Plate heat exchangers
Plate heat exchangers use two different processes. Firstly, there is the straightforward cross current process and secondly the cross countercurrent process. In the latter, a countercurrent portion inside the cross current heat exchanger increases the efficiency by lengthening the time during which the supply and extract airstreams are in contact. However, due to the internal pressure drop, this is only possible with lower airflow rates.
The operating principles, however, are the same for both processes, i.e. the airstreams are separated by finely incremented plates and routed past each other. The common separating surfaces enables heat to transfer directly from the warm to the cold airflow.
Construction prevents the supply air and extract airstreams to be separated with 100 % tightness. However, advanced plate heat exchangers keep this leakage rate to no more than 0.1 % of the nominal airflow rate, which is generally a negligible level.