Countercurrent exchange

The counter-current principle (also counter-current method ) is a method used in the heat or mass transfer, to be passed in two streams in opposite directions to each other. It is a fundamental principle of the process technology, but it is also found in nature, such as the oxygen uptake of some animals through lungs or gills, the concentration of the urine in the kidney, or in the circulation of water birds feet and the muscles in some bony fish, sharks and rays (in order to " warm-bloodedness ").

In the counter-current principle is allowed two streams - such as cold and warm air ( heat exchanger ) or flue gas and scrubbing liquid ( in a column ) - from the opposite direction past each flock and bring them such in contact with each other, that between them, the exchange of matter or heat is possible. Since by the current leadership there is always a temperature or concentration gradient between the streams, it is ( ideally) possible to transfer almost the entire heat or mass loading of a material stream to the other. This is in particular in the separation and in the heat recovery technique of great importance. The method was formulated by Werner von Siemens in 1857 and used by Carl von Linde in 1895 for the first time on a large scale.

In the adjacent figure, a heat exchanger is shown in the lower half, which operates according to the countercurrent principle. The lower air flow was the fresh air supply of a residential building which is to be pre-heated by the exhaust air of the same house. It can be seen that the fresh air takes during the passage of the heat exchanger from left to right after and after almost all the thermal energy of the exhaust air. At the end only lost a small part of the heat; this can not be prevented because the heat transfer is always a last big driving temperature gradient is necessary - the heat exchangers would otherwise have to be infinitely long. With a flow guide according to the uniflow principle, more than half of the heat can be recovered.

Material transferor methods such as thermal separation processes or membrane separation processes operate on analog principle.