In this slide, series and parallel configurations of multiple fans are examined. As this figure shows, when a system air flow/pressure curve is overlaid with a parallel performance curve, a higher system resistance results in a lower increase in flow from the parallel configuration. Therefore, parallel configuration should only be used when the fans can operate with a low air impedance near free delivery. With the same analysis, it is shown that the series configuration achieves the best results in a high air impedance system.