Heat Transfer Performance in Double-Pass Flat-Plate Heat Exchangers with External Recycle

The effect of external recycle on the performance in double-pass rectangular heat exchangers was investigated. Theoretical equations for heat transfer rate were derived base on the effectiveness-effect analysis. In constrast to a single-pass device without recycle, improvement in heat transfer of more than 20% is achievable if parallel-flow heat exchange is operated in a double-pass device of same size with external recycle which provides the increase of fluid velocity, resulting in reduction of heat-transfer resistance. It is found that the performances obtained in the double-pass devices with external recycle operating under concurrent and countercurrent flows are exactly the same.