Editing Countercurrent exchange (section)

==== 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.