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 kidneysBoth countercurrent exchange and countercurrent multiplication systems have been found in the kidneys. The latter in the loop of Henle, the first in the vasa recta 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.
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.
Cocurrent flow—half transfer
In the cocurrent flow exchange mechanism, the two fluids flow in the same direction.
As the cocurrent and countercurrent exchange mechanisms diagram showed, a cocurrent 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.
Cocurrent flow examples
A cocurrent heat exchanger is an example of a cocurrent flow exchange mechanism. Two tubes have a liquid flowing in the same direction. One starts off hot at , the second cold at . 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: , almost exactly between the two original temperatures ( and ). At the input end, there is a large temperature difference of and much heat transfer; at the output end, there is a very small temperature difference (both are at the same temperature of 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 cocurrent 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.
Countercurrent flow—almost full transfer
In countercurrent flow, the two flows move in opposite directions.
Two tubes have a liquid flowing in opposite directions, transferring a property from one tube to the other. For example, this could be transferring heat from a hot flow of liquid to a cold one, or transferring the concentration of a dissolved solute from a high concentration flow of liquid to a low concentration flow.
The counter-current exchange system can maintain a nearly constant gradient between the two flows over their entire length of contact. With a sufficiently long length and a sufficiently low flow rate this can result in almost all of the property transferred. So, for example, in the case of heat exchange, the exiting liquid will be almost as hot as the original incoming liquid's heat.
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 enters the top pipe. It warms water in the bottom pipe which has been warmed up along the way, to almost . 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 . Because the hot input is at its maximum temperature of , 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 , 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 ().
The result is that the top pipe which received hot water, now has cold water leaving it at , while the bottom pipe which received cold water, is now emitting hot water at close to . In effect, most of the heat was transferred.
Conditions for higher transfer results 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 cocurrent exchange systems, a variation in the gradient is expected because of a buildup of the property not being transferred properly.The specific heat capacity should be calculated on a mass basis, averaged over the temperature range involved. This is in keeping with the second law of thermodynamics
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). 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).
Countercurrent multiplication loop
A countercurrent multiplication loop 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 far end of the loop there is a 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, by allowing a natural buildup of concentration towards the tip inside the in-going tube, (for example using osmosis of water out of the input pipe and into the buffer fluid), and the use of many active transport pumps each pumping only against a very small gradient, during the exit from the loop, returning the concentration inside the output pipe to its original concentration.
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.
Theoretically a similar system could exist or be constructed for heat exchange.
In the example shown in the image, water enters at 299 mg/L (NaCl / H2O). Water passes because of a small osmotic pressure to the buffer liquid in this example at 300 mg/L (NaCl / H2O). 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 this example reaching 1200 mg/L. This is regulated by the pumping action on the returning tube as will be explained immediately.