Countercurrent exchange
From Free net encyclopedia
Countercurrent exchange is a mechanism used to transfer some property of a fluid from one flowing current of fluid to another across a semipermeable barrier between them. The property transferred could be heat, concentration of a chemical substance, or others. 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. The human kidneys use Countercurrent exchange to remove water from excrement so the body can retain water used to move the nitric waste products. Countercurrent exchange is also a key concept in chemical engineering thermodynamics and manufacturing processes for example in extracting sucrose from sugar beet roots.
The diagram presents a generic representation of a countercurrent exchange system, with two parallel tubes containing fluid separated by a permeable barrier. The property to be exchanged, whose magnitude is represented by the shading, transfers across the barrier in the direction from greater to lesser according to the second law of thermodynamics. With the two flows moving in opposite directions, the countercurrent exchange system maintain a constant gradient between the two flows over their entire length. With a sufficiently long length and a sufficiently low flow rate this can result in almost all of the property being transferred.
By contrast, in the concurrent (or co-current, parallel) exchange system the two fluid flows are in the same direction. As the diagram shows, a concurrent exchange system has a variable gradient over the length of the exchanger and is only capable of moving half of the property from one flow to the other, no matter how long the exchanger is.nl:tegenstroomprincipe de:Gegenstrom