本文使用有效性法探討三種不同流動方式(平行流、逆流及正交流)之雙級熱交換器系統的熱特性。文中假設流體之比熱及總體熱傳係數為定值。起初，在雙級熱交換器中以最小總熱傳面積為研究目的，找出適當的中間流體溫度。接著，於本研究提出設計之實例並討論ー些特殊情況。由實例的計算結果，可得到平行流、逆流及正交流等三種不同之雙級熱交換器系統的最佳操作狀態。

In this study, the fin effectiveness method is employed to investigate the thermal characteristics of parallel flow, counter flow and cross flow heat exchangers operated in series. The specific heat of the fluids and overall heat transfer coefficient are assumed constants. In the beginning, analysis is focused on the minimum heat transfer areas of heat exchangers. The temperature of the fluid flow in amid of the exchangers then is obtained. Subsequently, the design method is put into some special practical applications. From the calculated results, the optimum operating conditions of parallel flow, counter flow and cross flow heat exchangers are computed and proposed respectively.