Exact and approximate formulas for cross flow heat exchangers with unmixed fluids

This article addresses the calculation of the effectiveness of a single-pass cross-flow heat exchanger where the two fluids are not mixed. The author proposes an exact formula which is more convenient than using infinite series expansions as has been proposed until now. I also propose an approximate formula accurate to at least 2% for all the values of the thermal capacity ratio and for a number of transfer units ranging from 0 to 6. This new formula offers the decisive benefit of being able to be reversed to explicitly give the number of transfer units depending on the thermal capacity ratio and the effectiveness.