (or cooling) or solvent concentration while the returning tube has a constant small pumping action all along it, so that a gradual intensification of the heat or concentration is created towards the loop. Countercurrent multiplication has been found in the kidneys as well as in many other biological organs.

Three current exchange systems

Countercurrent exchange and cocurrent exchange are two mechanisms used to transfer some property of a fluid from one flowing current of fluid to another across a barrier allowing one way flow of the property between them. The property transferred could be heat, concentration of a chemical substance, or other properties of the flow.

Countercurrent exchange in biological systems

Countercurrent exchange is used extensively in biological systems for a wide variety of purposes. For example, fish use it in their gills to transfer oxygen from the surrounding water into their blood, and birds use a countercurrent heat exchanger between blood vessels in their legs to keep heat concentrated within their bodies. In vertebrates, this type of organ is referred to as a rete mirabile (originally the name of the organ in the fish gills).

Countercurrent exchange in industry and scientific research

Countercurrent Chromatography is a method of separation, that is based on the differential partitioning of analytes between two immiscible liquids using countercurrent or cocurrent flow. Evolving from Craig's Countercurrent Distribution (CCD), the most widely used term and abbreviation is CounterCurrent Chromatography (CCC), in particular when using hydrodynamic CCC instruments.