Editing Countercurrent exchange (section)

=== 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]].