Editing Countercurrent exchange (section)

== Three current exchange systems ==
[[Image:Heat exchanger.svg|thumb|right|320px|Three topologies of countercurrent 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 ===
[[Image:Comparison of con- and counter-current flow exchange.jpg|thumb|right|400px|A comparison between the operations and effects of a cocurrent and a countercurrent flow exchange system is depicted by the upper and lower diagrams respectively. In both it is assumed (and indicated) that red has a higher value (e.g. of temperature) than blue and that the property being transported in the channels therefore flows from red to blue. Channels are contiguous if effective exchange is to occur (i.e. there can be no gap between the channels).]]
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====
[[Image:Delta T 1.svg|thumb|right|200px|Cocurrent and countercurrent heat exchange]]
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 {{Convert|60|°C|abbr=on}}, the second cold at {{Convert|20|°C|abbr=on}}. 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: {{Convert|40|°C|abbr=on}}, almost exactly between the two original temperatures ({{Convert|20|°C|abbr=on}} and {{Convert|60|°C|abbr=on}}). At the input end, there is a large temperature difference of {{Convert|40|°C|abbr=on}} and much heat transfer; at the output end, there is a very small temperature difference (both are at the same temperature of {{Convert|40|°C|abbr=on}} 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 [[osmosis|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 ===
[[Image:Spiral-heat-exchanger-schematic-workaround.svg|thumb|right|130px|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, 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 {{Convert|60|°C|abbr=on}} enters the top pipe. It warms water in the bottom pipe which has been warmed up along the way, to almost {{Convert|60|°C|abbr=on}}. 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 {{Convert|60|°C|abbr=on}}. Because the hot input is at its maximum temperature of {{Convert|60|°C|abbr=on}}, 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 {{Convert|20|°C|abbr=on}}, 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 ({{Convert|21|°C|abbr=on}}).

The result is that the top pipe which received hot water, now has cold water leaving it at {{Convert|20|°C|abbr=on}}, while the bottom pipe which received cold water, is now emitting hot water at close to {{Convert|60|°C|abbr=on}}. 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 [[solvent]]s and [[Solution (chemistry)|solution]]s 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.<ref>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</ref>