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Countercurrent exchange along with Concurrent exchange comprise the mechanisms used to transfer some property of a fluid from one flowing current of fluid to another across a semipermeable membrane or thermally-conductive material between them. The property transferred could be heat, concentration of a chemical substance, or others. Countercurrent exchange is a key concept in chemical engineering thermodynamics and manufacturing processes, for example in extracting sucrose from sugar beet roots.

Contents

Overview

This diagram presents a generic representation of these two exchange systems, with two parallel tubes containing fluid separated by a semipermeable or thermoconductive membrane. The property to be exchanged, whose magnitude is represented by the shading, transfers across the barrier in the direction from greater to lesser and, in the case of heat, according to the second law of thermodynamics.

Concurrent exchange and countercurrent exchange.
  • Concurrent Flow - In this exchange system, the two fluids flow in the same direction. As the diagram shows, a concurrent exchange system has a variable gradient over the length of the exchanger. With equal flows in the two tubes, this method of exchange is only capable of moving half of the property from one flow to the other, no matter how long the exchanger is. If each stream changes its property to be 50% closer to that of the opposite stream's inlet condition, exchange will stop because at that point equilibrium is reached, and the gradient has declined to zero. In the case of unequal flows, the equilibrium condition will occur somewhat closer to the conditions of the stream with the higher flow.
  • Countercurrent Flow - By contrast, when the two flows move in opposite directions, the system can maintain a nearly 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. However, note that nearly complete transfer is only possible if the two flows are, in some sense, "equal". If we are talking about mass transfer, then this means equal flowrates of solvent or solution, depending on how the concentrations are expressed. For heat transfer, then the product of the average specific heat capacity (on a mass basis, averaged over the temperature range involved) and the mass flow rate must be the same for each stream. If the two flows are not equal (for example if heat is being transferred from water to air or vice-versa), then conservation of mass or energy requires that the streams leave with concentrations or temperatures that differ from those indicated in the diagram.

Examples

  • In a concurrent heat exchanger, the result is thermal equilibrium, with the hot fluid heating the cold, and the cold cooling the warm. Both fluids end up at around the same temperature, between the two original temperature. At the input end, we have a large temperature difference and lots of heat transfer; at the output end, we have a small temperature difference, and little heat transfer.
  • In a countercurrent heat exchanger, the hot fluid becomes cold, and the cold fluid becomes hot. At the hot end, we have hot fluid coming in, warming further hot fluid which has been warmed through the length of the exchanger. Because the hot input is at its maximum temperature, it can warm the exiting fluid to near its own temperature. At the cold end, because the cold fluid entering is still cold, it can extract the last of the heat from the now-cooled hot fluid in the other section, bringing its temperature down nearly to the level of the cold input fluid.

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 biology this is referred to as a rete mirabile. Mammalian kidneys use countercurrent exchange to remove water from urine so the body can retain water used to move the nitrogenous waste products (see countercurrent multiplication).

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Countercurrent exchange of heat in organisms

Countercurrent heat exchange (CCHE) is a highly efficient means of minimizing heat loss through the skin's surface because heat is recycled instead of being dissipated. This way, the heart does not have to pump blood as rapidly in order to maintain a constant body core temperature and thus, metabolic rate.

When animals like the leatherback turtle and dolphins are in colder water to which they are not acclimatized, they use this CCHE mechanism. Such CCHE systems are made up of a complex network of peri-arterial venous plexuses that run from the heart and through the blubber to peripheral sites (i.e. the tail flukes, dorsal fin and pectoral fins). Each plexus consists of a singular artery containing warm blood from the heart surrounded by a bundle of veins containing cool blood from the body surface. As these fluids flow past each other, they create a heat gradient in which heat is transferred. The warm arterial blood transfers most of its heat to the cool venous blood. This conserves heat by recirculating it back to the body core. Since the arteries give up a good deal of their heat in this exchange, there is less heat lost through convection at the periphery surface. [1]

Another biological example, with separated fluid flow rather than a single channel flowing back and forth, is in the legs of an arctic fox treading on snow. The paws are necessarily cold, but blood can circulate to bring nutrients to the paws without losing much heat from the body. Proximity of arteries and veins results in heat exchange, so that as the blood flows down it becomes cooler, and doesn't lose much heat to the snow. As the blood flows back up through the veins, it picks up heat from the blood flowing in the opposite direction, so that it returns to the torso in a warm state, allowing the fox to maintain a comfortable temperature.

See also

References


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