Editing Countercurrent exchange (section)

=== 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>