Countercurrent heat exchangers with both fluids subjected to external healing

A mathematical model was developed to study the performance of a counterflo w hear exchanger in which both fluid streams are exposed to external heatin g. It was found that under the external heat transfer condition, the effect iveness of the heat exchanger was drastically reduced. The hot fluid temper ature effectiveness increased as the external thermal conductance ratios, R -h and R-c, decreased. For effective operation and to avoid temperature cro ss, there exist a maximum possible NTU for a given R-h and R-c.