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Counter heat current exchange: Note the gradually declining differential and that the once hot and cold streams exit at the reversed temperature difference; the hotter entering stream becomes the exiting cooler stream and vice versa.
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.
The maximum amount of heat or mass transfer that can be obtained is higher with countercurrent than cocurrent (parallel) exchange because countercurrent maintains a slowly declining difference or gradient (usually temperature or concentration difference). In concurrent 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 .
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 [1] as well as in many other biological organs.
Contents
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1 Three current exchange systems o
1.1 Concurrent flow - half transfer
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1.1.1 Concurrent flow examples o
1.2 Countercurrent flow - almost full transfer
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1.2.1 Conditions for higher transfer results
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2 Countercurrent exchange in biological systems o
2.1 Countercurrent multiplier
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2.1.1 In the kidney
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2.1.2 History o
2.2 Countercurrent exchange of heat in organisms o
2.3 Countercurrent exchange in sea and desert birds to conserve water
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3 Countercurrent exchange in industrial and scientific systems
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4 See also
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5 External links
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6 References
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Three topologies of countercurrent exchange systems
Countercurrent exchange along with concurrent exchange and contra-current exchange comprise the 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.
When heat is transferred, a thermally-conductive membrane is used between the two tubes, and when the concentration of a chemical substance is transferred a semipermeable membrane is used.
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Concurrent and countercurrent exchange mechanisms
The magnitude of the property to be exchanged, is represented by shading. The direction of transfer across the barrier is from the greater to the lesser magnitude..
In the concurrent flow exchange mechanism, the two fluids flow in the same direction.

As the Concurrent and countercurrent exchange mechanisms 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 when the point of 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.
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Concurrent and Countercurrent heat exchange
A concurrent heat exchanger is an example of a concurrent flow exchange mechanism.
Two tubes have a liquid flowing in the same direction. One starts off hot at 60
°C, the second cold at 20 °C. A thermoconductive membrane or an open section allows heat transfer between the two flows.
The hot fluid heats the cold one, and the cold fluid cools down the warm one. The result is thermal equilibrium:
Both fluids end up at around the same temperature: 40
°C, almost exactly between the two original temperatures (20 and 60
°C). At the input end, there is a large temperature difference of 40 °C and much heat transfer; at the output end, there is a very small temperature difference (both are at the same temperature of
40
°C or close to it), and very little heat transfer if any at all. If the equilibrium - where both tubes are at the same temperature - is reached before the exit of the liquid from the tubes, no further heat transfer will be achieved along the remaining length of the tubes.
A similar example is the concurrent concentration exchange . The system consists of two tubes, one with brine (concentrated saltwater), the other with freshwater (which has a low concentration of salt in it), and a semi permeable membrane which allows only water to pass between the two, in an osmotic process . Many of the water molecules pass from the freshwater flow in order to dilute the brine, while the concentration of salt in the freshwater constantly grows (since the salt is not leaving this flow, while water is). This will continue, until both flows reach a similar dilution, with a concentration somewhere close to midway between the two original

dilutions. Once that happens, there will be no more flow between the two tubes, since both are at a similar dilution and there is no more osmotic pressure .
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Spiral counter-current heat exchange schematic
In countercurrent flow, the two flows move in opposite directions.
Two tubes have a liquid flowing in opposite directions. The top tube starts off hot, the bottom cold. 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 heat transferred.
Countercurrent flow examples
In a countercurrent heat exchanger, the hot fluid becomes cold, and the cold fluid becomes hot.
In this example, hot water at 60
°C enters the top pipe. It warms water in the bottom pipe which has been warmed up along the way, to almost 60
°C. A minute but existing heat difference still exists, and a small amount of heat is transferred, so that the water leaving the bottom pipe is at close to 60
°C. Because the hot input is at its maximum temperature of 60
°C, and the exiting water at the bottom pipe is nearly at that temperature but not quite, the water in the top pipe can warm the one in the bottom pipe to nearly its own temperature. At the cold end - the water exit from the top pipe, because the cold water entering the bottom pipe is still cold at 20
°C, it can extract the last of the heat from the now-cooled hot water in the top pipe, bringing its temperature down nearly to the level of the cold input fluid (21
°C).
The result is that the top pipe which received hot water, now has cold water leaving it at 20
°C, while the bottom pipe which received cold water, is now emitting hot water at close to 60
°C. In effect, most of the heat was transferred.
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It should be noted that nearly complete transfer in systems implementing countercurrent exchange, is only possible if the two flows are, in some sense, "equal".
For a maximum transfer of substance concentration, an equal flowrate of solvents and solutions is required. For maximum heat transfer, the average specific heat capacity 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, similar to concurrent exchange systems, a variation in the gradient is expected because of a buildup of the property not being transferred properly.
[2]
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Rete mirabile = RM
Countercurrent exchange in biological systems was discovered and studied following the discovery of Countercurrent multiplication systems by Werner Kuhn .
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). 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 multiplier ).
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Counter current multiplier diagram
A countercurrent multiplier is a system where fluid flows in a loop so that the entrance and exit are at similar low concentration of a dissolved substance but at the tip of the loop there is a very high concentration of that substance. A buffer liquid between the incoming and outgoing tubes receives the concentrated substance. The incoming and outgoing tubes do not touch each other.
The system allows the buildup of a high concentration gradually, with the use of many active transport pumps each pumping only against a very small gradient.

Theoretically a similar system could exist or be constructed for heat exchange.
The incoming flow starting at a low concentration has a semipermeable membrane with water passing to the buffer liquid via osmosis at a small gradient. There is a gradual buildup of concentration inside the loop until the loop tip where it reaches its maximum.
In the example image water enters at 299 mg/L (NaCL / H
2
O). Water passes because of a small osmotic pressure to the buffer liquid in this example at 300 mg/L (NaCL / H
2
O). Further up the loop there is a continued flow of water out of the tube and into the buffer, gradually raising the concentration of NaCL in the tube until it reaches 1199 mg/L at the tip. The buffer liquid between the two tubes is at a gradually rising concentration, always a bit over the incoming fluid, in our example reaching 1200 mg/L. This is regulated by the pumping action on the returning tube as explained immediately.
The tip of the loop has the highest concentration of salt (NaCL) in the incoming tube - in the example
1199 mg/L, and in the buffer 1200 mg/L. The returning tube has active transport pumps, pumping salt out to the buffer liquid at a low difference of concentrations of up to 200 mg/L more than in the tube. Thus when opposite the 1000 mg/L in the buffer liquid, the concentration in the tube is 800 and only 200 mg/L are needed to be pumped out. But the same is true anywhere along the line, so that at exit of the loop also only 200 mg/L need to be pumped.
In effect, this can be seen as a gradually multiplying effect - hence the name of the phenomena: a
'countercurrent multiplier' or the mechanism: Countercurrent multiplication .
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Loop of Henle (Grey's Anatomy book)
A circuit of fluid in the Loop of Henle - an important part of the kidneys allows for gradual buildup of the concentration of urine in the kidneys, by using active transport on the exiting ' nephrons ' (tubules carrying liquid in the process of gradually concentrating the urea). The active transport pumps need only to overcome a constant and low gradient of concentration, because of the countercurrent multiplier mechanism [3]
Various substances are passed from the liquid entering the Nephrons until exiting the loop (See the Nephron flow diagram). The sequence of flow is as follows:
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Renal corpuscle : Liquid enters the nephron system at the Bowman's capsule.
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Proximal convoluted tubule : It then may reabsorb urea in the thick descending limb .
[5] Water is removed from the nephrons by osmosis (and Glucose and other ions are pumped out with active transport ), gradually raising the concentration in the nephrons.
[6]
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Loop of Henle Descending : The liquid passes from the thin descending limb to the thick ascending limb.
Water is constantly released via osmosis.
[7] Gradually there is a buildup of osmotic concentration, until
1200 mOsm is reached at the loop tip, but the difference across the membrane is kept small and constant.

For example, the liquid at one section inside the thin descending limb is at 400 mOsm while outside it's
401. Further down the descending limb, the inside concentration is 500 while outside it is 501, so a constant difference of 1 mOsm is kept all across the membrane, although the concentration inside and outside are gradually increasing.
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Loop of Henle Ascending : after the tip (or 'bend') of the loop, the liquid flows in the thin ascending limb .
[8] Salt - Sodium Na + and Chlorine Cl ions are pumped out of the liquid [9] gradually lowering the concentration in the exiting liquid, but, using the countercurrent multiplier mechanism, always pumping against a constant and small osmotic difference.
For example, the pumps at a section close to the bend, pump out from 1000 mOsm inside the ascending limb to 1200 mOsm outside it, with a 200 mOsm across. Pumps further up the thin ascending limb, pump out from 400 mOsm into liquid at 600 mOsm, so again the difference is retained at 200 mOsm from the inside to the outside, while the concentration both inside and outside are gradually decreasing as the liquid flow advances.
The liquid finally reaches a low concentration of 100 mOsm when leaving the thin ascending limb and passing through the thick one [10]
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Distal convoluted tubule : Once leaving the loop of Henle the thick ascending limb can optionally reabsorb and re increase the concentration in the nephrons.
[11]
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Collecting duct : The collecting duct receives liquid between 100 mOsm if no re-absorption is done, to 300 or above if re-absorption was used. The collecting duct may continue raising the concentration if required, by gradually pumping out the same ions as the Distal convoluted tubule, using the same gradient as the ascending limbs in the loop of Henle, and reaching the same concentration.
[12]
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Ureter : The liquid urine leaves to the Ureter .